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Institute of Medicine (US) Committee on Damp Indoor Spaces and Health. Damp Indoor Spaces and Health. Washington (DC): National Academies Press (US); 2004.

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Damp Indoor Spaces and Health.

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2Damp Buildings

Almost all buildings experience excessive moisture, leaks, or flooding at some point. If dampness-related problems are to be prevented, it is essential to understand their causes. From a technologic viewpoint, one must understand the sources and transport of moisture in buildings, which depend on the design, operation, maintenance, and use of buildings in relation to external environmental conditions such as climate, soil properties, and topography. From a societal viewpoint, it is necessary to understand how construction, operation, and maintenance practices may lead to dampness problems. The interactions among moisture, materials, and environmental conditions in and outside a building determine whether the building may become a source of potentially harmful dampness-related microbial and chemical exposures. Therefore, an understanding of the relationship of building moisture to microbial growth and chemical emissions is also critical.

This chapter addresses those issues to the extent that present scientific knowledge allows. It starts with a description of how and where buildings become wet; reviews the signs of dampness, how dampness is measured, and what is known about its prevalence and characteristics, such as severity, location, and duration; discusses the risk factors for moisture problems; reviews how dampness influences indoor microbial growth and chemical emissions; catalogs the various agents that may be present in damp environments; and addresses the influence of building materials on microbial growth and emissions.

The chapter does not review effects of building dampness that are unrelated to indoor air quality or health. However, dampness problems often cause building materials to decay or corrode, to become structurally weakened or lose their thermal capacity, and thus to reduce their useful life. Dampness also causes building materials and furnishings to develop an unacceptable appearance. The societal cost of such structural and visual effects of dampness may be high.

As discussed below, there is no single, generally accepted term for referring to “dampness” or “damp indoor spaces.” This chapter and the remainder of the report adopts the terminology of the research being cited or uses the default term “dampness.”


Studies use various qualitative terms to denote the presence of excess moisture in buildings. These include dampness, condensation, building dampness, visible dampness, damp patches, damp spots, water collection, water ponding, and moisture problem. Dampness—however it is expressed—is used to signify a wide array of signs of moisture damage of variable spatial extent and severity. It may represent visual observations of current or prior moisture (such as water stains or condensation on windows), observed microbial growth, measurement of high moisture content of building materials, measurement of high relative humidity in the indoor air, moldy or musty odors, and other signs that can be associated with excess moisture in a building. Some studies make separate observations of dampness and mold, and both observed dampness and visible mold have been weakly associated with measured concentrations of fungi (Verhoeff et al., 1992). Chapter 3 discusses the various signs and measurements of dampness, moisture, or mold that have been used in studies and lists several examples.

Numerous technical terms are also used to describe characteristics of moisture and moisture physics, including absorption, adsorption, desorption, diffusion, capillary action, capillary height, convection, dew point, partial pressure, and water vapor permeability. A complete discussion of all the terms is beyond the scope of this study, but some that are used in the report are defined below.

The amount of water present in a substance is expressed in relation to its volume (kg/m3), or to its oven-dry weight (kg/kg). The former is referred to as moisture content (MC), and the latter as percentage moisture content (%-MC). MC is directly proportional to %-MC and to the density of the substance (Björkholtz, 1987).

Relative humidity (RH) is the existing water vapor pressure of the air, expressed as a percentage of the saturated water vapor pressure at the same temperature. RH reflects both the amount of water vapor in air and the air temperature. For example, if the temperature of a parcel of air is decreased but no water is removed, the RH will increase. If the air is cooled sufficiently, a portion of the gaseous water vapor in the air will condense, producing liquid water. The highest temperature that will result in condensation is called the “dewpoint temperature.” “Humidity ratio” is another technical term used to characterize the moisture content of air. The humidity ratio of a parcel of air equals the mass or weight of water vapor in the parcel divided by the mass or weight of dry (moisture-free) air in the parcel. Humidity ratio, unlike RH, is independent of air temperature. The indoor–outdoor humidity ratio can be used to estimate the rate of interior water vapor generation, or more qualitatively to indicate if a building has sources. Water generation rate can be computed from a moisture mass balance equation; however, the rate of outdoor air ventilation must be known. If the building has a dehumidifier or an air conditioner that dehumidifies, the rate of water removal via this device must be factored. Sorption and desorption of water and from indoor surfaces also complicates the estimation of the internal water vapor generation rate. Monthly mean water activity level has been proposed as a metric for evaluating whether mold growth will occur on surfaces of newly-designed buildings (TenWolde and Rose, 1994) but there is reason to be skeptical about its practicality because the level varies throughout a building and is not easily measured at all relevant locations (for example, in wall cavities).

The temperature of air and materials in a building varies spatially; therefore, RH also varies spatially. In the winter for example, the temperature of the interior surface of a window or wall will normally be less that the temperature of air in the center of a room. Air in contact with the window or wall will cool to below the central room temperature, increasing the local relative humidity. If the surface has a temperature below the dewpoint temperature of adjacent air, water vapor will condense on the surface, producing liquid water.

Without a source to moisten building material continuously, the MC of the material depends on temperature and the RH of the surrounding air. The RH of the atmosphere in equilibrium with a material that has a particular MC is known as the equilibrium relative humidity (ERH) (Oliver, 1997). Different materials have different distributions of pore size and degrees of hygroscopicity so materials that have the same ERH may have different MC. For example, at an ERH of 80%, the MC for mineral wool is about 0.3 kg/m3, for concrete can be 80 kg/m3, and for wood is about 90 kg/m3 (Nevander and Elmarsson, 1994).


Water exists in three states: solid (ice), liquid, and gas (water vapor). The molecules in liquid water and water vapor move freely; molecules in ice are bound into a crystal matrix and are unable to move except to vibrate. Liquid water is a cohesive fluid; when it interacts with other materials, it is affected by forces that originate in the new material. If a drop attaches to a surface that has a strong affinity for water, like wood, it will spread out across the surface. The attraction may be great enough that water will run along the bottom of a horizontal material—a roof truss, for example—until it comes to an air gap or a downward projection where gravity pulls it away from the surface and it falls.

Many building materials are porous, and the size of the pores affects their permeability. If the pores are small enough to keep both liquid water clusters and water vapor molecules from passing, the material is impermeable; metal foils are examples of such materials. Materials with slightly larger pores (building papers like Tyvek™ and builders felt) will shed liquid water but be relatively permeable to water vapor. If the material has pores that are large enough for tiny clusters of liquid water to enter, it will be permeable to both liquid water and water vapor. As a result of intermolecular forces, liquid water is drawn into the pores of such materials by capillary suction. Water drawn in that way is said to be absorbed by the porous material. Water migration through porous materials is a complex interaction of forces. Water molecules clinging to the surface of a solid material are bound to that surface by intermolecular forces. They cannot move about as freely as liquid water molecules or water vapor molecules and are in what is sometimes referred to as the adsorbed state. Water must accumulate on surfaces to a depth of four or five molecules before it begins to move freely as a liquid (Straube, 2001). Adsorbed water cannot be removed by drainage. In the adsorbed state, water molecules are less available for chemical and biologic purposes than they are in a nonadsorbed state.

It does not take a great deal of moisture to cause problems with sensitive materials like paper or composite wooden materials. Moisture sources in buildings include rainwater, groundwater, plumbing, construction moisture, water use, condensation, and indoor and outdoor humidity (Lstiburek, 2001; Straube, 2002). The first three are sources of liquid-water problems, construction moisture may result in both liquid-water and water-vapor problems, and condensation associated with humidity involves water vapor as well as liquid water. Moisture problems begin when materials stay wet long enough for microbial growth, physical deterioration, or chemical reactions to occur. Those may happen because of continual wetting or intermittent wetting that happens often enough to keep materials from drying. As discussed below, the important moisture-related variables in determining whether fungal growth occurs are those which affect the rate of wetting and the rate of drying (Lstiburek, 2002a).

The most damaging water leaks are those which are large enough to flood a building or small enough not to attract notice but large enough to wet or humidify a cavity space or material for a long time. Thus, the “best” leak is one that is large enough to be noticed right away but small enough that the wetting does not promote microbial growth or affect materials. Both floods and slow leaks can result in large areas of fungal growth. Condensation sometimes occurs over a large area and can also result in extensive mold growth.

Rainwater and Groundwater

Placing a building on a site does not change how much rain falls each year—it changes the path that rainwater takes on its journey through the hydrologic cycle. When building designs work properly, rainwater is collected and redirected so that it does not intrude into the buildings themselves. When collection and redirection fail, rainwater wets buildings. Buildings have been protected from rainwater for centuries by using gravity, air gaps, and moisture-insensitive materials to direct and drain water away from other materials that can be damaged by water through corrosion, microbial contamination, or chemical reaction (Lstiburek, 2001). Weakness in rainwater protection can be found in the detailing of the roof, walls, windows, doors, decks, foundation, and site. Rainwater leaks may take a long time to become noticeable because the water often leaks into cavities that are filled with porous insulation. Insulation may retain the water, keeping materials wet longer than would empty cavities.

Many roofing materials are impermeable to liquid water and can be repeatedly wetted and dried without damage. Wooden shingles and thatched roofs are exceptions. They drain the bulk of rainwater away from the interior but also absorb some of it. An air gap beneath then forms a moisture or water break and allows drying of the shingle or thatch by evaporation from inner and outer surfaces. Roof leaks typically occur at joints and penetrations; parapet walls, curbs for roof-mounted equipment and sky-lights, intersections between roofs and walls, and roof drains are common leakage sites. These leaks are often the result of failures in design or of installation of flashings and moisture or water breaks.

In climates that receive substantial snowfall, water can intrude through roofs as the result of melting snow. Ice dams occur when there is snow on a roof and roof temperatures reach 33°F (1°C) or higher at times when the outdoor air temperature is below freezing. Snow on the warm part of the roof melts and then follows the drainage path until it reaches roofing that is chilled below freezing by outdoor air. The water then freezes on this part of the roof and causes ice dams and icicles. Aggravating conditions for ice dams include sources of heat that warm snow-covered sheathing (air leaks and conductive heat loss from the building, recessed lighting fixtures in insulated ceilings, and uninsulated chimneys passing through attics) and valley roofs, which may collect water from a large surface area and drain it to one small location. Several design approaches are available for preventing ice dams:

  • Air-seal and heavily insulate the top of the building so that escaping heat does not reach the roofing.
  • Ventilate the roof sheathing from underneath with outdoor air. (In combination with the air sealing and insulation, this keeps roofing cold, so melting does not occur or is minimized to rates that do not result in ice problems).
  • Avoid heat sources in the vented attic or vent bays (for example, do not use recessed lights in insulated ceilings).

Rainwater protection in walls is accomplished largely with three basic methods: massive moisture storage, drained cladding, and face-sealed cladding (Lstiburek, 2001; Straube and Burnett, 1997). Historically, walls capable of massive moisture storage have been built of thick masonry materials (such as stone in older churches). Exterior detailing channels rainwater away from entry through such walls. The walls are also able to store a large amount of water in the adsorbed state, and their storage capacity is sufficient to accommodate rainwater wetting and drying cycles without causing problems.2 Rainwater intrusion problems occur in these walls when a pathway wicks water from the exterior to the interior, where moisture-sensitive materials are. Wooden structural members in masonry pockets, interior-finish walls made of wood or paper products, and furnishings composed of fabrics, adhesive, or composites are typical materials that may be affected by rainwater transported through walls by bridging or capillary suction.

