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WHO Guidelines for Indoor Air Quality: Dampness and Mould. Geneva: World Health Organization; 2009.

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WHO Guidelines for Indoor Air Quality: Dampness and Mould.

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2Building dampness and its effect on indoor exposure to biological and non-biological pollutants

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2.1. Frequency of indoor dampness

A review of studies in several European countries, Canada and the United States in 2004 indicated that at least 20% of buildings had one or more signs of dampness (Institute of Medicine, 2004). This estimate agrees with those of a study of 16 190 people in Denmark, Estonia, Iceland, Norway and Sweden, which gave an overall prevalence of indoor dampness of 18%, with the lowest prevalence in Göteborg, Sweden (12.1%), and the highest in Tartu, Estonia (31.6%) (Gunnbjörnsdóttir et al., 2006). Dampness was defined on the basis of self-reported indicators, such as water leakage or damage, bubbles or discoloration of floor coverings, and visible mould growth indoors on walls, floors or ceilings. From several studies conducted in the United States, Mudarri and Fisk (2007) estimated the prevalence of dampness or mould in houses to be approximately 50%.

Although few data are available for low-income countries, several studies suggest that indoor dampness is also common in other areas of the world. For example, a study of 4164 children in rural Taiwan, China, showed that 12.2% of the parents or guardians considered their dwelling to be damp, 30.1% reported the presence of visible mould inside the house in the past year, 43.4% reported the appearance of standing water, water damage or leaks, and 60% reported at least one of these occurrences (Yang et al., 1997a). In a study in Singapore of 4759 children, the prevalence of dampness in the child’s bedroom was 5% and that of mould was 3% (Tham et al., 2007); the overall prevalence of mould and damp in the rest of the house was not given. About 11% of parents of 10 902 schoolchildren in a study in three cities in China (Beijing, Guangzhou and Hong Kong Special Administrative Region) reported mould on the ceilings and walls (Wong et al., 2004). In a study in Japan of the residents of 98 houses built within the past 4 years, condensation on window panes or walls was reported by the residents in 41.7% of all houses, and 15.6% had visible mould (Saijo et al., 2004). Indoor damp was reported by 13% of 3368 adults living in Nikel in the Arctic area of the northern Russian Federation (Dotterud, Falk, 1999). A case-control study of asthma in the West Bank and Gaza Strip, involving participants in villages, cities and refugee camps, showed that 62 of 110 dwellings (56%) had visible mould on the walls and ceilings (El Sharif et al., 2004). The prevalence of houses characterized as damp with visible mould was highest in the refugee camps, with an estimated 75% of houses affected. Another study in the West Bank in 188 randomly selected houses in the Al-Ama’ri refugee camp south of Ramallah City showed that 78.2% of the houses had damp problems, leaks or indoor mould (Al- Khatib et al., 2003).

As dampness is more likely to occur in houses that are overcrowded and lack appropriate heating, ventilation and insulation (Institute of Medicine 2004), the prevalence of indoor damp in low-income communities can be substantially higher than the national average. For example, in a study of 1954 young mothers in the United Kingdom, those who lived in owner-occupied or mortgaged accommodations (relatively affluent) reported damp (52%) and mould (24%) significantly less often than those who lived in council houses or rented accommodations (relatively deprived), 58% of whom reported damp and 56% of whom reported mould (Baker, Henderson, 1999). Similarly, a study of 25 864 schoolchildren in eastern Germany showed that the children of parents with a low educational level were 4.8 times more likely (95% confidence interval (CI), 3.4–5.4) to live in damp houses than those of parents with a high level of education. Children whose parents had received education at the intermediate level were 1.8 times as likely to live in a damp house (95% CI, 1.6–2.1) (du Prel, Kramer, Ranft, 2005).

Indoor damp is likely to remain an important issue in less affluent countries and neighbourhoods, particularly since an increasing shortage of affordable housing provides little incentive for landlords to improve rental accommodation. Similarly, the substantial costs involved in remediation may dissuade low-income home owners from improving substandard housing conditions. This will add to the already high burden of poor health in those communities.

Most of the estimates of prevalence are based on self-reports and may therefore be biased. To address this problem, self-reports of dampness were compared in several studies with results obtained through inspections by trained personnel. Although in most of these studies the occupants reported more dampness than the trained surveyors (Bornehag et al., 2001), some studies showed that occupants reported less dampness (Williamson et al., 1997; Nevalainen et al., 1998). Studies by Engman, Bornehag and Sundell (2007) and Sun, Sundhell and Zhang (2007) also showed poor agreement between self-reported and inspectors’ observations of dampness and mouldy odour. Douwes et al. (1999), however, found that occupants’ reports of damp spots or fungal spots correlated better with an objective measure of indoor fungi than investigators’ reports of these visible signs; a similar finding was reported by Sun, Sundhell and Zhang (2007). This result was not confirmed by another study, in which objective measures of fungal concentrations and questionnaire reports were compared (Dales, Miller, McMullen, 1997).

The exact prevalence of home dampness cannot be established in the absence of a gold standard, but occupants’ and inspectors’ reports indicate that it is likely to be in the order of 10–50% in the most affluent countries. Although the evidence is more limited for less affluent countries, the magnitude of the problem appears to be similar, the prevalence sometimes even exceeding 50% (e.g. in refugee camps in the West Bank and Gaza Strip, see above). As problems of damp are commonest in deprived neighbourhoods, a substantial proportion of that population is at risk of adverse health effects associated with damp indoor environments (see Chapter 4).

Climate change and its effects on the weather (i.e. storms and heavy rainfall) and the subsequent rise in sea level and increased frequency and duration of floods are likely to increase the proportion of buildings with damp problems further, particularly in flood-prone areas such as river valleys and coastal areas. In addition, high energy costs will prevent adequate heating in winter in many houses (so-called fuel poverty), leading to increased condensation and indoor dampness.

Dampness and indoor mould also occur in school buildings, day-care centres, offices and other buildings (Mudarri, Fisk, 2007). The health risks associated with dampness-related exposures in these building are likely to be similar to those in damp houses; however, no systematic surveys have been conducted to assess the prevalence of dampness and mould in these establishments.

2.2. Effects of dampness on the quality of the indoor environment

Indoor environments contain a complex mixture of live (viable) and dead (non-viable) microorganisms, fragments thereof, toxins, allergens, volatile microbial organic compounds and other chemicals. The indoor concentrations of some of these organisms and agents are known or suspected to be elevated in damp indoor environments and may affect the health of people living or working there (see Chapter 4). In particular, it has been suggested that dust mites and fungi, both of which favour damp environments, play a major role. Dust mites and several fungi produce allergens known to be associated with allergies and asthma; many fungi also produce toxins and irritants with suspected effects on respiratory health.