Cladding (a protective, insulating, or decorative covering) with air gaps and a drain plane is another historical answer to rainwater intrusion. A drained-cladding wall has an exterior finish that intercepts most of the rainwater that strikes it but is backed by an air gap and water-resistant drainage material to keep any water that gets past the cladding from entering the wall beneath. Wooden clapboard, wooden shingles, board and bat, brick or block veneer, and traditional stucco are examples of cladding used in some climates in the United States that has historically been backed by an air gap and drainage layer. Asphalt-impregnated felt paper, rosin paper, and high-permeability spun-plastic wraps are examples of materials that are used as the drainage layer. Foam board and foil-faced composite sheathing have also been used as drain planes beneath cladding (Lstiburek, 2000). The most frequent problems in these walls occur when moisture-sensitive sheathings—such as oriented strand board (OSB), plywood, and low-density fiberboards—are not protected by a drainage layer.

Face-sealed walls are made of materials that are impermeable to water and are sealed at the joints with caulking or gaskets (Straube, 2001). Structural glazing, metal-clad wooden or foam panels, and corrugated metal siding are examples of face-sealed cladding. The intention is to seal the joints between the panels well enough to prevent rainwater entry. Rainwater intrusion occurs when the seals fail. Seals on some face-sealed walls need to be renewed every 4–5 years.

The unavoidable weakness in rainwater protection for any wall is at the penetrations—windows, doors, light fixtures, the roofs of lower portions of the structure, decks, balconies, and porches. Rainwater leaks through poorly detailed, designed or installed flashing are most common. Common errors include failure to provide detailed instructions for flashing in construction documents, providing two-dimensional details for situations that require three-dimensional flashing, installing head flashings on top of building paper rather than installed underneath, and ignoring leaks in the window itself. Wall drain papers for windows must be installed in the same way that a raincoat is worn: over, not tucked into rain pants. Pan flashing beneath windows can prevent leaks, even of poorly installed windows, from wetting the wall below (Lstiburek, 2000).

Foundations are typically protected from moisture problems by being constructed of materials that are resistant to water problems (stone, concrete, and masonry) and having rainwater diverted away from them (Lstiburek, 2000, 2001). (In some old buildings, foundation structures could be constructed of wooden piers, which might have to be kept wet.) Excessive moisture in foundations is often the result of poorly managed rainwater, but it may also result from groundwater intrusion, plumbing leaks, ventilation with hot humid air, or water in building materials (such as concrete) or in exposed soil (for example, saturated ground in a crawl space foundation). Rainwater is diverted by sloping the finish grade away from the building; rainwater and groundwater are diverted with subsoil drainage. Drainage systems use stone pebbles, perforated drain pipe, sand and gravel, or proprietary drainage mats. Stone pebbles and perforated pipe are typically enclosed in a filter fabric to prevent clogging by fine soil particles. Below-grade foundations are coated with dampproofing to provide a capillary break. Water problems occur if rainwater collected on the roof is drained to the soil next to the foundation. This may happen if the site is inadvertently contoured to collect rainwater and drain it into the building or if paving does so. Other problematic scenarios include a drainage pipe that is missing, is installed improperly, or does not drain to daylight or a sump pump; a drainage system that fills with silt carried by water percolating through the soil and that then clogs; and a failure to install a capillary break, which would keep water from being wicked through concrete products to the interior.

Foundations may be slab on grade (or near grade), full basements, crawl spaces, piers, or a combination of these types. A slab-on-grade foundation consists of a concrete slab that constitutes the first floor of the building. The perimeter of the slab may be thickened and reinforced, or it may be bound by a perimeter wall that extends some distance into the soil. The most common water problems with slab-on-grade foundations are caused when rainwater from the roof or site wets the foundation and the water is wicked up through concrete to wall or flooring materials. If air ducts are placed in or beneath the slab, these may flood with poorly managed rainwater.

A basement is made by excavating a large, pond-like hole in the ground and constructing walls and a floor in the bottom of the hole. A basement floor slab is wholly or partially below grade. Some basement floors are at grade on one side and below grade on another. A drainage system is placed on the bottom of the hole around the perimeter of the walls, and a capillary break in the form of stone pebbles or polyethylene film is placed beneath the floor. Walls are coated with some form of dampproofing to make a capillary break. Free-draining material is placed against the walls to divert water from the foundation into footing drains. Many potential causes of dampness problems in full basements result from vagaries of weather and defects in design, construction, and maintenance. Rainwater from the roof or site can easily saturate the soil near the foundation and make it more likely for liquid water to seep or run into the basement. A more subtle problem occurs when water wicking through the walls or slab evaporates into the basement, leaving the walls dry but over-humidifying the space. Placing framing, insulation, paneling, or gypsum board against a basement wall creates a microclimate between finished wall and basement wall. In fact, if the outdoor-air dewpoint is higher than the temperature in this space, ventilating air will add moisture to the cavity, not dry it and this can result in conditions favorable for microbial growth. A solution to this problem is to insulate the foundation wall on the outside. If the foundation is insulated on the inside, a material with high insulating value and low water-vapor permeability should be used; this will keep the warm humid basement air away from earth-chilled walls. Plastic foam insulation meets this criterion. If the water vapor permeability of the insulation is low enough, it will reduce drying from the foundation wall into the basement. Placing insulation beneath the floor slab can prevent basement floors from “sweating” during hot humid weather because it thermally isolates the concrete slab from the cool earth below.

A crawl space is constructed in the same way as a basement foundation except that it is shorter and often the floor is not covered by a concrete slab. Many crawl spaces have air vents through the walls intended to provide passive ventilation. Because crawl spaces are not intended for occupancy, drainage detailing around them is often lacking or poorly implemented. Rainwater intrusion is common. In addition, the floor is often exposed soil, which creates the potential for evaporation into the crawl space. Vents placed too close to the ground sometimes become rainwater intakes. When the outdoor-air dewpoint is higher than the temperature of the soil and foundation surfaces, ventilating air wets the crawl space rather than drying it (Kurnitski, 2000).

Pier foundations (concrete or crushed-stone footings for posts that constitute the major structural support for a building) are the most resistant to rainwater problems. Piers extend from the ground to above the surface of the soil to support the lower structure of a building. The most common water problem for this type of foundation occurs if a depression in the ground beneath the structure collects water and exposes the underside of the building to prolonged high humidity.

Plumbing and Wet Rooms

Most water intentionally brought into buildings is used for drinking, cooking, or cleaning. The bulk of this water passes harmlessly through drains to public or private treatment and is then released to the hydrologic cycle from which it was diverted. The pathway followed by such water consists of pipes, tubs, sinks, showers, dish and clothes washers, driers, and ventilating air. Most of the materials used in the pathway are moisture-insensitive—able to withstand dampness without decomposing, dissolving, corroding, hydrolyzing, or supporting microbial growth. Moisture problems occur when water leaks from pipes or from sinks, tub or shower enclosures, washing machines, ice machines, or other fixtures and appliances that have water hookups.

Pipes leak when joints are incorrectly made or fail, water freezes in them, the pipe material corrodes or decomposes, or a screw or nail is driven through them. Joints may not be correctly soldered, gasketed, cemented, or doped. Water lines lose integrity when they are exposed to acidic or caustic water or—in the case of rubber or plastic lines to washing machines—the polymers break down from oxidation or ultraviolet (UV) light exposure. Corrosive water may lead to mold growth if a large number of small leaks result. Pipes in exterior walls or unheated crawl spaces or attics may freeze and crack during subfreezing weather. A screw or nail driven through a pipe may not leak for some time, because the fastener seals the hole it made; after thermal expansion and contraction and corrosion work for some time, the pipe may begin to leak.

Drains and water traps are vulnerable to leaks. Overflows and careless installation and renovation practices also contribute to problems with fixtures and appliances that use water. The materials that surround tubs and showers—typically ceramic tiles and fiberglass panels—receive regular wettings. They must be constructed, sealed, and maintained to protect the wall and floor materials beneath them. As with rainwater protection, most problems occur at the joints. Grout between ceramic tiles often does not adequately serve as a capillary break and water wicks through to the base. In ceramic tile surrounds with paper-covered gypsum board as the base, mold growth may occur beneath the grout and on the backside of the gypsum board where water wicks through the paper facing the wall cavity. Depending on the detailing, water may also be wicked through the gaps where fiberglass panels overlap and meet tubs or shower pans. The shower pan in stand-alone showers is another weak spot. Essentially, these are basins that must hold a small depth of water. Leaks are most common at the drain penetration. Pans that are constructed on site have more joints to leak than prefabricated pans that are molded into a single piece. Poorly designed, incorrectly installed, and carelessly used shower curtains and doors are another source of problems. Tub surrounds and shower enclosures can be constructed of materials that are poor substrates for fungal growth; for example, fiber-cement board, rather than paper-covered gypsum board, can be used as the base for ceramic tile. Such steps reduce, but do not eliminate, the possibility of microbial contamination.

Construction Moisture

In newly constructed buildings, a large amount of water vapor can be released by wet building materials such as recently cast concrete, and wet wooden products (Christian, 1994). Manufactured products that were originally dry can become extensively wetted by exposure to rain during transportation, storage, and building construction. Case studies have attributed microbial contamination to the use of wet building materials or to wetting during building construction (Hung and Terra, 1996; Salo, 1999). Large areas of mold growth may occur when a floor enclosing an earth-floored crawl space is installed because the soil may be a reservoir of rainwater; the humidity in such a crawl space quickly becomes high when the floor deck is applied over moist earth. Floor decks made from OSB or plywood are vulnerable to mold growth during extended periods (23 days for OSB, 42 days for plywood) of RH greater than 95% (Doll, 2002).

Condensation and High Humidity

Condensation necessarily involves water-vapor transport. The two important variables for condensation are chilled surfaces and sources of water vapor. Materials chilled below the indoor or outdoor air temperature accumulate water molecules in the adsorbed state and are at risk for condensation; those chilled below the local dew point will begin to accumulate liquid water. Porous materials can hold more water vapor than impermeable ones before liquid water appears. The combination of high RH in indoor or outdoor air and cooled building materials increases the risk of dampness problems and microbial growth. Even without condensation, the local RH of air at the surface of cool material can be very high, leading to high moisture content in the material.

Figure 2-1 illustrates how much air needs to be cooled before the difference between the air temperature and dewpoint temperature equals zero and condensation occurs. Regardless of the initial air temperature, when the relative humidity is very high only a few degrees of cooling will result in condensation. For example, if the bulk of the air in a room has a RH of 80%, condensation will occur on a surface that is only about 7oF (4oC) cooler than the bulk room air temperature. Therefore, whenever cool surfaces are present due to cold outdoor temperatures or air conditioning of a building, high humidity poses a condensation risk. However, at present, there is no generally accepted upper limit for indoor RH level, based on the need to prevent dampness problems. Acceptable RH levels vary with climate and building features.