Dampness may also promote bacterial growth and the survival of viruses, but this has received little attention in the literature (see sections 2.3.2 and 2.3.7). In addition, dampness is an indicator of poor ventilation, which may result in increased levels of a wide range of other potentially harmful indoor pollutants (see Chapter 3). Excess moisture may also result in increased chemical emissions from building materials and floor covers (see section 2.3.6). Furthermore, standing water may attract cockroaches and rodents, which can transmit infectious diseases and are also a source of indoor allergens. These pests are not specific to damp buildings, however, and are therefore not be discussed further in this document. Damp indoor environments (particularly damp soil and wood) may also attract termites, which can cause substantial damage to buildings, significantly compromising the integrity of the structure and therefore the health and safety of its occupants. Also, the presence of termites may indirectly affect the indoor environment by inducing the use (and misuse) of potentially hazardous pesticides. As termites are not known to affect indoor air quality directly, they are not discussed further in this document. Although outbreaks of legionellosis are commonly associated with water sources in buildings, they are not typically associated with damp buildings and are also not discussed further.

2.2.1. Dust mites

As indicated in Chapter 1, the focus of these guidelines is dampness, mould and microbial agents (see below). The close association between damp indoor environments and dust mites is, however, discussed briefly. The reader is referred to the report of the Institute of Medicine (2000) on house dampness and asthma for a more detailed overview of the literature on the associations between indoor damp, house dust mite allergens and asthma.

House dust mites are arachnids, and many different species have been identified; however, only a few are of concern with respect to damp indoor environments (see section 2.3.1). The natural food source of house-dust mites includes skin scales, although many other sources may be used. Therefore, in most houses, nutrition is abundantly available, particularly in mattresses and carpets or rugs. Laboratory studies have shown that most dust mites require a relative humidity in excess of 45–50% for survival and development, but they feed and multiply more rapidly at higher relative humidity (Arlian, 1992). Indoor humidity is therefore the main influence on the presence and propagation of house dust mites, as confirmed in several field studies (van Strien et al., 1994; de Andrade et al., 1995; Simpson et al., 2002; van Strien et al., 2004; Zock et al., 2006). Damp houses therefore significantly increase exposure to dust-mite allergens, at least in populations living in mild and temperate climates.

2.2.2. Fungi

Fungi are ubiquitous eukaryotic organisms, comprising an abundance of species. They may be transported into buildings on the surface of new materials or on clothing. They may also penetrate buildings through active or passive ventilation. Fungi are therefore found in the dust and surfaces of every house, including those with no problems with damp. Once fungi are indoors, fungal growth can occur only in the presence of moisture, and many fungi grow readily on any surface that becomes wet or moistened; that is, virtually all fungi readily germinate and grow on substrates in equilibrium with a relative humidity below saturation (i.e. below 100%).

The species that grow on a given substrate depends largely on the water activity of the substrate. Water activity is a measure of water availability and is defined as the ratio of the vapour pressure above a substrate relative to that above pure water measured at the same temperature and pressure. The minimum water activity required for fungal growth on building surfaces varies from less than 0.80 to greater than 0.98 (Grant et al., 1989; see also Table 1). On the basis of their water requirements, indoor fungi can be divided into: (1) primary colonizers, which can grow at a water activity less than or equal to 0.80; (2) secondary colonizers, which grow at a water activity level of 0.80–0.90; and (3) tertiary colonizers, which require a water activity greater than 0.90 to germinate and start mycelial growth (Grant et al., 1989; see also Table 1). Although high levels of humidity and some surface and interstitial condensation may be sufficient for most primary and secondary colonizers, tertiary colonizers generally require serious condensation problems. These problems may be due to construction faults, including inadequate insulation, in combination with poor ventilation, or water damage from leaks, flooding and groundwater intrusion.

Table 1. Moisture levels required for growth of selected microorganisms in construction, finishing and furnishing materials.

Table 1

Moisture levels required for growth of selected microorganisms in construction, finishing and furnishing materials.

Fungi also need nutrients, which may include carbohydrates, proteins and lipids. The sources are diverse and plentiful, ranging from plant or animal matter in house dust to surface and construction materials (such as wallpaper and textiles), condensation or deposition of cooking oils, paint and glue, wood, stored products (such as food), and books and other paper products. Nutrients are therefore generally not a limiting factor for indoor fungal growth. In fact, fungi are known to grow even on inert materials such as ceramic tiles and can obtain sufficient nutrients from dust particles and soluble components of water. As most indoor fungi grow at 10–35 °C, common indoor temperatures are also not a limiting factor; however, although temperature and nutrients are not critical, they may affect the rate of growth (see section 3.3) and the production of certain allergens and metabolites (Nielsen et al., 1999; Institute of Medicine, 2000). Thus, water remains the most critical factor in indoor fungal growth, as also indicated in field studies, which show elevated numbers of fungi and fungal spores in damp houses (Gallup et al., 1987; Waegemaekers et al., 1989; Douwes et al., 1999). House dampness therefore significantly contributes to fungal spores, fragments and allergens (see section 2.3).

Fungi not only have adverse effects on health but also cause considerable damage to buildings, the wood-rotting fungi being particularly destructive to (wooden) building structures. The commonest and possibly the most destructive wood decay fungus found in buildings in temperate regions, including Australia, Europe and Japan, is the dry-rot fungus Serpula lacrymans (previously known as Merulius lacrymans) (Singh, 1999). This fungus can grow quickly and may spread throughout a building from one timber to another, potentially causing devastating effects in the whole building. There are many other dry- and wet-rot fungi that can cause wood decay and subsequent damage to the built environment (reviewed by Singh, 1999). They have also been implicated in the causation of hypersensitivity pneumonitis (extrinsic allergic alveolitis).

2.2.3. Bacteria

Bacteria are ubiquitous prokaryotic single-cell organisms, comprising an abundance of species. They can be found in the dust and on the surfaces of every house, including those with no damp problems. The main sources of bacteria in the indoor environment are outdoor air, people and indoor bacterial growth. Bacteria from outdoor air and those originating from people are considered to be fairly harmless; bacteria growing actively or accumulating in the indoor environment, however, may affect health, but this has not been studied extensively.

As described for fungi, water is a critical requirement for bacterial growth. In fact, bacteria require higher water activities than most fungi. The temperature and nutrient demands are generally met in most indoor environments. Surprisingly, few studies have been conducted on bacterial growth in damp houses. Those that have suggest that bacteria grow in the same areas as fungi. In particular, Streptomycetes (Gram-positive spore-forming bacteria that are not normal indoor flora in urban environments) may grow on damp or wet building materials (Hyvärinen et al., 2002). Their presence in indoor air may therefore indicate that a building has a moisture problem (Samson et al., 1994). Although no clear association with dampness has been found, it has been suggested that endotoxins from Gram-negative bacteria occur at increased levels in damp buildings (Bischof et al., 2002; Solomon et al., 2006).