FIGURE 2-1. The difference between air and dewpoint temperatures needed for condensation to occur, expressed as a function of relative humidity, for three indoor air temperatures.


The difference between air and dewpoint temperatures needed for condensation to occur, expressed as a function of relative humidity, for three indoor air temperatures.

During periods of cooling in air-conditioned buildings, indoor materials are colder than the outdoor air. Ventilation is then a source of indoor moisture—not a removal process—unless the incoming ventilation air is first dehumidified (Harriman et al., 2001). If nonconditioned outdoor air is accidentally drawn across a surface that is chilled sufficiently by air-conditioned indoor air, condensation will occur. In the cooling season, that is most likely to happen when an exhaust fan or the return side of an air handler lowers indoor air pressure in rooms or depressurizes wall or ceiling cavities (Brennan et al., 2002). Outdoor air is drawn in by the lower air pressure and carries water vapor with it. Water vapor in this accidental outdoor airflow may condense on the backside of gypsum board or in cabinets that have holes for wire or plumbing. The backside of interior gypsum board and the underside of vinyl wallpaper on exterior walls are common locations for mold growth resulting from this process (Lstiburek, 2001). When buildings are air-conditioned, a combination of wind-driven rain and water-vapor transport can also result in condensation and mold growth beneath vinyl wallpaper, on the backside of gypsum board on an exterior wall, or on the backside of interior foam board (Lstiburek and Carmody, 1996). Those materials act as accidental vapor retarders on the cool side of the wall. Furnishings and wall decorations—such as pictures, cabinets, mirrors, and chalkboards—can also act as accidental vapor retarders.

Materials may be chilled by outdoor air when it is cold outdoors (Brennan et al., 2002). Cold-weather condensation is often observed on the interior side of windows. Because windows usually have a lower insulating value than solid walls, the room-side surface of the glass is cooler than the surface of the surrounding walls. Indeed, if there is condensation or frost on the window, the glass temperature is necessarily below the indoor-air dew point. Condensation may also occur on cold pipes and on the bottom side of roof sheathing (the side facing the attic rather than the sky), the inside of exterior-wall sheathing, and the back side of claddings (clapboards, stone slabs, concrete panels, plywood panels, and the like).

Foundations constructed of concrete, masonry, stone, and wood are often chilled by contact with the earth. If the indoor-air dew point is higher than the temperature of the earth-chilled surfaces, water will begin to condense (Brennan et al., 2002). As water condenses on capillary materials, such as concrete or wood, it is wicked away by capillary action. Hygroscopic concrete and stone foundation materials can store moisture in a relatively harmless state until they become saturated, at which time liquid water will appear. If the materials are coated with a vapor-impermeable material, such as sheet floor covering or many paints, condensation will immediately collect under the proper conditions. Water vapor in basements or crawl spaces may come from water passing through the foundation materials as liquid or vapor or from the ventilating air when outdoor-air dew points are high, or it may be dominated by water vapor from exposed soil (Kurnitski, 2000).

Moisture-related problems become more likely when basement areas are finished (CMHC, 1996). Many of the materials used to finish basements allow water vapor to diffuse through them but are relatively good thermal insulators; thus, the materials inhibit heating of the foundation by warm indoor air but allow moisture to reach the cool surfaces. When a wooden stud wall with fiberglass insulation covered with gypsum board is placed against a concrete foundation and no vapor retarder is used, water vapor can easily pass through the wall section via the permeable materials and through gaps. However, there is very little drying potential under these conditions. Vapor pressure moves water vapor from the basement into the wall and sometimes from the outdoors into the wall. Very little air is moving behind such walls so drying by airflow cannot be achieved. The “microclimate zone” behind the stud wall stays moist throughout the cooling season, while wooden studs and paper-covered gypsum provide nutrient for mold growth. A vapor retarder in the wall will not prevent migration through the gaps and holes but will reduce the drying potential of the wall and thus increase the importance of small rainwater or plumbing leaks. Carpet systems on floor slabs produce a similar phenomenon, unless there is an insulating, low-vapor-permeability layer in the system. Finished basements may have substantial mold growth because of those phenomena. Warming the surfaces of the earth-chilled materials, by insulating them or heating them prevents condensation. Insulating material placed inside the foundation must prevent vapor in the indoor air from reaching the chilled foundation materials and present a warm surface to the indoor air. As noted above, that is best accomplished by using a material with high insulating value and low vapor permeability. Insulating material placed on the outside of the foundation must resist biologic, chemical, and physical deterioration when exposed to soil and liquid water. Condensation on earth-chilled surfaces can also be avoided by dehumidifying the indoor air to lower the dew point to below the foundation surface temperatures.

Occupants as Sources of Moisture

High humidity indoors can originate in moisture emissions from cooking, washing clothes, bathing, and keeping living plants indoors. Respiration and perspiration by building occupants contribute to humidity, as does the use of humidifiers. In improperly ventilated building spaces, those sources can account for substantial problems. In addition to plumbing leaks and flooding by water overflow, wicking along wall surfaces from poor wet-mopping practices is a problem in some indoor environments.

The practices of cooking, bathing, and drying of clothes and the density of occupation vary among cultural and economic groups. In some homes, internal moisture is high because of nearly continuous simmering of foods or extensive indoor drying of clothes. Anecdotal evidence indicates that such activities can lead to high indoor humidity and associated microbial growth. In low-rise residential buildings, a damp foundation may contribute as much water vapor as all the rest of the sources combined (Angell, 1988).

Moisture in Heating, Ventilating, and Air-Conditioning Systems

Although relatively little attention has been directed to dampness and mold growth in heating, ventilating, and air-conditioning (HVAC) systems, there is evidence of associated health effects. Pollutant emissions linked to moisture and microbial growth in HVAC systems are one of several potential explanations for the consistent association of air-conditioning systems with an increased prevalence of nonspecific health symptoms, called sick building syndrome, experienced by office workers (Seppänen and Fisk, 2002). The presence of air conditioning in homes has also been associated with statistically significant increases in wheezing and other symptoms of current asthma (Zock et al., 2002). Mendell et al. (2003) analyzed data on 80 office buildings where complaints had been made and found an increased prevalence of lower respiratory symptoms associated with poor draining of water from the drain pans beneath cooling coils of HVAC systems (OR 2.6; CI 1.3–5.2). In contrast, a preliminary analysis of data on a representative set of 100 large U.S. office buildings found that dirty cooling coils, dirty or poorly draining drain pans, and standing water near outdoor air intakes were not associated with reports of mucus membrane symptoms, lower respiratory symptoms, or neurologic symptoms (Mendell and Cozen, 2002).

Liquid water is often present at several locations in or near commercial-building HVAC systems, facilitating the growth of microorganisms that may contribute to symptoms or illnesses. Outdoor air is often drawn from the rooftop or from a below-grade “well” where water (and organic debris) may accumulate. Raindrops, snow, or fog can be drawn into HVAC systems with incoming outside air, although systems are usually designed to prevent or limit this moisture penetration.

In both commercial and residential air-conditioning units, moving the supply-air stream in the direction of airflow leads it to the cooling coil where moisture condenses (as a consequence of cooling the air or intentionally for dehumidification). Ideally, that moisture drips from the surfaces of the coil into a drain pan with a drainage pipe. Drain pipes may become clogged with the remains of microbial growth. Occasionally, drain pans contain stagnant water because they do not slope toward the drain line. In drawthrough systems, drains may also be plugged or otherwise nonfunctional because air-pressure differences prevent drainage, sometimes causing the drain pan to overflow with water. If the velocity of air passing through the cooling coils is too high, water drops on the surface of the cooling coil can become entrained in the supply-air stream and deposit in the HVAC system downstream of the cooling coil. Air leaving the cooling coils is often nearly saturated with water vapor, and the high humidity of this air increases the risk of microbial growth. HVAC systems sometimes have a humidifier that uses steam or an evaporation process to add moisture. Humidifiers, used predominantly in colder climates, may have reservoirs of water or surfaces that are frequently wetted, or they may produce water drops that do not evaporate. Thus, there are many potential sources of liquid water and high humidity in HVAC systems.

Microbial growth in HVAC systems can be limited by (Ottney, 1993)

  • Using sloped drain pans with drains at the low point.
  • Correctly trapping drains or using critical orifice drains that work against negative pressure in the system.
  • Providing easy access to coils, drain pans, and the downstream side of cooling coils for inspection and cleaning.
  • Making inner surfaces of the air-conveyance systems of materials that are impermeable to water penetration and are easy to clean.
  • Protecting the system from particle buildup by using filters with greater than 25% dust spot efficiency.

Microbial contamination of HVAC systems has been reported in many case studies and investigated in a few multibuilding research efforts (Battermann and Burge, 1995; Bencko et al., 1993; Martiny et al., 1994; Morey, 1994; Morey and Williams, 1991; Shaughnessy et al., 1998). Sites of reported contamination include outside air louvers, mixing boxes (where outside air mixes with recirculated air), filters, cooling coils, cooling-coil drain pans, humidifiers, and duct surfaces. The porous insulating and sound absorbing material called duct liner that is used in some HVAC systems may be particularly prone to contamination (Morey, 1988; Morey and Williams, 1991). Bioaerosols from contaminated sites in an HVAC system may be transported to occupants and deposited on previously clean surfaces, making microbial contamination of HVAC systems a potential risk factor for adverse health effects.

A 2003 study investigated the health impact of such contamination by examining the association between ultraviolet germicidal irradiation (UVGI) of drip pans and cooling coils in buildings ventilation systems and indoor microbial concentrations and self-reported symptoms in occupants (Menzies et al., 2003). The researchers systematically turned UVGI lamps installed in the HVAC systems of three office buildings on and off over the course of a year and collected environmental and occupant data. Fungi, bacteria, and endotoxin concentrations were measured, and building occupants who were unaware of the operating condition of the UVGI lamps filled out questionnaires on their health. Other environmental data (temperature, humidity, air velocity, HVAC recirculation; and CO2, NOx, O3, formaldehyde, and total volatile organic compound concentrations) and occupant data (participants' assessment of thermal, physical, and air quality; and demographic, personal, medical, and work characteristics) were also collected. Occupants reported significantly fewer work-related mucosal symptoms (adjusted OR 0.7; 95% CI 0.6–0.9) and respiratory symptoms (0.6; 0.4–0.9) when the UVGI lamps were on. Reports of musculoskeletal symptoms (0.8; 0.6–1.1) and systemic symptoms (headache, fatigue, or difficulty concentrating) (1.1; 0.9–1.3) were not significantly different. Although median concentrations of viable microorganisms and endotoxins were reduced by 99% (CI 67%–100%) on surfaces exposed to UVGI, there were no significant decreases in airborne concentration. The results suggest that limiting microbial contamination of HVAC systems may yield health benefits, and follow-up research is recommended.



Table 2-1 provides examples of published data on the prevalence of signs of dampness in buildings. The studies address a variety of locations and climates. Different dampness metrics were used; most data were collected with occupant-completed questionnaires. The reported prevalence of signs of dampness ranges from 1% to 85%. In most datasets, at least 20% of buildings have one or more signs of a dampness problem. But because moisture metrics and data-collection methods varied among studies, comparisons of prevalence data from different studies can be only qualitative.

TABLE 2-1. Examples of Reported Prevalence of Signs of Building Dampness.


Examples of Reported Prevalence of Signs of Building Dampness.

Figure 2-2, which is based on the biennial U.S. Census American Housing Survey, plots the prevalence of water leaks in U.S. houses by year. The data indicate that the prevalence of water leaks generally decreased over the period 1985–2001 and that more leaks are from external sources of water—for example, rain—than from internal water sources—plumbing leaks and the like. In 2001, 11.8% and 9.4% of houses had water leakage from exterior and interior sources, respectively.

FIGURE 2-2. Prevalence of reported housing water leaks during the preceding 12 months, 1985–2001.


Prevalence of reported housing water leaks during the preceding 12 months, 1985–2001. SOURCE: Biannual American Housing Survey for the United States, Bureau of the Census, U.S. Department of Commerce, http://www.census.gov/hhes/www/ahs.html.

Most of the available data on dampness prevalence are related to houses and apartments, but some are related to other indoor environments. A study by Jaakkola et al. (2002) found that the self-reported prevalence of signs of dampness was similar in the workplaces and homes of subjects in the Finnish Environment and Asthma Study. In a study of 100 U.S. office buildings (Mendell and Cozen, 2002), 85% reported past water damage and 43% current water damage. Those prevalences are high relative to those typically reported in studies of homes; although localized water damage in a large office building may significantly influence exposures only of workers near the damage. Nonetheless, these data suggest that dampness in workspaces should not be ignored.

The committee did not identify any large systematic surveys of dampness in U.S. classrooms,3 but anecdotal reports of dampness in classrooms are common, and school data have been collected as part of broader characterizations of children's exposures. A survey of the condition of U.S. schools by the General Accounting Office (GAO) did not contain a specific question about dampness or water leaks; however, the documentation of GAO's visits to 41 schools included many references to water leaks (U.S. GAO, 1995). Daisey and Angell (1998) reviewed 49 health-hazard evaluations of educational facilities performed by the National Institute for Occupational Safety and Health in response to indoor air quality (IAQ) complaints; 28 of the evaluations reported water leaks in the building shell. Thus, the available evidence suggests that classrooms also commonly have dampness problems.

Table 2-1 includes data from two studies (Haverinen et al., 2001a; Nevalainen et al., 1998) that used inspections by trained personnel to assess the prevalence of signs of dampness. Only a few studies have analyzed differences between occupant-reported and investigator-verified prevalence of moisture and mold observations in buildings. Douwes et al. (1999) found that occupant's reports of damp spots or mold spots were better correlated with a measure of indoor mold than investigator's reports of these visible signs. Bornehag et al. (2001) concluded that, although in most studies occupants had reported more dampness than investigators had, this was due to the occupants' longer time perspective than the investigators' “snapshot” observations. A conflicting study by Williamson et al. (1997) found that occupants reported dampness less often than trained surveyors. Nevalainen et al. (1998) reported similar results, suggesting that the explanation was a result of a trained eye and of knowledge of what represents a critical problem. Dharmage et al. (1999a) examined the validity and reliability of interviewer-administered questionnaires against observations and measurements made by an independent researcher. Among 44 items examined for validity (defined as correspondence between occupant reports and independent observations or objective measurements), there was perfect or almost perfect agreement on 21 and substantial agreement on 19 others. Among 10 items examined for reliability (defined as correspondence between interviews conducted 1 year apart), there was perfect or almost perfect agreement on nine items and substantial agreement on the other one. They concluded that the data collected with questionnaires were both reliable and valid. In another study, Dales et al. (1997) concentrated on the validity and determinants of reported home dampness and molds. They established associations between occupant-reported water damage, mold and mold odors, and objectively measured concentrations of viable indoor fungi in dust. However, they found little association between questionnaire responses and an objective measure of total airborne fungal matter (ergosterol concentration) and there was evidence that—in the presence of low concentrations of viable fungi in dust—respondents reporting allergies were more likely to report visible mold growth than asymptomatic respondents. The authors therefore recommended that objective exposure measures, not questionnaires, be used in studies of the health effects of indoor fungi.

Information on the prevalence or severity of moisture damage reported by occupants is likely to be highly subjective. The validity and reliability of data gathered from questionnaires are affected by several survey factors, such as sample size, response rate, recall period, and factors related to the design of the questionnaire. Underreporting, overreporting, and systematic reporting bias that would not be corrected by increasing sample sizes are possible. But questionnaires are a relatively cost-effective method of collecting information on perceived indoor-air quality, especially if the sample is large, and questionnaire responses collected from the occupants themselves provide first-hand information; occupants' perceptions are also important in assessing the condition of a building.

Trained building inspectors have experience in observing and evaluating structures and may also be more objective than occupants, who have a personal relationship with the building. Those advantages would not, however, exclude the subjectivity of trained investigators. Thorough building investigations need both expert assessment of the building's condition and occupant knowledge of its history and current problems to complement each other.

Chapter 6 has further information on this topic, addressing the evaluation of moisture problems in the context of identifying sources and planning remediation.


Most studies have not attempted to quantify the severity of dampness or of damage associated with dampness. It is clear that the severity of dampness varies widely, from occasional minor condensation on windows to the wetting of a large portion of a building during a flood. The evaluation of the severity or magnitude of moisture problems can use several criteria, most of which are subjective. Excess moisture in a building environment may induce physical damage, but it may also manifest biologic or chemical damage. Direct, immediate impacts include structural, microbiologic, chemical, or aesthetic effects. Indirect consequences include health effects and remediation or repair costs. Because of the complexity of the evaluation, there is no agreed-on basis for determining the severity of damage from either the engineering or the health point of view.

In buildings that have moisture-induced damage, people can be exposed to a complex mixture of microorganisms, organic and inorganic dust, and volatile chemicals (Husman, 1996). It is difficult to measure and distinguish between the various agents and their effects, and exposures have often been defined indirectly and cumulatively as “damp housing” or living in a “water damaged” or “moldy” building. However, as noted in this chapter, there is no generally accepted definition of dampness or of what constitutes a dampness problem, and no generally accepted metric for characterizing dampness.

Several factors might be considered in evaluating the severity of moisture damage. Four of these are discussed below: the size of the damaged area, the presence of visible signs of moisture damage, the duration of its presence, and the building material on which the damage is observed.

The size or extent of damage is an important moisture-damage characteristic assumed to be related to source strength. It is reasonable to expect larger or more extensive damage to be associated with higher potential exposure. However, the literature does not provide much information on the estimation of damage size. Williamson et al. (1997) used a subjective grading of the extent of visible mold on a four-point scale: 0 = absent, 1 = trace, 2 = obvious but localized, and 3 = obvious and widespread.

Subjectively, the extent of visible mold contamination on surfaces in buildings has been taken into account in guidelines for cleanup procedures issued by government and professional organizations. Chapter 6 discusses those guidelines and their recommendations.

In a study intended to seek insights into the type of moisture damage that could be critical as a risk indicator for adverse health effects, a random sample of residential buildings was inspected for signs of moisture damage (Haverinen et al., 2001a). Trained building inspectors estimated the size of observed damage in square meters, and a dose-dependent association with respiratory infections and lower respiratory symptoms was observed. A later study used multivariate Poisson regression models to examine respiratory symptoms (Haverinen et al., 2003); the relative importance of a variable characterizing the size of moisture damage appeared to be high, and the authors concluded that the size of the damage is an important characteristic related to the severity of damage. It should be remembered, however, that estimation of the size of damage is difficult and that estimation accuracy varies because damage is often hidden.


Intuition suggests that the location of moisture damage or mold growth might be important in evaluating exposure because it will be related to the amount of pollutants that may come into contact with a person. However, few studies have examined it in any detail. Some have concentrated on the more frequently or densely occupied locations within a home, such as bedrooms and living rooms (Dharmage et al., 1999b; Reponen et al., 1989; Su et al., 1992; Verhoeff et al., 1994; Wickman et al., 1992). Ross et al. (2000) examined the association between asthma symptoms and indoor bioaerosols in an area where severe flooding had taken place. The study focused on locations on the basis of how they potentially influenced the exposure: bedrooms (location in relation to exposed people) and basements (location in relation to pollutant source). Little between-room variability was observed. Of the 44 homes evaluated, 26 showed no difference in concentrations between rooms; only eight of the remaining 18 had significantly higher concentrations in one room than the home average.


The period of or duration of moisture damage might also be expected to be important, but little research has investigated it. “Ongoing damage”—defined as damage resulting from either recent wetting or a lack of change in moisture conditions within 6 months of construction—has been associated with higher concentrations of culturable fungi in building materials than “dry damage”—past damage due to high moisture conditions where the materials had subsequently dried without remediation (Pasanen et al., 2000a). However, the time-frame of the damage has not been associated with health effects in a straightforward manner (Haverinen et al., 2001a).

The definition of duration of damage and examination of its possible influence on occupant health deserves more consideration. Well-designed studies could allow important data to be gathered on whether time-related characteristics of moisture-induced deterioration of materials influence the manifestation of health effects. The resulting information could be used to guide prevention or remediation strategies.


Building Characteristics

Indoor moisture is linked with some building characteristics. Reported dampness has been associated with age of the building, lack of central heating, humidifiers, and pets (Spengler et al., 1994; Tariq et al., 1996). Low temperatures and high RH indoors can result from cold climatic conditions or from such building characteristics as the lack of thermal insulation and heating. Evans et al. (2000) found a linear association between reported indoor dampness and low temperature and adult health. Older buildings tend to be colder (Hunt and Gidman, 1982) and therefore to have higher RH. Thus, the age of the building can indirectly be associated with both indoor temperature and dampness, all else equal. Martin et al. (1987) found a relationship between damp housing and overcrowding, but not duration of occupancy, household income, use of gas heating, or occupant smoking behavior.

Microbial growth has also been associated with building characteristics. In residences, measures of microbial contamination have been found to be positively correlated with indoor temperature and humidity, age and size of buildings, use of wood stoves and fireplaces, absence of mechanical ventilation, and presence of pets and old wall-to-wall carpeting (Dharmage et al., 1999b, Lawton et al., 1998). Garrett et al. (1998) found airborne fungal concentrations and signs of moisture damage (including musty odor, water intrusion, and high RH) to be associated with smaller amount of thermal insulation, cracks in cladding, and poor ventilation. A factor analysis found an association between airborne concentrations of soil fungi and a dirt-floor, crawl space type of basement in residences (Su et al., 1992). The same study measured increased concentrations of water-requiring fungi in the air of residences where water accumulation was observed. Lawton et al. (1998) developed a “calculated internal moisture source strength” metric that was associated with high biologic contamination and age of houses but not with RH or number of occupants. Verhoeff et al. (1994) found an association between number of fungal propagules in settled dust and type of flooring; no association with other characteristics—such as ventilation and heating facilities, building materials, insulation, and observed dampness—was identified. In another study, Verhoeff et al. (1992) found that indoor viable mold propagules were weakly correlated with several risk factors for moisture problems (age of building, moisture-retaining building materials, and the presence of a crawl space) and observed dampness (damp spots, mold growth, wood rot, silverfish or sowbugs, stale odor, and wet crawl space).