2.2.4. Protozoa

It has been proposed that protozoa play a role in the complex mixture of exposures that contribute to adverse health effects associated with damp indoor environments (Yli-Pirila et al., 2004). In their study, amoeba were detected in 22% of 124 material samples from moisture-damaged buildings. Most of the 11 samples of materials that were badly damaged by moisture were found to contain ciliates and flagellates. No field studies have been conducted to assess the airborne concentrations of protozoa or fragments thereof. Although a study in vitro suggested that amoeba enhance the pro-inflammatory properties of certain microbes (Yli-Pirila et al., 2007), it is not clear whether protozoa in damp buildings contribute to ill health.

2.3. Dampness-related indoor pollutants

In this section, we describe the most important indoor pollutants that have been associated with damp indoor spaces and related health effects. The health effects associated with these pollutants are discussed in Chapter 4.

2.3.1. Allergens

All agents that can induce specific immune responses (resulting in the production of specific antibodies) are also potential allergens. The term allergen can refer to a single molecule, a mixture of molecules or a particle from which allergen molecules can be eluted. The latter can be dead material, like mite faecal particles, or viable propagules, such as bacteria or mould spores. Thus, allergens comprise a large variety of macromolecular structures, ranging from low-relative-molecular- mass sensitizers (mainly chemicals such as formaldehyde) to high-relativemolecular- mass sensitizers (such as polymeric carbohydrates and proteins). In damp indoor environments, those with a high relative molecular mass are most relevant, in particular house dust mite allergens and fungal allergens.

2.3.1.1. House dust mite allergens

Dust mites produce the predominant inhalation allergens in many parts of the world. The most common mite species that produce allergens are Dermatophagoides pteronyssinus and Dermatophagoides farinae. The major allergens produced by D. pteronyssisus (called Der p I and Der p II) are proteases, which are present in large amounts in faecal pellets (Institute of Medicine, 2000). The major allergen produced by D. farinae is Der f I. Elevated levels of these allergens have been detected in house dust, mattress dust and bedding in damp houses (van Strien et al., 1994; Simpson et al., 2002; van Strien et al., 2004). As the focus of these guidelines is indoor dampness and microorganisms, dust mite allergen levels are not discussed further.

2.3.1.2. Fungal allergens

Many fungal species produce type I allergens, and immunoglobulin (Ig)E sensitization to the commonest outdoor and indoor fungal species, like Alternaria, Penicillium, Aspergillus and Cladosporium spp., is strongly associated with allergic respiratory disease, especially asthma (Chapter 4). Fungi are also well-known sources of type III (or IgG-inducing) allergens. The species involved include many common genera such as Penicillium and Aspergillus, which can be found in most houses. At high concentrations, fungi may also be involved in combined type III and IV allergic reactions, including hypersensitivity pneumonitis.

Many fungal allergens are glycopeptides with enzymatic properties. They are found in spores, hyphae and fungal fragments but are released in greater amounts during germination and mycelial growth, which may occur inside the airways (Green et al., 2006). The viability of spores is therefore important for allergenic expression, as confirmed in some studies in experimental animals. Although non-viable fungal spores and hyphae release allergens at lower concentrations, they are still likely to play an important role in fungi-related allergies and respiratory effects. Non-viable fungal spores and fungal fragments also contain potentially harmful compounds such as (1→3)-β-D-glucans and mycotoxins (see sections 2.3.4 and 2.3.5). Species of the genera Cladosporium, Alternaria and Aspergillus have been shown to produce a variety of allergens, including several major ones: Cla h I (Cladosporum herbarum), Alt a I and Alt a II (Alternaria alternata) and Asp f I and Asp f III (Aspergillus fumigatus). Nonetheless, the most potent allergenic proteins, identified as major allergens in fungal extracts produced in vitro, might not be the same as those to which people are actually exposed in indoor environments. This might explain the negative skin-prick test and IgE results in people with asthma (see Chapter 4).

Commercial assays are available for only a limited number of indoor fungal allergens (including Alternaria allergens, see section 2.4.3) because of difficulties in the manufacture and standardization of fungal allergen extracts. Therefore, little information is available on exposure to these allergens.

Many fungi and some yeast replicate by producing numerous spores that are well adapted to airborne dispersal. Spores are typically 2–10 μm in length. They can stay airborne for long periods and may deposit in the respiratory system, some smaller spores reaching the alveoli (Eduard, 2006). Fungi also release even smaller fungal fragments (Gorny, 2004), which are derived from broken or fractured spores and hyphae and can be categorized into submicron particles (< 1 μm) or larger fungal fragments (> 1 μm). Even more fungal fragments than spores may be deposited in the respiratory tract (Cho et al., 2005); like spores, they are known to contain allergens (Green et al., 2006) and mycotoxins (Brasel et al., 2005a). Both spores and fungal fragments may therefore be involved in mould-related adverse health effects.

The aerosolization of fungal matter and its subsequent inhalation are only partly understood, but two mechanisms are believed to be of particular importance: (1) release of spores or fungal fragments by active discharge, indoor air movement or physical disturbance caused by people or pets; and (2) resuspension of settled fungal matter due to human activities. Factors that may affect the rate of release of spores or fungal fragments include air velocity, time, colony structure, desiccation stress, moisture condition and vibration. These factors may affect the rate of aerosolization of spores and fungal fragments differently (Gorny, 2004).

Fungal spores are ubiquitous in outdoor air, the levels ranging from less than 100 to more than 105 spores/m3. The indoor levels are usually lower than those outdoors but may be increased by unintended fungal growth in damp buildings (Flannigan, Samson, Miller, 2001). Studies in damp indoor environments have shown a wide diversity of fungal species and genera, probably due to differences in climate, indoor temperature and humidity, and building materials, as well as differences in sample collection and subsequent culture. Fungi are often found on wet window frames and damp walls of bedrooms, living rooms and kitchens. Mattresses constitute an important reservoir for mould, with measured concentrations of 103–107 spores/g of dust (Verhoeff et al., 1994a). Several extensive reviews on fungal species found in damp indoor environments have been published (e.g. Flannigan, Samson, Miller, 2001).

The airborne concentrations of viable fungi in indoor environments are usually in the order of a few to several thousand colony-forming units (CFUs) per cubic metre of air. In a given space, concentrations of fungi are highly variable and depend on such factors as: climate and season, type of fungus, construction, age and use of the building, and ventilation rate. They also depend largely on the sampling and analytical methods used, making valid comparisons between studies difficult.