Barriers to Prevention

Information on controlling moisture in residences and larger buildings has been developed and published (Lstiburek, 2001, 2002a,b; Lstiburek and Carmody, 1996; Rose, 1997), but the high incidence of indoor dampness suggests that it is not consistently applied by those designing, constructing, or maintaining buildings.

A number of institutional barriers hinder good practice. One may be that building professionals do not have the knowledge needed to design and build structures to minimize moisture problems. Systematic surveys of curricula are lacking, but generally there is minimal instruction in moisture-control principles for architects and structural engineers4 and a lack of formal training in moisture-protective building techniques and materials for the construction workforce. Indeed, increased interest in dampness issues has resulted in workshops, continuing education, and new design tools that are addressing this need (Karagiozis, 2001; ORNL/IBP, 2003).

Additional barriers result from building-code requirements that inadvertently or indirectly increase the risk of moisture problems. Most codes require passive or active ventilation of crawl spaces. That requirement makes it difficult to construct a crawl space that is included as part of the conditioned space or simply inside the thermal envelope (Advanced Energy, 2001). If rainwater and groundwater are kept out, sealed insulated crawl spaces are often drier than ventilated ones. The entry of warm, humid outdoor air into ventilated crawl spaces, which are often cooler than outdoors, serves as a moisture source for the crawl spaces. Some codes, such as the 2000 International Residential Code, contain exceptions that provide a path to constructing sealed insulated crawl spaces (ICBO, 2000). Sealing crawl spaces can reduce moisture problems there, but sealing can increase concentrations of radon in a crawl space and the associated house. Because radon exposure increases the risk of lung cancer, sealed crawl spaces may be inappropriate in locations where radon concentrations tend to be high.

Building-code requirements for vapor retarders on the interior side of exterior walls and ceilings may also have an impact on building dampness. Adherence to some codes may result in condensation problems when air conditioning is used and—in combination with low-permeability exterior sheathings—reduce the drying potential of a wall section. When a building is air-conditioned, the vapor-pressure gradient is from the exterior toward the interior, where condensation on the back side of the intentional vapor retarder may occur. The situation is aggravated if the building's cladding is composed of a material, such as brick or split-face block veneer, that absorbs rainwater, because the vapor-pressure driving force is greatly increased when the sun raises the temperature of the veneer to that of liquid water. A similar situation occurs when the building interior is depressurized relative to the outdoors; depressurization causes warm air to be drawn through the building envelope, and it washes the backside of chilled surfaces with humid air from which water may condense. Such circumstances point to the need for building codes and design and construction recommendations that take climate into account. Lstiburek and Pettit (2000, 2001, 2002a,b), for example, have produced a series of books that offer design and construction advice specific to various housing types and climatic conditions found in the United States, including advice on avoiding water intrusion and excessive indoor dampness.

Finally, building codes—which guide new construction—may sometimes also apply to renovations. The advocacy group Smart Growth in America asserted that in the late 1990s there were conditions in which even a simple repair could trigger requirements to bring an entire building up to code in Maryland (Smart Growth in America, 1999). Such circumstances could make upgrades uneconomical and limit the funds available for remediation.

Chapter 7 addresses other barriers to preventing and remediating moisture problems.


Dampness and other excess moisture accumulation in buildings are closely connected to observations of mold, mildew, or other microbial growth. The behavior of moisture and air movements can be characterized with physical parameters, but the biological phenomena take place according to a complicated network of regulating factors. Several phenomena make up the microbial ecology of an indoor environment.

Buildings as Microbial Habitats

In principle, common saprophytic environmental microorganisms5 and their spores are present everywhere and they start to grow wherever their basic needs for growth are met. They differ enormously in their needs for environmental conditions and some fungi or bacteria always do well in practically any indoor microenvironmental conditions. As previously noted, one important factor is the availability of moisture. Many environmental microorganisms easily start growing on any surface that becomes wet or moistened. The minimal moisture need for microbial growth may be characterized in terms of the water activity of the substrate, aw, which is the ratio of the moisture content of the material in question to the moisture content of the same material when it is saturated. In a situation where the material is in equilibrium with surrounding air that has a RH of 100%, aw = 1.

The lowest aw at which the most tolerant, so-called xerophilic fungi may grow is 0.7, which corresponds to an ERH of 70%. A few species—such as Penicillium brevicompactum, Eurotium spp., Wallemia sebi, and Aspergillus versicolor—may start growing in these conditions. At higher moisture levels, such intermediate species as Cladosporium sphaerospermum, C. cladosporioides, and Aspergillus flavus may germinate and start their mycelial growth. Most fungi and bacteria require nearly saturated conditions; that is, aw of at least 0.85–0.90 (Grant et al., 1989). Examples of such fungi are Mucor plumbeus, Alternaria alternata, Stachybotrys atra, Ulocladium consortiale, and yeasts (Flannigan and Miller, 2001).

Determinants of Microbial Growth Indoors

Along the life span of a building, weather changes and other events often cause at least temporary wetting of some of its parts. Signs of microbial growth can thus be detected on many parts of a structure. Airborne spores and cells also accumulate in the parts of the structure that are in contact with soil or outdoor air, especially parts that act as sites of infiltration of intake air. Accumulated spores may or may not grow in these sites, depending primarily on moisture conditions.

Because their growth is regulated by the available resources, conditions, and competing organisms, the development of a microbial community may be slow in slowly changing conditions or fast whenever there is a sudden increase in one or more of the limiting factors. Examples of such incidents are floods, firefighting, and acute water damage (Pasanen et al., 2000b; Pearce et al., 1995; Rautiala et al., 2002).

The time it takes for fungi to grow on a particular material depends on the material's characteristics, the fungal species, and the amount of moisture (Doll, 2002). Molds are also capable of producing large quantities of spores within a short time. Rautiala et al. (2002) reported massive fungal growth within a week after firefighting efforts. According to Pasanen et al. (1992a), a fungus can grow and sporulate within a day in moist conditions and within a week on occasionally wet indoor surfaces. Viitanen (1997) modeled the time factor in the development of fungi and found that at RH above 80% for several weeks or months, mold can grow in wood when the temperature is 40–120°F (5–50°C). At RH above 95%, mold can be seen within a few days. In wetted gypsum board inoculated with spores, fungal growth started within 1–2 weeks (Murtoniemi et al., 2001). Chang et al. (1995) reported a latent period of 3 days for fungal growth on ceiling tiles, during which the germination and mold growth could be arrested.

Besides water, microorganisms need proper nutrients and temperatures to grow; some also need particular light conditions.6 Those circumstances are usually met in buildings. Even if modern building materials do not appear to be readily biodegradable, they may support microbial action.

Microbial nutrients may be carbohydrates, proteins, lipids and other biologic molecules and complexes, or they may be nonbiologic compounds. Nutrients are provided by house dust and available moisture and by many surface and construction materials, such as wallpapers, textiles, wood, paints, and glues. Even nonbiodegradable material, such as ceramic tiles and concrete, may support microbial growth (Hyvärinen et al., 2002) by providing a surface for colonies. That explains why fungal colonies may be found on mineral fiber insulation—a material that would not seem hospitable to microbial growth (Wålinder et al., 2001; Hyvärinen et al., 2002).

Prevailing temperatures in living spaces and other sections of buildings are usually 32–130°F (0–55°C), that is, greater than freezing and less than the temperature at which the denaturalization of proteins would start. That range permits the growth of most environmental microorganisms even if the temperature is not optimal for a particular genus or species. Many environmental microorganisms are not especially strict in their temperature demands, in contrast with many pathogenic microorganisms that need the human body temperature to be able to grow.

Time is another integral element in the assessment of microbial growth in buildings. Growth may be slowed by decreasing or increasing temperatures or other limiting factors, and the time window that must be considered in building microbiology is weeks, months, or even years. It is known that microbial degradation normally consists of a chain of events, in which different groups of microorganisms follow each other (Grant et al., 1989), but present knowledge of building microbial ecology does not allow accurate estimation of the age of microbial damage on the basis of the particular fungal or bacterial flora observed.


Microorganism is a catch-all term that refers to any form of life of microscopic size. This section focuses on fungi and bacteria associated with damp indoor spaces. Other microorganisms that may be found in such environments—notably, house dust mites—are not addressed here, although their presence may have important effects on occupants; the health effects of exposure to them and to others more generally related to indoor environments are covered in detail in the Institute of Medicine (IOM) reports Clearing the Air (IOM, 2000) and Indoor Allergens (IOM, 1993) which discuss asthma and general allergic responses, respectively. Larger organisms, such as cockroaches, also inhabit damp spaces and may be responsible for some of the health problems attributed to these spaces; they are also addressed in the IOM reports cited above.

Fungi and Bacteria in Outdoor and Indoor Air

The fungi have (eukaryotic) cells like animals and plants, but are a separate kingdom. Most consist of masses of filaments, live off of dead or decaying organic matter, and reproduce by spores. Visible fungal colonies found indoors are commonly called mold or sometimes mildew. This report, following the convention of much of the literature on indoor environments, uses the terms fungus and mold interchangeably to refer to the microorganisms.

Filamentous fungi, yeasts, and bacteria are common in outdoor soil and vegetation, and outdoor air is an important transport route to the indoor environment for spores and other particles of microbial origin. Spores are often monitored outdoors with direct microscopic counting instead of culturable methods; when so measured, total spore counts may often reach an order of magnitude of 104 spores/m3 (Mullins, 2001). Microorganisms from outdoor air often enter indoor environments through open doors and windows and through ventilation intakes.

Spores of common molds, bacteria, and other microbial particles are regularly found in indoor air and on surfaces and materials—no indoor space is free of microorganisms. They are continuously deposited and removed by various mechanisms, such as gravitational settling on surfaces, by exhaust ventilation,7 and by diffusion to vertical surfaces and cavities. Deposited spores are also removed or released by cleaning, vibration, filtration, accidental ventilation, fan-powered outdoor air (which may pressurize the building and squeeze air out rather than exhaust it), and thermophoresis. Those mechanisms depend primarily on the size of the particle: the larger the particle, the faster the gravitational settling. Small microbial particles (<5 µm) may not settle on surfaces before they are removed by ventilation. After settling on surfaces, microbial particles integrate with other house dust, and they may be removed by cleaning. Part of the settled house dust is resuspended into the air as a result of occupants' movements and other mechanical disturbance (Buttner and Stetzenbach, 1993; Thatcher and Layton, 1995).