Indoor fungal fragments are not commonly measured in field studies, but a study with an aerosolization chamber showed that submicron fungal fragments from culture plates and mould-contaminated ceiling tiles aerosolized simultaneously with spores but at substantially higher concentrations (320–514 times higher) (Gorny et al., 2002; Cho et al., 2005). This suggests that indoor exposure to fungal fragments is at least as important as exposure to fungal spores.

2.3.2. Bacteria

As mentioned above, few studies have addressed bacteria in damp indoor environments. Some identified Streptomycetes on damp or wet indoor surfaces (Hyvärinen et al., 2002; Rintala, Nevalainen, Suutari, 2002; Rintala et al., 2004). Streptomycetes are Gram-positive, spore-forming actinobacteria, which are typical soil organisms that produce a wide range of metabolites, including some toxins, such as valinomycin (Andersson et al., 1998). The metabolites produced are substrate-dependent (Hirvonen et al., 2001; Roponen et al., 2001; Murtoniemi et al., 2001a, 2003). Mycobacteria have also been shown to be common in moisture damaged buildings, their presence increasing with the degree of fungal damage (Torvinen et al., 2006). Cell wall components of mycobacteria are known to be highly immunogenic, and exposure to mycobacteria may cause inflammatory responses (Huttunen et al., 2000, 2001).

The concentrations of total viable bacteria in indoor environments may range between 101 and 103 CFU/m3 (Gorny, Dutkiewicz, Krysinska-Traczyk, 1999), probably representing the degree of occupancy of the building and the efficiency of its ventilation. The literature does not provide typical airborne concentrations in damp indoor environments as compared with non-damp environments. Also, although the presence of Streptomycetes and mycobacteria may be an indication of bacterial growth, normal levels have not been established.

2.3.3. Endotoxins

Endotoxins are integral components of the outer membrane of Gram-negative bacteria and are composed of proteins, lipids and lipopolysaccharides. The term endotoxin refers to a toxin on the bacterial cell wall, which is often liberated as a result of cell lysis. In the environment, airborne endotoxins are usually associated with dust particles or aqueous aerosols. They have a broad size distribution, but the levels may be higher in the coarse fraction (Schwarze et al., 2007). Heavy exposure to endotoxins can cause respiratory symptoms, including non-allergic asthma, but moderate-to-low exposure may protect against allergies and asthma (Douwes, Pearce, Heederik, 2002). It has been hypothesized that endotoxins play a role in the pathogenisis of rheumatic diseases in damp buildings (Lorenz et al., 2006).

Lipopolysaccharides are a class of pure lipid carbohydrate molecules (free of protein and other cell-wall components) and are responsible for most of the immunological properties of bacterial endotoxins. Lipopolysaccharides are stable, water-soluble, non-allergenic molecules composed of a lipid and a polysaccharide and have not been found in the cell walls of Gram-positive bacteria, mycobacteria or fungi (Morrison, Ryan, 1979; Rietschel et al., 1985). The lipid moiety of lipopolysaccharides, called lipid A, is responsible for their toxic properties. Although the terms endotoxins and lipopolysaccharides technically have different meanings, they are often used interchangeably in the scientific literature.

The concentrations of endotoxins in the indoor environment range from a few to several thousand endotoxin units per milligram of house dust (Douwes, Pearce, Heederik, 2002). When the concentration is expressed per square metre, it varies even more. The endotoxin levels in different studies vary only moderately, regardless of the geographical area, which is remarkable, as the analytical methods used were not standardized. Few studies have focused on airborne concentrations in the indoor environment. Park et al. (2000) reported a mean airborne endotoxin level of 0.64 units/m3 in 15 houses in Boston, United States, and mean levels of endotoxins in dust of 44–105 units/mg. The mean inhalable level of endotoxins measured in nine houses in Belgium was similar, i.e. 0.24 units/m3 (Bouillard, Devleeschouwer, Michel, 2006). Indoor concentrations may be higher in damp houses, but a study conducted in areas of Louisiana affected by hurricane Katrina and the subsequent flooding did not confirm this (Solomon et al., 2006). Other studies also found no evidence for a relationship between endotoxins in house dust and observed dampness or mould (Chen et al., 2007; Giovannangelo et al., 2007).

2.3.4. Fungal (1→3)-β-D-glucans

(1→3)-β-D-glucans are non-allergenic, water-insoluble structural cell-wall components of most fungi, some bacteria, most higher plants and many lower plants (Stone, Clarke, 1992). They consist of glucose polymers with variable relative molecular mass and degree of branching (Williams, 1997) and may account for up to 60% of the dry weight of the cell wall of fungi (Klis, 1994). In the fungal cell wall, (1→3)-β-D-glucans are linked to proteins, lipids and carbohydrates such as mannan and chitin, and they contain (1→6)-β-glucan side-branches, which may connect with adjacent (1→3)-β-D-glucan polymers (Klis, 1994). The (1→3)-β-D-glucan content of fungal cell walls has been reported to be relatively independent of growth conditions (Rylander, 1997a; Foto et al., 2004). (1→3)-β-D-glucans have immunomodulating properties and may affect respiratory health (Douwes, 2005; see also section 4.2.1).

The methods used to analyse (1→3)-β-D-glucans in environmental (settled or airborne) dust samples have not been standardized, and studies are therefore not comparable. In Sweden and Switzerland, the concentrations in buildings with fungal problems ranged from about 10 to more than 100 ng/m3 in a Limulus amoebocyte lysate assay in airborne dust samples generated by rigorous agitation of settled dust in the buildings (Rylander, 1999) (see section 2.4 for a description of analytical methods). The concentrations in buildings with no obvious fungal problems were close to 1 ng/m3. The mean levels of (1→3)-β-D-glucans in house dust in Germany and the Netherlands, determined with a specific enzyme immunoassay, were comparable: 1000–2000 μg/g dust and 500–1000 μg/m2 (Douwes et al., 1996, 1998, 2000; Gehring et al., 2001; Chew et al., 2001). Samples were taken in houses that were not selected because of fungal problems and were analysed in the same laboratory with identical procedures. No airborne samples were taken.

2.3.5. Mycotoxins

Mycotoxins, or fungal toxins, are low-relative-molecular-mass biomolecules produced by fungi, some of which are toxic to animals and human beings. Mycotoxins are known to interfere with RNA synthesis and may cause DNA damage. Some fungal species may produce various mycotoxins, depending on the substrate. In the case of Penicillium, one such compound is penicillin, a strong antibiotic. Several mycotoxins, e.g. aflatoxin from Aspergillus flavus and Aspergillus parasiticus, are potent carcinogens. Many mycotoxins are immunotoxic, but the trichothecene mycotoxins are immunostimulating at low doses (Eduard, 2006). Numerous mycotoxins have been classified by their distinct chemical structures and reactive functional groups, including primary and secondary amines, hydroxyl or phenolic groups, lactams, carboxylic acids, and amides.

The mycotoxins that have perhaps received most attention are the trichothecenes, produced by Stachybotrys chartarum. Bloom et al. (2007) showed that several mycotoxins produced by S. chartarum and Aspergillus versicolor (i.e. macrocyclic trichothecenes, trichodermin, sterigmatocystin and satratoxin G) could be present in most samples of materials and settled dust from buildings with current or past damage from damp or water. Charpin-Kadouch et al. (2006) compared the levels of macrocyclic trichothecenes in samples from 15 flooded dwellings known to be contaminated with S. chartarum or Chaetomium, and a group of nine dwellings without visible mould. The level of macrocyclic trichothecenes was significantly higher in floor dust from the mouldy houses than from the reference dwellings; the levels in wall samples from mouldy houses were also higher (of borderline statistical significance), but no statistically significant difference in air concentrations was observed. In a study by Brasel et al. (2005a) in seven buildings known to be contaminated with S. chartarum, the airborne level of macrocyclic trichothecenes was significantly higher than that in four control buildings (i.e. with no detectable S. chartarum or history of water damage). The same authors also showed that S. chartarum trichothecene mycotoxins can become airborne in association with both intact conidia and smaller fungal fragments (Brasel et al., 2005a,b). Sterigmatocystin was shown to aerosolize from a finishing material (Moularat, Robine, 2008), at an airflow rate of 100 cm/s and a relative humidity of 30%. These studies demonstrate that mycotoxins are present in the indoor environment and that the levels may be higher in buildings affected by mould or damp. It is still not clear, however, whether the levels of airborne mycotoxins in damp buildings are sufficiently high to cause adverse health effects (see section 4.2.2).

2.3.6. Microbial and other volatile organic compounds

Several fungi produce volatile metabolites, which are a mixture of compounds that can be common to many species, although some also produce compounds that are genera- or species-specific. Microbial volatile organic compounds are often similar to common industrial chemicals. To date, more than 200 of these compounds derived from different fungi have been identified (Wilkins, Larsen, Simkus, 2000, 2003), including various alcohols, aldehydes, ketones, terpenes, esters, aromatic compounds, amines and sulfur-containing compounds. As few of those compounds are specific to fungi, measuring (microbial) volatile organic compounds is therefore of limited use in identifying indoor fungal growth. Detection of specific organic compounds does, however, permit a conclusion of fungal growth (visible or hidden), even if the results are not quantitative (Moularat et al., 2008a,b). No larger field studies have been conducted to compare mouldy and control buildings, and microbial volatile organic compounds have been measured only rarely in health surveys. Data on airborne concentrations are therefore scarce.

Some exposures with adverse health effects associated with damp indoor environments include emissions of volatile organic compounds from damp and mouldy building materials (Claeson, Sandstrom, Sunesson, 2007). Emissions are a consequence of competition between moisture and some chemicals for adsorption sites. Volatile organic compounds can be similar to microbial ones, as both often occur in the same environment. The main difference is the source of emission, i.e. mould or building materials. Damp concrete floors have been shown to increase chemical degradation of the plasticizer in polyvinyl chloride floor coatings and glues, resulting in emissions of volatile organic compounds such as 2-ethyl-1-hexanol (Norbäck et al., 2000; Tuomainen, Seuri, Sieppi, 2004). Similarly, damp concrete floors may emit ammonia from the self-levelling flooring compound used in the late 1970s and early 1980s in Europe. Furthermore, the offgassing of formaldehyde from composite wood products and the rate of formation of ozone increase with relative air humidity (Arundel et al., 1986; Godish, Rouch, 1986). Formaldehyde concentrations may also be elevated in damp indoor environments because moist air holds more formaldehyde. The levels of semi-volatile compounds, such as pentachlorophenol (a wood preservative) and other pesticides, may also be elevated in damp indoor environments. No studies have systematically addressed the link between levels of volatile organic compounds and dampness in indoor environments.

2.3.7. Viruses

It has been hypothesized that damp indoor environments with sufficiently high air humidity prolong the survival of respiratory viruses, so that the occupants are at greater risk of respiratory infection and, possibly, the onset of allergic disease (Hersoug, 2005). Although some experimental evidence shows significantly better survival times for several common cold-causing viruses, no real-life data are available. It is therefore unclear whether exposure to viruses should be considered as a risk associated with damp indoor spaces.

2.4. Exposure assessment

Exposure can be defined as an event during which people come into contact with a pollutant at a certain concentration during a certain length of time (WHO Regional Office for Europe, 2006a). In most circumstances, however, exposure defined in this way cannot be determined confidently, and exposure indicators are used instead. Thus, when the word exposure is used without qualification in this document, it refers to indicators. The indicators of exposure in indoor environments used most commonly are derived from answers to questionnaires. A more objective approach (but not necessarily a more valid one; see below) might be to measure the airborne or surface concentrations of indoor pollutants, such as the amount of pollutant per cubic metre of air or per gram of house dust. These are, however, generally relatively crude proxies of the true exposure and thus lead to at least some misclassification of exposure and subsequent bias.

The relative lack of knowledge about the role of specific exposures in health problems related to house dampness is due mainly to a lack of valid, quantitative methods for assessing exposure, particularly of bioaerosols. This may explain the relatively large number of studies that have failed to demonstrate a direct association between bioaerosol concentrations and health effects in damp indoor environments (see Chapter 4). This section discusses the issues of exposure assessment related to observed and perceived dampness and of bioaerosol measurements. Measurement of humidity in the air and of the moisture content of building materials is discussed in Chapter 3.

2.4.1. Measurement of indicators of dampness

Occupants’ perceptions are the basis used for assessing house dampness in most epidemiological studies, and questionnaires are therefore often the method chosen. The questions typically elicit information on whether conditions such as leaks, flooding, wet basements, window condensation, visible fungal growth or mouldy odours are or have been present. Sometimes, the extent of water damage and damp is also assessed. Prevalence estimates may vary widely, however, depending on the way in which such questions are framed, the type of question, the level of detail requested and the judgement of the people filling in the questionnaire.

Reliance on self-reporting, which is by definition subjective, may be a source of error in cross-sectional studies, as demonstrated by Dales, Miller and McMullen (1997), who reported that under some conditions people with allergies are more likely than non-allergic people to report visible fungal growth. Other studies have shown that such bias is unlikely (Verhoeff et al., 1995; Zock et al., 2002). To overcome the problems associated with the bias of self-reporting, trained inspectors have been used in several studies to visit houses and assess indoor dampness, including its severity. This method has the advantage of being more objective and allows a more standardized approach; nevertheless, it is often a single snapshot, which lacks the longer perspective of the occupants. These differences in approaches may lead to different estimates of the prevalence of house dampness, as demonstrated in several comparisons of the two methods (see section 2.1).