Common fungal genera found in outdoor air include Cladosporium, Aspergillus, Penicillium, Alternaria, and Saccharomyces (yeasts) (Mullins, 2001), but the overall diversity of outdoor fungi is great. The genus Aspergillus, for example, has over 185 known species, including Aspergillus fumigatus, A. versicolor, A. flavus, A. penicilloides, and A. niger. Among the other fungal genera observed in outdoor air are Acremonium, Aureobasidium, Cunninghamella, Curvularia, Drechslera, Epicoccum, Fusarium, Geotrichum, Hyalodendron, Leptosphaeria, Neurospora, Paecilomyces, Rhinocladiella, Trichoderma, Tritirachium, and such basidiomycete genera as Coprinus and Ganoderma (Mullins, 2001; Shelton et al., 2002).

The concentrations and diversity of outdoor-air fungi vary with the geographic area, climate, season, weather conditions, and individual sources, such as agricultural activities. In temperate climates, the concentrations are usually highest in summer and fall and lowest in winter and spring (Shelton et al., 2002). Variation is also reflected in the counts and mycoflora of the indoor environment. Indoor concentrations of fungi are usually lower than the corresponding outdoor concentrations, but they vary considerably with the same range as outdoor air: 100–104 cfu/m3 (Shelton et al., 2002). Thus, it is difficult to give any “typical” counts of airborne fungi that would apply to more than a specific, defined set of conditions.

Fungal contamination of the indoor environment creates a source of spores, fungal fragments, and other products that may become airborne and cause changes in the microbial status of the environment outside the range of “normal” conditions. Measurements of airborne fungi are often used to detect such contamination. However, even an actively growing mold mycelium does not release spores continuously; release depends on many physiologic and environmental factors, and it is not possible to detect the presence of such a source solely from the fungal-spore content of the indoor air.

Sampling methods also cause variation in the data collected on fungal concentrations and speciation. For airborne fungi, the characteristics of the sampling device—such as its cutoff size and collection efficiency—influence the recovery of fungal particles (Reponen et al., 2001). Fungal counts are obtained either by direct microscopic counting or by culturing the spores into colonies, which are then counted and identified according to their morphologic features. Direct counting usually allows a rough genus- or group-level identification, although some species can be identified by microscopic examination of spore trap plates or tape lifts (such as Stachybotrys chartarum, Cladosporium sphaerospermum, C. cladosporioides, Alternaria alternata, and Aspergillus niger). Instead, the culturing results depend on the growth media and conditions selected. Ren et al. (1999a) noted that the type and concentrations of fungi measured in house-dust samples were not representative of those isolated in indoor air. No sampling and analytic technique will cover all the fungi and allow their equal detection and identification. Therefore, reported profiles also depend on the sampling, counting, and culturing methods used (ACGIH, 1999). Chapter 3 discusses sampling methods in detail.

Table 2-2 summarizes studies that have aimed at differentiating buildings with and without moisture damage by fungal counts of the indoor air. As can be seen, there is no general pattern whereby a characteristic fungal concentration is associated with either moisture-damaged or nondamaged homes, and the variation in measured quantities is large in both cases. Some studies have shown that increased airborne concentrations of fungi are associated with moisture damage in a building, and others have failed to show any such pattern. Taken together, the studies indicate that the fungal counts alone provide little information about the microbial status of an indoor environment.8 However, information about the species found is useful in assessing whether the microbial constituents of a given indoor environment differ from what are considered typical in those particular conditions.

Indoor concentrations of fungi are usually lower than outdoor concentrations, but the indoor concentrations follow the outdoor ones (Shelton et al., 2002). The large variation in and sometimes dominating effect of outdoor-air fungal concentrations cause difficulties in interpreting measurements made in indoor environments. It is common to use the indoor:outdoor (I/O) concentration ratios to reflect the presence of indoor sources of microorganisms. Because fungal spores circulating in indoor air deposit on surfaces and are caught by air filtration, I/O ratios are typically less than 1.0. However, if there is a strong microbial source indoors, the ratio can exceed 1.0. In a compilation of data from indoor air quality investigations in the United States, Shelton et al. (2002) found I/O ratios of 0.1–200. However, the ratios in most cases were well under 1.0. Species-level identification of the fungi allows even more accurate assessment. Where the I/O ratio of an individual species is repeatedly over 1.0, it suggests the presence of an indoor source of the species. It should be remembered, though, that where the numbers of both indoor and outdoor spores are low, ratios may yield misleading values.

Most fungi found indoors come from outdoor sources, but bacteria have outdoor and indoor sources. Occupants of a building are a major source of bacteria, although the large majority of bacteria shed by people are not considered harmful to other people (Burge et al., 1999). Bacteria of human origin include gram-positive cocci, such as micrococci and staphylococci. Among typical outdoor-air bacteria are Bacillus, Corynebacterium, Flavobacterium, Micrococcus, Pseudomonas, Streptomyces, and other actinomycetes. Like that of fungal flora, the genus and species diversity of outdoor-air bacteria is large.

Environmental bacteria also grow in all wet spaces and are found in most cases where there is mold growth (Hyvärinen et al., 2002), but the profile of bacterial genera and species growing on moist building materials differs from that originating from humans.

Fungi and Bacteria on Building Materials

Most fungi and bacteria that grow on moistened building materials can also be found in outdoor natural habitats and air. However, the rank order of the most prevalent species in indoor growth sites is generally different from that of species normally found in outdoor air, and otherwise unusual species may prevail indoors. Table 2-3 lists examples of fungal genera that have been isolated from “moldy” building materials or surfaces. Most fungal genera have several species, many of which occur on moldy building materials. Therefore, the species diversity is far more extensive than the genus diversity shown in the table.

TABLE 2-3. Examples of Fungal Genera Found in Infested Building Materials.


Examples of Fungal Genera Found in Infested Building Materials.

TABLE 2-2. Summary of Studies of Airborne Fungal Concentrations in Residences in Relation to Building Dampness Characteristics.


Summary of Studies of Airborne Fungal Concentrations in Residences in Relation to Building Dampness Characteristics.

Some fungi are considered “typical” or “indicators” of mold growth on building materials because they are often isolated from mold samples. However, the mere presence of a fungus at a low concentration does not necessarily indicate mold damage. Instead, the simultaneous presence of several otherwise unusual or indicator fungi at concentrations that exceed the background concentrations in outdoor air or other reference samples can be regarded as an indication of indoor mold colonization.

Although there is no general international consensus on which species should be regarded as indicators of the presence of mold, several fungi are often isolated from moldy areas. Table 2-4 shows examples of such fungi.

TABLE 2-4. Examples of Fungi and Other Microorganisms Often Associated with Dampness or Mold Growth in Buildings.


Examples of Fungi and Other Microorganisms Often Associated with Dampness or Mold Growth in Buildings.

Mold growth on materials is usually accompanied by bacterial growth (Hyvärinen et al., 2002). Such bacteria have been studied much less than fungi, but they are a part of the phenomenon of dampness and microbial growth on materials and therefore among the agents occupants may be exposed to, so they deserve attention. Bacteria that have been identified in samples of moldy-building materials are shown in Table 2-5.

TABLE 2-5. Bacterial Genera Isolated from Moldy Building Materials.


Bacterial Genera Isolated from Moldy Building Materials.

Components of Microbial Agents

Some studies of fungi and bacteria examine specific microbial components found in damp indoor environments. Among the components characterized so far are spores and hyphal fragments of fungi, spores and cells of bacteria, allergens of microbial origin, structural components of fungal and bacterial cells (such as β(1→3)-glucans of fungi, endotoxins produced by gram-negative bacteria, and peptidoglycans of bacteria), and such products as microbial volatile organic compounds (MVOCs) and toxic products of microbial secondary metabolism. Information on those agents is briefly summarized below. Chapter 3 discuss exposures to these agents in more detail.

Spores and Fragments of Fungi

Fungi produce and release spores that are cells with well-developed resistance to environmental stresses, such as desiccation and UV radiation. They are the essential means of distribution of filamentous fungi. The particle size of most fungal spores is roughly 2–10 µm, so they are easily transported by winds and air currents, and they may enter the respiratory system (Reponen et al., 2001). Fungal types vary remarkably in their capacity to produce and release spores. Penicillium and Aspergillus typically produce large numbers of spores that are easily released into the air. Stachybotrys and Chaetomium are examples of fungi that produce fewer spores and release them only occasionally. Penicillium and Aspergillus spores are regularly found in air samples, and Stachybotrys and Chaetomium spores are rarely found in the air, even in environments where they are growing (Andersen and Nissen, 2000).

Fungi also release smaller particles (<1 µm) from the mycelium, as experimentally shown by Górny et al. (2002). The microbial origin of the small fragments was verified with antigen characterization. In the experimental study, the smaller particles were released in greater numbers than whole spores, but the concentrations of the small fragments in indoor environments have not yet been characterized. Their small size makes them capable of penetrating deeply into the alveolar region. However, their specific role—if any—in adverse health outcomes has not been studied.

Spores and Cells of Bacteria

Like fungi, spore-forming bacteria release spores from the growth site into the air. Among spore-forming bacteria are Bacillus spp. and actinomycetes, such as Streptomyces. Bacterial spores are smaller than those of fungi—about 1 µm—but bacterial growth may release fragments smaller than the spores (Górny et al., 2003).

Non-spore-forming bacteria may also enter the air as a result of various processes, but these bacteria have no specific mechanism that causes them to become aerosolized. As mentioned above, humans shed bacteria from their skin and respiratory system. Waterborne gram-negative bacteria may enter the air via aerosolization or other mechanical disturbances of standing water. Gram-negative bacteria are also common in house dust, soil, and plants, and they are probably carried indoors on pets and dust.

Allergens of Microbial Origin

Fungi produce an enormous array of potentially allergenic compounds; each fungus produces many allergens of different potencies. Table 2-6 lists the major defined allergens isolated from fungi. Others have been identified but are clinically “minor” (few patients react to them); still others remain to be identified. Fungal allergen production varies with the isolate (strain), species, and genus (Burge et al., 1989). Different allergen amounts and profiles are contained in spores, mycelium, and culture medium (Cruz et al., 1997; Fadel et al., 1992). In addition, the substrate strongly influences the amount and patterns of allergen production. Fungi, for example, release proteases during germination and growth, and fungal extracts contain sufficient protease to denature other allergens in mixtures.

TABLE 2-6. Major Defined Allergens Isolated from Fungi.


Major Defined Allergens Isolated from Fungi.

Microbial allergens are addressed in detail in the IOM reports Clearing the Air (IOM, 2000) and Indoor Allergens (IOM, 1993), which should be consulted for additional information.

Structural Components of Fungi and Bacteria

Some components of microbial cells have been investigated for their possible role in human health effects. Three have attracted particular attention from researchers.

Fungal cell walls are composed of acetylglucosamine polymer fibrils embedded in a matrix of glucose polymers formally referred to as β(1→3)-glucans. Potent T-cell adjuvants, the β(1→3)-glucans have been investigated as antitumor agents (Kiho et al., 1991; Kitamura et al., 1994; Kraus and Franz, 1991). They increase resistance to gram-negative bacterial infection by stimulating macrophages and effecting the release of tumor-necrosis factor α mediated by endotoxin (Adachi et al., 1994a,b; Brattgjerd et al., 1994; Saito et al., 1992; Sakurai et al., 1994; Zhang and Petty, 1994). Soluble glucans have an effect in the lung similar to that of endotoxin (Fogelmark et al., 1994).