Measurements of humidity in air, the moisture content of building materials, their interrelationships and their relationship with indoor climate dynamics more generally are discussed in Chapter 3.

2.4.2. Measurement of microorganisms and microbial agents

The assessment of indoor concentrations of microorganisms presents distinct challenges. Pathogenic microorganisms may be hazardous at extremely low levels, while other organisms may become important health hazards only at concentrations that are orders of magnitude higher. Some organisms and spores are extremely resilient, while others are inactivated during sampling. Certain fungal spores are easily identified and counted, while many bacteria are difficult to characterize. Sensitive, specific methods are available for quantifying some microbial agents, while there are no good methods for others. Many of the newly developed methods (e.g. measurement of microbial agents such as fungal (1→3)-β-D-glucans or fungal extracellular polysaccharides; see below) have not been well validated and are often unavailable commercially. Even with some well-established methods (e.g. the Limulus amoebocyte lysate assay for measuring bacterial endotoxins; see below), significant variations in concentrations have been found (Thorne et al., 1997; Chun et al., 2000; Reynolds et al., 2002). Issues about the storage and transport of bioaerosol samples have often not been addressed, although these conditions can affect the activity of some biological agents, such as endotoxins (Thorne et al., 1994; Douwes et al., 1995; Duchaine et al., 2001). Furthermore, not all the biological agents that might be associated with damp indoor environments and their health effects may have been identified.

Most studies of dampness and health have focused on visible fungi or water damage, and in most of these studies exposure was assessed from questionnaires. The extent to which questionnaire reports of fungal growth correlate with actual exposure to relevant fungal components is, however, not known. In studies with objective measurements of fungal concentrations, spores were generally cultured from indoor air (Garrett et al., 1998) or from settled dust (Jacob et al., 2002). The section below gives the options available for measuring concentrations of microorganisms (particularly fungi) in indoor air.

2.4.2.1. Culture-based methods

Airborne concentrations of microorganisms can be studied by counting culturable propagules in air samples or settled dust samples. Sampling of culturable microorganisms is based on impaction (in which microorganisms are collected from the airstream due to an inertial force that deposits them onto a solid or semisolid collection surface), liquid impingement (in which inertia as a principle force collects microorganisms in a liquid medium) or air filtration (separation of microorganisms from the airstream by passage through a porous medium such as a filter). After sample collection, colonies of bacteria and fungi are grown on culture media at a defined temperature for the length of time required for colony development (usually 3–7 days). Colonies are counted manually or by image analysis techniques. To date, no standard methods are available for detecting and enumerating fungi in indoor environments, which significantly limits the potential for comparing data from different studies. International standards are, however, being prepared by the International Organization for Standardization (ISO) technical committee 147/SC on indoor air for sampling by filtration and impaction and for the cultivation of fungi (ISO 16000-16, -17,-18).

Counting culturable microorganisms has some serious limitations. These include poor reproducibility; selection of certain species because of, for example, the choice of sampling method, culture media or temperature chosen; and the lack of detection of non-culturable and dead microorganisms, cell debris and microbial components, although they too may have toxic or allergenic properties. In addition, no good methods for sampling personal air for culturable microorganisms are available, and air sampling for more than 15 minutes is often not possible, whereas air concentrations usually vary widely over time (see section 2.4.5). Nevertheless, counting culturable microorganisms is potentially a very sensitive technique, allowing the identification of many different species.

Traditional culture methods have proven to be of limited use for quantitative assessment of exposure. Culture-based techniques thus usually provide qualitative rather than quantitative data. The former can, however, be important in risk assessment, as not all fungal and bacterial species pose the same hazard. Furthermore, a qualitative comparison of indoor and outdoor microbiota (in samples collected at the same time) may provide important information about potential indoor sources of contamination. More extensive reviews of techniques for sampling and culturing microorganisms are available (Eduard, Heederik, 1998; Macher, 1999).

2.4.2.2. Non-culture-based methods

In non-culture-based methods, organisms are enumerated regardless of their viability. Non-culturable microorganisms are generally sampled by air filtration or liquid impingement methods. Microorganisms can be stained with a fluorochrome, such as acridine orange, and counted under an epifluorescence microscope (Thorne et al., 1994).

Slit impaction on glass slides and staining with lactophenol blue is a common method for microscopic determination of the total concentration of fungal spores. The possibility of classifying microorganisms taxanomically is limited because little structure can be observed. Electron microscopy or scanning electron microscopy allows better determination (Eduard et al., 1988; Karlsson, Malmberg, 1989). Bacteria collected on impingers or filters can be counted by flow cytometry after staining with 4′,6-diamino-2-phenylindole or by fluorescent in situ hybridization (Lange, Thorne, Lynch, 1997).

The main advantage of microscopy and flow cytometry is that both culturable and non-culturable microorganisms can be quantified, selection effects are limited, personal air sampling is possible, the sampling time can – for many microorganisms – be varied over a wide range, and results are available quickly. The disadvantages include the unknown validity of these techniques, lack of detection of possibly relevant toxic or allergenic components or cell debris, limited possibilities for determining microorganisms, laborious and complicated procedures, and high cost per sample of the more advanced methods. An extensive review of microscopy and flow cytometry methods for counting nonculturable micro organism has been published (Eduard, Heederik, 1998). Little or no experience has been gained in non-industrial indoor environments with more advanced non-culture-based methods, such as scanning electron and epifluorescence microscopy and flow cytometry. Therefore the usefulness of these methods for indoor risk assessment is unknown.

2.4.2.3. Methods for assessing microbial constituents

Constituents or metabolites of microorganisms can be measured to estimate microbial exposure, instead of counting culturable or non-culturable microbial propagules. Toxic (e.g mycotoxins) or pro-inflammatory components (e.g endotoxins) can be measured, and non-toxic molecules can be used as markers of large groups of microorganisms or of specific microbial genera or species. The availability of methods, such as those based on the polymerase chain reaction (PCR) and immunoassays, has opened new avenues for detection and identification of species, regardless of whether the organisms are culturable.

Markers for the assessment of fungal biomass include ergosterol, measured by gas chromatography–mass spectrometry (Miller, Young, 1997), and fungal extracellular polysaccharides, measured in specific enzyme immunoas says (Douwes et al., 1999). These allow partial identification of the mould genera present. Volatile organic compounds produced by fungi, which may be suitable markers of fungal growth (Dillon, Heinsohn, Miller, 1996; Moularat et al., 2008b), are usually measured in air samples by gas chromatography with or without mass spectrometry or high-pressure liquid chromatography. Other agents, such as (1→3)-β-Dglucans (Aketagawa et al., 1993; Douwes et al., 1996) and bacterial endotoxins, are measured because of their toxic potency. Endotoxins are measured with a Limulus amoebocyte lysate test, prepared from blood cells of the horseshoe crab, Limulus polyphemus (Bang, 1956). Analytical chemistry techniques with gas chromatography–mass spectrometry for quantifying lipopolysaccharides have also been developed (Sonesson et al., 1988, 1990); however, these methods require special extraction procedures and have not been widely used. Two methods for measuring (1→3)-β-D-glucans have been described, one of which is based on the Limulus amoebocyte lysate assay (Aketagawa et al., 1993) and the other on an enzyme immunoassay (Douwes et al., 1996).

PCR techniques are available for the identification of species of bacteria and fungi in air (Alvarez et al., 1994; Khan, Cerniglia, 1994), and several quantitative PCR methods have been validated for use in the indoor environment. For example, real-time PCR methods have been described to detect and quantify Cladosporium (Zeng et al., 2006) and Aspergillus (Goebes et al., 2007) at the genus level. Similar methods have been developed for measuring species of common indoor fungi (Vesper et al., 2005; Meklin et al., 2007). PCR methods allow the assessment of large groups of microorganisms. For instance, a quantitative PCR method is available for measuring 36 indicator species commonly associated with damp houses in the United States and has been used to define an “environmental relative mouldiness index” for houses in that country (Vesper et al., 2007). PCR methods for quantitative assessment of exposure to fungi and other microorganisms have significant advantages, including sensitivity and specificity. Also, they can be used for quantitative assessment; they provide results relatively quickly; they can be used to measure a wide range of microorganisms, both genus and species; and they are independent of the culturability of the organism.

Most methods for measuring microbial constituents (with the exception of that for bacterial endotoxins) are experimental and have yet to be used routinely or are unavailable commercially. Important advantages of these methods include the stability of most of the measured components, allowing longer sampling times for airborne measurements and frozen storage of samples before analysis; the use of standards in most of the methods; and the possibility of testing for reproducibility. These methods do not, however, leave fungal isolates for further investigation.

2.4.3. Measurement of indoor allergens

Antibody-based immunoassays, particularly enzyme-linked immunosorbent assays, are widely used to measure aeroallergens and allergens in settled dust in buildings. These assays involve use of antibodies specific to the target allergen and an enzymic reaction with a substrate for detection. In radioimmunoassays, radiolabelling is used for detection. The house dust mite allergens Der p I, Der f I and Der p/f II have been widely investigated and the methods well described (Luczynska et al., 1989; Price et al., 1990; Leaderer et al., 2002). Methods for assessing exposure to allergens from rodents (Swanson, Agarwal, Reed, 1985; Schou, Svendson, Lowenstein, 1991; Hollander, Heederik, Doekes, 1997), cockroaches (Pollart et al., 1994) and storage mites (Iversen et al., 1990) have been published.

Methods for measuring fungal allergens are not widely available, mainly because of difficulties in manufacturing and standardizing fungal allergen extracts (see section 2.3.1). Nonetheless, some enzyme-linked immunosorbent assays have been described in the literature, and a commercial assay is available for A. alternata allergen (Alt a I). A comparison of several monoclonal and polyclonal antibody-based assays for measuring Alt a I (including a commercially available method) showed, however, wide disparity (Barnes et al., 2006). It is therefore unclear whether these assays provide valid estimates of the true Alternaria allergen concentrations in indoor samples.

2.4.4. Strategies for monitoring exposure

In addition to questionnaires, personal or environmental monitoring is commonly used for exposure assessment. Although monitoring can potentially result in a more valid, accurate assessment, this may not always be the case. Validity is strongly dependent on the sampling strategy chosen, which in turn depends on a large number of factors, including: the type of exposure and disease or symptoms of interest; whether the health outcomes are acute or chronic (e.g. exacerbation versus development of disease); whether the approach is population- or patient-based; suspected variations in exposure over both time and space and between diseased and reference populations; the methods available to assess exposure; and the costs of sampling and analysis.

2.4.4.1. What should be measured?

With regard to health problems associated with indoor air, many exposures should be considered, as it is often unclear which microorganisms or agents are causing the symptoms or diseases. Some studies are conducted specifically to assess which exposures are contributing to the development of symptoms. In practice, both funding and the availability of methods for measuring agents are limited, as many methods are not commercially available and are used only in research, severely limiting the possibility of measuring all agents of interest.

2.4.4.2. How useful are routinely collected data?

Data collected for use in monitoring may be of limited value in epidemiological studies. For example, monitoring is often done in areas where the concentrations are likely to be highest, to ensure compliance with exposure limits. In contrast, epidemiological studies require information on average concentrations. Special surveys may therefore be necessary, with random sampling, rather than relying on data collected during monitoring.

2.4.4.3. When should sampling be done?

To the extent possible, samples should be taken so that they represent the true exposure at an appropriate time. For acute effects, exposure measured shortly before the effects occur is the most useful. The situation is more complicated for chronic effects, as, ideally, exposure should be assessed before the effects occur and preferably at the time they are biologically most relevant – that is, when the exposure is considered the most problematic or when people are most likely to be exposed. This is possible only in prospective cohort studies or in retrospective cohort studies in which information on past exposure is available; even then, it is often unclear when people are most likely to be exposed to the agent of interest. In cross-sectional studies, exposure measurement can be valuable for assessing past exposure, but only when the environment has not changed significantly.

2.4.4.4. How many samples should be taken?

Measures of exposure should be sufficiently accurate and precise that the effect on disease can be estimated with minimal bias and maximum efficiency. Precision can be gained (i.e. measurement error can be reduced) by increasing the number of samples taken, either by increasing the number of people for whom exposure is measured or by increasing the number of measurements per person. In population studies, repeated sampling is particularly effective for exposures known to vary more widely over time within people than among people. If the within-people variation is lower than that between people, repeated measurements will not reduce the measurement error significantly. If there is known within- and between-person variation (from previous surveys or pilot studies, for example), the number of samples required to reduce bias in the risk estimate by a specific amount can be computed in the manner described by Cochran (1968) (see also section 2.4.5).

2.4.4.5. Should settled dust or airborne samples be taken?

In many studies, reservoir dust from carpets or mattresses is collected, and the concentrations are usually expressed in either weight per gram of sampled dust or weight per square metre. Although both measures are generally accepted, the latter may better reflect actual exposure (Institute of Medicine, 2004). The advantage of settled dust sampling is the presumed integration over time that occurs in deposition of the pollutant on surfaces (Institute of Medicine, 2000). Micro organisms can also proliferate in carpets, provided there is sufficient access to water; however, surface samples allow only a crude measure that is probably only a poor surrogate for airborne concentrations.