Endotoxins—biologically active lipopolysaccharides—are components of some bacterial cell walls that are released when the bacteria die or the cell walls are damaged. They are responsible for some characteristic toxic effects of gram-negative bacteria. Endotoxin exposure has been associated with occupational lung disease among workers exposed at high levels (Douwes and Heederik, 1997; Milton, 1999). Rylander's literature review (2002) notes that studies report both adverse and beneficial effects from low-level exposure to endotoxins, and suggests further research to clarify the role of other agents found in connection with them—β(1→3)-glucans in particular—in health outcomes attributed to endotoxin exposure.

Peptidoglycans are the chemical substances that make up the rigid cell walls of eubacteria (bacteria with rigid cell walls, also called true bacteria). They are a major component of the cell walls of gram-positive bacteria and, like endotoxins, may be released into the environment when the cells die or are damaged. One study of classrooms in two elementary schools noted that high concentrations of a biomarker of the presence of peptidoglycans were associated with a teacher's perception of the severity of indoor-air quality problems (Liu et al., 2000). Their possible role in adverse health outcomes related to damp indoor environments is otherwise unexplored.

Microbial Volatile Organic Compounds

MVOCs are small-molecule, volatile substances that are typically released by growing fungi and bacteria as end products of their metabolism. They are often odorous, causing the typical smell of “mold,” “cellar,” or organic soil. Chemically, they are usually alcohols, aldehydes, ketones, esters, lactones, hydrocarbons, terpenes, and sulfur and nitrogen compounds (Korpi, 2001). However, most of them have sources in addition to microbial growth, so their occurrence is not specific for damp indoor environments with microbial growth. Among the several substances generally considered MVOCs are 3-methylfuran, 3-methyl-1-butanol, 1-octen-3-ol, 2-methylisoborneol, and geosmin (Smedje et al., 1996).

Although the odor of mold has often been associated with respiratory symptoms in damp buildings, the specific role of individual MVOCs or their mixtures in adverse health outcomes has not been studied.

Toxic Products of Microbial Secondary Metabolism9

Many fungi and bacteria are able to produce compounds called secondary metabolites. The compounds are not produced in all growth conditions but are often produced in cases of nutrient starvation, in the presence of other environmental stressors, or in the presence of competing organisms. Many secondary metabolites are toxic or otherwise biologically active. Commonly known microbial secondary metabolites are mycotoxins, bacterial toxins, antibiotics, and antimicrobial agents (Demain, 1999). Microbial toxins are not volatile, but they may be carried by spores (Sorenson et al., 1987).

Numerous studies have examined the fungi and bacteria that may produce toxins while growing on building materials (Andersson et al., 1997; Nielsen et al., 1998; Nikulin et al., 1994; Pitt et al., 2000; Tuomi et al., 2000). The same bacterial strain has been shown to express different degrees of toxicity and inflammatory potential while growing on different building materials (Roponen et al., 2001); this supports the view that the substrate is important in the regulation of secondary metabolism.

Production of secondary metabolites, including microbial toxins, may vary within a single toxigenic strain (Jarvis and Hinckley, 1999; Larsen et al., 2001; Vesper and Vesper, 2002; Vesper et al., 2001). Variable production of toxins while microorganisms are growing on building materials has been shown experimentally (Murtoniemi et al., 2002, 2003a,b; Ren et al., 1999b). The identity of a mold species thus is insufficient information on which to predict its toxic potential.

Mycotoxin production depends on a number of factors, including the availability of nutrients and water activity of the substrate on which the mold grows, temperature (Gqaleni et al., 1997), the sporulation cycle of the organisms (Larsen and Frisvad, 1994), and the presence of other organisms that are in competition for the moisture, nutrients, and other aspects of the growth environment (Wicklow and Shotwell, 1983). The presence of competing organisms appears to be important, and toxins seem to be produced to inhibit the growth of or kill competitors (Wicklow and Shotwell, 1983). Smith and Moss (1986) found that some molds stop making toxins after a few generations when grown in isolation; if generally true, this suggests that testing to determine whether a microorganism might have produced mycotoxins is best conducted in the early stages of growth after isolation from their environment.

The time in the organism's life cycle also appears to influence toxin production. Aspergillus and Penicillium species are known to produce potent toxins with sporulation (Larson and Frisvad, 1994). The large energy demands of sporulation require an available supply of nutrients and precursors for structural molecules, such as proteins, nucleic acids, and lipids. Germination of spores likewise requires a large amount of energy. Reducing competition for nutrients, water, oxygen, or other resources by inhibiting the growth of other occupiers of the mold's ecologic niche gives a toxigenic mold a competitive edge toward survival of its offspring (Wicklow and Shotwell, 1983).

One potentially toxigenic fungus found in water-damaged buildings is Stachybotrys chartarum, formerly referred to as Stachybotrys atra or Stachybotrys alternans. It is a cellulose-degrading fungus that grows well on wetted paper, gypsum board, and the paper liner and gypsum core of plasterboard (Hyvärinen et al., 2002; Murtoniemi et al., 2002; Nielsen et al., 1998). Stachybotrys may also occur on other types of materials, although less frequently (Hyvärinen et al., 2002). The cellulolytic properties of the fungus explain its occurrence on the wetted paper liner of plasterboard, but it is not fully understood why the gypsum core alone also supports the growth of Stachybotrys and its toxin production, as assessed by the in vitro cytotoxicity of the spores (Murtoniemi et al., 2002). A study did find a decrease in S. chartarum growth and sporulation (compared with a reference board) when desulfurization gypsum was used in the core and when the liner was treated with biocide or starch was removed from the plasterboard (Murtoniemi et al., 2003b). The same study found that treating plasterboard liner with biocide did not decrease growth and sporulation but did increase the cytotoxicity of the spores produced.10

Other possibly toxigenic fungi found in buildings or building materials include Aspergillus versicolor, A. fumigatus, A. flavus, and some species of Penicillium, Trichoderma, Fusarium, and Chaetomium (Gravesen et al., 1994). Their toxins have been isolated in mold-infested building materials (Nielsen et al., 1999; Tuomi et al., 2000) and in house dust or carpet dust of damp houses (Engelhart et al., 2002; Richard et al., 1999). However, toxin-producing fungal species produce toxins of varied potency (Abbas et al., 2002; Jarvis, 2002; Nielsen et al., 2002).

Some bacteria found in damp indoor environments are also capable of producing toxins. Among the bacterial types that are potentially toxic while growing on building materials are species of Streptomyces, Bacillus, and Nocardiopsis (Andersson et al., 1998; Jussila et al., 2001; Peltola et al., 2001a,b).

Although mycotoxins or bacterial toxins have often been shown to occur in mold-infested materials, as well as house dust in damp buildings, they have seldom been isolated directly from the air. Spores and fragments of toxigenic fungi may carry these toxins (Sorenson et al., 1987), and this speaks for possible airborne exposure, but little information is available on the degree to which the occupants might be exposed to them. That is partly because of methodologic problems of exposure assessment in general (see Chapter 3). Chapter 4 addresses toxins produced by microbial agents in greater detail.

Gaps in Building Microbiology Science

The great variations in environmental mycoflora in indoor spaces and the large number of variables that affect their occurrence and measurement are among the factors that make it difficult to set quantitative or qualitative guidelines or standards for the microbial quality of indoor air. However, there is evidence of clear differences in harmful potential between different microbes (Huttunen et al., 2003) and more such research would elucidate connections between agents and effects. Chapters 4 and 5 address those concepts in greater detail.

Building Materials and Microbial Growth

High moisture content is commonly observed in building materials (Haverinen et al., 2001b). That is not necessarily abnormal, nor does it necessarily mean that there will be microbial exposure. Care must be exercised in the interpretation of indications of high moisture content. Some signs of moisture may indicate old damage, already dried out, that may or may not still be a possible source of exposure. The signs may also indicate problems below the surface or periodic problems. Visible mold, although not a precise measure of exposure, is probably the clearest risk indicator for potential exposure.

Building materials differ in the degree to which their constituents support microbial growth. Below are brief descriptions of the characteristics of some common materials that influence microbial growth.

Wood has a cellular structure, the cell walls being made up of two natural polymers, cellulose and lignin. Water in wood is present as free water in cell cavities and in combination with cellulose in the cell walls. Wood is hygroscopic and always tends to achieve a moisture content in balance with its environment (Oliver, 1997). Wood is also used in various composite products, which traditionally are poor at resisting moisture unless they are bound together with waterproof products, such as glues. The variability in the properties of those products is high; factors that affect their susceptibility to moisture include environmental conditions, component properties, manufacturing processes, preservative treatments, and chemical modification of raw materials (Wang, 1992).

Many fungi use cellulose as a source of nutrients. However, they vary in their ability to degrade cell walls of wood. Fungal growth on wooden material depends on species, surface characteristics of the material, air humidity, and temperature (Viitanen, 1994, 2002). Pasanen et al. (2000b) studied microbial growth in wood-based materials collected from buildings with moisture problems and found high median concentrations of viable fungi in all wood-based materials regardless of whether the damage was considered current or complete. Tuomi et al. (2000) analyzed the occurrence of mycotoxins in moisture-damaged material samples and found them in most of the material categories tested, but 82% of the mycotoxin-positive samples contained cellulose matter, such as paper, board, wood, or paper-covered gypsum board.

Chang et al. (1995) evaluated growth of fungi on cellulose ceiling tiles and found that although dust deposited on old used tiles provided valuable nutrients, even new ceiling tiles could support growth when ERH was above 85%; fungal growth could be limited only if the wetted tiles were dried quickly and thoroughly. Doll (2002) did not observe growth on ceiling tile kept for 8 weeks in an environmental chamber at 85% RH and 72°F (22°C), but did see growth after 3–6 weeks at 95% RH. The samples were not inoculated with fungi in the laboratory—contamination was from natural sources.

Insulation materials include a wide array of wood-based, mineral, and organic materials. The wood-based materials are more hygroscopic than the mineral or organic materials; that is, their moisture content is much higher at a given ERH. Therefore, the moisture behavior may vary substantially among the materials (Nevander and Elmarsson, 1994). Pasanen et al. (2000a) studied the occurrence of microbial growth in insulation-material samples, including glass wool, polystyrene foam, and granulated cork. They observed a correlation between total spores and fungal concentration and the RH of the materials but usually not the %-MC of the materials. Ezeonu et al. (1994) observed no fungal colonization in fiberglass insulation below 50% RH and delayed colonization below 90% RH.

Compared with wood-based materials, masonry and cementitious materials are low in nutrients and biologically inert. That does not necessarily mean that they are immune to problems. Clay brick, for example, is a fast-wetting material because of its powerful capillary suction, and cementitious materials are hygroscopic and slow in drying. Therefore, if wetted, those materials may support microbiologic and chemical deterioration through their interaction with other materials (Oliver, 1997). Pasanen et al. (2000b) showed that the culturable fungal concentrations correlated with %-MC but not with the RH of the material in concrete, cement, mortar, and plaster-based finishing coating. Stone or mineral-based materials are commonly used for interior finishing in facilities with high moisture loads. Those materials are resistant to microbial growth and are not biodegradable. However, nutrients from water and air can accumulate on them and support microbial growth.