Airborne sampling requires very sensitive analytical methods. In addition, for an accurate assessment, large numbers of samples must be collected, as the temporal variation in airborne concentrations is probably very high (see section 2.4.5). Airborne sampling after agitation of settled dust has been used in some studies (Rylander et al., 1992, 1998; Rylander, 1997b; Thorn, Rylander, 1998), but it is questionable whether this results in a more valid exposure assessment. Therefore, exposure assessment is generally uncertain and this may obscure exposure–response relationships in epidemiological studies.

The recently described dustfall collector, a simple passive tool for long-term collection of airborne dust, combines the two methods to some extent (Würtz et al., 2005). Collectors are placed on shelves or cupboards at least 1.5 m above the floor and receive airborne dust by sedimentation; they can be used for up to several months. This method is not affected by short-term temporal variance in airborne concentrations and is probably a better surrogate for airborne exposures relevant to indoor health. The dustfall collector is cheap to produce and simple to use, and the microbial levels measured with this device appear to correlate with the degree of moisture in school buildings. Although the initial results look promising, more validation is required to assess the usefulness of the device for measuring indoor exposure.

2.4.4.6. Should ambient or personal airborne sampling be conducted?

In general, personal measurements best represent the risk of the relevant exposure, and personal sampling is therefore preferred to area sampling. Modern sampling equipment is now sufficiently light and small that it can be used for personal sampling, and several studies of chemical air pollution have demonstrated its feasibility both indoors and outdoors (Janssen et al., 1999, 2000). Personal sampling might not always be possible, however, for practical reasons, such as being too cumbersome for the study participants or a lack of portable equipment for making the desired measurements (of viable microorganisms, for example). Nonetheless, it is expected that greater use of new, sensitive exposure assessment methods, including quantitative PCR techniques to measure indoor microbial concentrations (see section 2.4.2), will overcome some of these constraints, in particular when used in combination with passive personal samplers.

2.4.5. Problems in measuring indoor exposure

Exposure to microorganisms in the indoor environment is most frequently assessed by counting culturable spores in settled dust or the air, but this approach has serious drawbacks (see section 2.4.2). Perhaps the most important problem, which has rarely been acknowledged in the literature, is that air sampling for more than 15 minutes is often not possible, since air concentrations usually vary a great deal over time. The few studies in which repeated measurements were made of fungi in air or in settled dust showed considerable temporal variation in concentrations, even over short periods (Hunter et al., 1988; Verhoeff et al., 1994b). The variation in the concentrations of isolated genera was even more substantial (Verhoeff et al., 1994b; Chew et al., 2001).

It has been suggested that in order to achieve a ratio of 3–4 for within- and between-house variation in concentration, which appears to be realistic for culturable indoor fungi (Verhoeff et al., 1994b), 27–36 samples should be taken per house. This is necessary for reliable estimates of the average concentration in an epidemiological study with less than 10% bias in the relationship between a health end-point and the exposure (Heederik, Attfield, 2000; Heederik et al., 2003).

Thus, unless many samples are taken per house, sampling of culturable organisms will probably result in a poor quantitative measure of exposure, leading to a nonspecific bias towards the null. This might explain why most studies that included measurements of culturable fungi found no association with symptoms (in contrast to reported mould). The issue is particularly relevant for measurements of viable microorganisms; nonetheless, similar problems may exist for airborne measurements of other bioaerosols, such as house dust mite allergens, endotoxins and fungal (1→3)-β-D-glucans, as the airborne concentrations of these agents are also likely to be characterized by high temporal variation. This problem can be overcome by increasing the sampling time (up to several days or weeks), which is feasible for most bioaerosols, except viable microorganisms. This is often considered to be impractical, and therefore most exposure measurements continue to involve surface sampling, which is generally less affected by temporal variation. Surface sampling, however, may be a poor proxy for airborne concentrations (see above).

As no health-based exposure limits for indoor biological agents have been recommended, interpretation of concentrations is difficult, particularly in case studies. Therefore, strategies to evaluate indoor concentrations (either quantitatively or qualitatively) should include comparisons of exposure data with background levels or, better, comparisons of the exposure levels of symptomatic and non-symptomatic persons or in damp and non-damp buildings. A quantitative evaluation involves comparisons of concentrations, whereas a qualitative evaluation could consist of comparisons of species or genera of microorganisms in different environments. Because of differences in climatic and meteorological conditions and in the measurement protocols used in different studies (e.g. viable or non-viable sampling or by type of sampler or analysis), reference material in the literature can seldom be used.

2.5. Summary and conclusions

The prevalence of indoor damp is estimated to be in the order of 10–50%. It is highest in deprived neighbourhoods, where it often significantly exceeds the national average. Many case reports have also shown dampness and mould problems in office buildings, schools and day-care centres, but it is unclear what proportion of these buildings is affected. High air humidity, condensation and water damage promote the survival and growth of dust mites and fungi, resulting in increased exposure to mite and fungal allergens and fungal toxins and irritants. Damp indoor environments may also contain bacteria, bacterial endotoxins and other microorganisms, such as amoeba, but less information is available about these agents and further research is required. Damp building materials may increase their chemical degradation, resulting in more emissions of volatile organic compounds, including formaldehyde, further deterioration of building materials and structural integrity and subsequent use (and misuse) of potentially hazardous chemicals such as pesticides. Although it is plausible that the exposures listed above are the main causal factors of the health effects associated with damp buildings, this has not been proven.

Risk assessment is seriously hampered by a lack of valid methods for quantitative assessment of exposure. The usual culture methods for quantifying exposure to microbes have major limitations. Non-culture methods for assessing microbial constituents (e.g. microbial DNA, allergens, endotoxins, (1→3)-β-D-glucans and fungal extracellular polysaccharides) appear more promising, although experience with these methods is generally limited. Therefore, more research is needed to establish better exposure assessment tools, and newly developed methods must undergo rigorous validation.

If exposure in indoor environments is to be monitored and valid conclusions are to be drawn, it is important also to include measurements of people without symptoms and measurements in buildings without damp. Furthermore, interpretation of airborne sampling should be based on multiple samples, as space–time variation in the environment is high. Proper interpretation of indoor measurements also requires detailed information about sampling and analytical procedures (including quality control) and awareness of the problems associated with these procedures. If methods for culturable organisms are used, comparisons with outdoor microbiota might provide further qualitative evidence of potential indoor sources of contamination.

Copyright © 2009, World Health Organization.

All rights reserved. The Regional Office for Europe of the World Health Organization welcomes requests for permission to reproduce or translate its publications, in part or in full. Address requests for publications of the WHO Regional Office for Europe to: Publications, WHO Regional Office for Europe, Scherfigsvej 8, DK-2100 Copenhagen Ø, Denmark. Alternatively, complete an online request form for documentation, health information, or for permission to quote or translate, on the Regional Office web site (http://www.euro.who.int/pubrequest).

Bookshelf ID: NBK143945

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