Polyvinyl chloride (PVC) materials are among the most frequently used wall and floor finishing materials because they provide inexpensive, easy-to-clean surfaces. They typically resist microbial growth, but (as discussed below) they may degrade in the presence of moisture.

Paint, varnish, and similar materials are often used to protect other materials from water absorption, as well as for aesthetic reasons. Different types of paints differ in their water permeability and capacity to tolerate moisture (Oxley and Gobert, 1994). Peeling or blistering of a painted surface is often a sign of excess moisture in the structure underneath.


Apart from mold, bacteria, and mite-related contaminants, moisture sometimes contributes to the release of nonmicrobial chemicals into the indoor air. It has been known for many years that the rate of release of formaldehyde from composite building materials that contain urea-formaldehyde resins, such as particle board, increases with the humidity of the surrounding air (van Netten et al., 1989). The emission of formaldehyde occurs, in part, as a consequence of hydrolysis of the resin. In chamber studies, Andersen et al. (1975) found that increasing the RH from 30% to 70% doubled the rate of formaldehyde emission from particle board.

There have been numerous anecdotal reports of indoor odor and irritation complaints associated with moist building materials, particularly plastic materials on moist alkaline substrates, such as concrete. The phenomenon has also been investigated in a few scientific studies. Offermann et al. (2000) reported increased emission rates of potentially odorous and irritating alcohols from the PVC backing of carpet tiles placed on a concrete slab that had a high water content. Lorenz et al. (2000) reviewed four case studies of health symptoms thought to be caused by chemicals that were emitted when high moisture content was combined with building materials that contained plasticizers. When materials were moistened and heated, they measured high emission rates of alcohols, phthalic anhydride, and other compounds thought to be irritating. In a study of four geriatric hospitals, dampness-related and moisture-related degradation of a plasticizer in PVC flooring was strongly associated with asthma symptoms (OR, 8.6; CI, 1.3–57) (Norbäck et al., 2000). In the same study, the dampness and degradation of the plasticizer were less strongly but still statistically significantly associated with increases in ocular symptoms, nasal symptoms, and lysozyme in nasal lavage (an indicator of inflammation) and with a decrease in tear-film stability (Wieslander et al., 1999). The degradation of the plasticizer was indicated by increased indoor airborne concentrations of 2-ethyl-1-hexanol. Wålinder et al. (2001) also reported a higher concentration of 2-ethyl-1-hexanol in the air of a water-damaged office building with PVC flooring, relative to a control office building in the same complex.

More recently, Sjoberg and Nilsson (2002) offered a theoretical analysis of how heating systems embedded in concrete slabs can exacerbate emissions from alkaline hydrolysis of floor coverings. Two problematic scenarios were identified. In the first, the heating system drives construction moisture out of the concrete slab to moisten the floor covering. In the second, moisture from the soil is driven through the concrete slab because of the temperature gradient that occurs when the heating system is turned off, for example, in the summer after an extended period of heating that has warmed the soil beneath the slab.

It can be concluded only that dampness-related emissions of chemicals have been confirmed and linked in a few studies with health symptoms and odor complaints. The available data are too sparse to support conclusions about health implications of dampness-related emissions of chemicals from materials.


Moisture and microbial growth are present in all buildings, and there is no widely accepted definition of the conditions that constitute a “dampness problem.” Moisture-damage observations may include visual observations of dampness or microbial growth, readings of moisture measurements, and other signs that can be associated with excess moisture in building construction. The reported prevalence of signs of dampness in buildings varies widely. In most datasets, at least 20% of buildings have signs of a dampness problem. The available dampness data are primarily from studies of homes; however, some data suggest that dampness in workplaces, schools, and HVAC systems should not be ignored. The extent, location, and duration of building dampness are important for an understanding of its role in health problems, but there has not been much research to evaluate their influence.

Water problems in buildings originate in rainwater, groundwater, plumbing, construction, water use by occupants, and condensation of water vapor. Moisture problems begin when materials stay wet long enough for microbial growth, physical deterioration, or chemical reactions to occur. The important variables are the rate of wetting and the rate of drying. A complex set of moisture-transport and air-transport processes related to building design, construction, operation, and maintenance and to climate determine whether a building will have a moisture problem. Below-grade spaces are particularly prone to moisture problems (Lstiburek, 2002a).

Dampness has been associated with an array of building characteristics, including age of the building, lack of central heating, humidifiers, presence of pets, low temperatures, and crowding. During the life span of a building, weather changes and other events cause at least temporary wetting of some of its parts; signs of microbial growth can thus be detected on many parts of a structure. Microbial growth is regulated by the available resources, conditions, and competing organisms. Indoor microbial concentrations are also influenced by indoor temperature and humidity, building materials, type of foundation, and HVAC system characteristics.

Environmental microorganisms are diverse. They differ enormously in their needs for particular environmental conditions, so there will almost always be some fungi or bacteria that do well in any microenvironmental conditions. In buildings, the general microbial needs of temperature, nutrients, oxygen, and light are usually met; therefore, the availability of moisture is the primary limiting factor in microbial growth. The materials used in the building determine both the amount of moisture needed to support growth and the type of microorganisms whose growth will be favored. Moisture may also trigger degradation of building materials and so contribute to the release of nonmicrobial chemicals into the indoor air.

Research on the biologic and human health effects of microbial and other agents associated with damp indoor environments is discussed in the following chapters. It should be noted that there is very little documentation of interactions of these various agents. While it is evident that various pollutants occur simultaneously in these environments, an overall risk assessment of the combined exposures is not possible with the present knowledge.


On the basis of the review of the papers, reports, and other information presented in this chapter, the committee has reached the following findings and recommendations and identified the following research needs regarding damp buildings. The committee's discussion of the public health response to damp indoor spaces (Chapter 7) provides additional observations on how some of the recommendations for actions might be accomplished.


  • The term dampness has been used to define a variety of moisture problems in buildings, including high RH, condensation, and signs of excess moisture or microbial growth. However, there is no generally accepted definition of dampness or of what constitutes a “dampness problem” and no generally accepted metric for characterizing dampness.
  • Dampness—as defined and documented in studies using a wide variety of metrics—is prevalent in residential housing in a wide array of climates. The prevalence and significance of dampness are less well understood in nonresidential buildings like office buildings and schools than in residential buildings. Relatively little information is available on the prevalence and importance of dampness and microbial growth in HVAC systems.
  • Environmental microorganisms require moisture and nutrients to grow. The range of temperatures in buildings permits the growth of many microorganisms even if it is not optimal for a particular genus or species.
  • Dampness increases the risk of microbial contamination and can cause or exacerbate the release of chemical emissions from building materials and furnishings.
  • Dampness problems in buildings result from failures in design, construction, operation, maintenance, and use. The prevalence and nature of dampness problems suggest that what is known about their causes and prevention is not consistently applied in building design, construction, maintenance, and use.
  • The prevalence of dampness problems appears to increase as buildings age and deteriorate, but some modern construction techniques and materials and the presence of air-conditioning probably increase the risk of dampness problems. Scientific studies have not, in general, provided data to confirm or refute this idea.
  • Changes in building design, operation, maintenance, and use are the key to preventing the manifestation of dampness-related building damage and microbial growth.


  • Precise, agreed-on definitions of dampness should be developed to allow important information to be gathered about mechanisms by which dampness and dampness-related effects and exposures affect occupant health. More than one definition may be required to meet the specific needs of health researchers (epidemiologists, physicians, and public health practitioners) in contrast with those involved in preventing or remediating dampness (architects, engineers, and builders). However, definitions should be standardized to the extent possible. Any efforts to establish common definitions should be international in scope.
  • Increased attention should be paid to HVAC systems as a potential site for the growth and dispersal of microbial contaminants that may result in adverse health effects in building occupants.
  • Building professionals (architects, home builders, facility managers and maintenance staff, code officials, and insurers) should receive better training in how and why dampness problems occur and their prevention.
  • Current building codes should be reviewed and modified as necessary to reduce dampness problems.

Research Needs

As noted above, standardized dampness metrics and associated dampness-assessment protocols should be developed to characterize the nature, severity, and spatial extent of dampness. Using the standardized metrics, the determinants of dampness problems in buildings should be studied to ascertain where to focus intervention efforts and health-effects research.

In addition, the committee identified the following research needs:

  • Economic research is needed to determine the societal cost of dampness problems and to quantify the economic impact of design, construction, and maintenance practices that prevent or limit dampness problems.
  • New and continuing research is needed to better characterize
    • —The presence and health effects of bacteria that grow on damp materials indoors.
    • —Dampness-related emissions of spores, bacteria, and smaller particles of biologic origin, dampness-related chemical emissions from building materials and furnishings and any role these emissions may have in adverse health outcomes.
    • —The nature and significance of dampness-related microbial contamination in HVAC systems.
    • —The microbial ecology of buildings, that is, the link between dampness, different building materials, microbial growth, and microbial interactions.
    • —The impact of the duration of moisture damage on materials and its possible influence on occupant health.
    • —The effectiveness of various changes in building designs, construction methods, operation, and maintenance in reducing dampness problems.
  • Research should be performed to develop design, construction, and maintenance practices for buildings and HVAC systems that reduce moisture problems.

Chapter 7 operationalizes some of these research needs by suggesting specific actions and actors to implement them.


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Material in this section and later in the chapter has been adapted or excerpted from a dissertation by Dr. Ulla Haverinen-Shaughnessy (Haverinen, 2002) that was written under the supervision of one of the committee members. It is used here with the permission of the author.


Condensation is not typically a problem, because, unlike many composite structures, such walls have relatively even distribution of water-vapor permeability.


Meklin et al. (2003) have assessed the occurrence of moisture damage, fungi, and airborne bacteria in schools in Finland, focusing on the impact of main building frame material (wooden vs concrete or brick). An earlier paper (Meklin et al., 2002) examined respiratory symptoms in the children attending those schools.


It must be noted that the broad array of subject competencies required of architects and structural engineers may leave little time for such focused training. Design teams might thus need specialists in moisture dynamics and control.


The term “environmental microorganism” is used here to distinguish the microorganisms that are usually found in indoor or outdoor spaces from those more typically found in humans or other living hosts.


Light is needed for the growth of many fungi and bacteria but the lack of light does not prevent microbial action. Thus, in general, light is not a critical factor in building microbiology.


Chapter 10 of Clearing the Air (IOM, 2000) provides greater detail on building ventilation and air cleaning and their effect on exposure to indoor pollutants.


The “Sampling Strategy” section of Chapter 3 also addresses the use of fungal counts in the assessment of indoor microbial contamination.


Chapter 4 addresses the toxic potential of fungi and bacteria found in damp indoor environments in greater detail.


When Murtoniemi et al. (2003c) examined growth of the bacteria Streptomyces californicus on various plasterboards, they found that removal of starch from the liner and core inhibited growth and sporulation almost completely; spore cytotoxicity was not affected by the presence of a biocide.

Copyright 2004 by the National Academy of Sciences. All rights reserved.
Bookshelf ID: NBK215649


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