1.1. Background
Problems of indoor air quality are recognized as important risk factors for human health in both low-income and middle- and high-income countries. Indoor air is important also because populations spend a substantial fraction of time within buildings. In residences, day-care centres, retirement homes and other special environments, indoor air pollution affects population groups that are particularly vulnerable due to their health status or age. Microbial pollution involves hundreds of species of bacteria and fungi that grow indoors when sufficient moisture is available. Exposure to microbial contaminants is clinically associated with respiratory symptoms, allergies, asthma and immunological reactions.
Indoor air plays a special role as a health determinant, and management of indoor air quality requires approaches that differ from those used for outdoor air. For these reasons, the working group preparing the global update of the WHO air quality guidelines (WHO Regional Office for Europe, 2006a) recommended that WHO also prepare guidelines for indoor air quality. This is in line with the recommendations of an earlier WHO working group that formulated The right to healthy indoor air and, in particular, with its Principle 6, which states that “Under the principle of accountability, all relevant organizations should establish explicit criteria for evaluating and assessing building air quality and its impact on the health of the population and on the environment” (WHO Regional Office for Europe, 2000a).
The subsequent working group meeting in Bonn in October 2006 acknowledged the applicability of the existing WHO guidelines for air quality (WHO Regional Office for Europe, 2000b, 2006a) to indoor air and identified a number of chemical substances for which specific indoor air guidelines was recommended. The working group also recommended guidelines for two additional categories of risk factors of particular importance to health in indoor environments: biological agents and indoor combustion (WHO Regional Office for Europe, 2006b).
Biological agents of relevance to health are widely heterogeneous, ranging from pollen and spores of plants (mainly from outdoors), to bacteria, fungi, algae and some protozoa emitted outdoors or indoors. They also include a wide variety of microbes and allergens that spread from person to person. There is strong evidence regarding the hazards posed by several biological agents that pollute indoor air; however, the WHO working group convened in October 2006 concluded that the individual species of microbes and other biological agents that are responsible for health effects cannot be identified. This is due to people often being exposed to multiple agents simultaneously, to complexities in accurate estimation of exposure and to the large numbers of symptoms and health outcomes due to exposure. The exceptions include some common allergies, which can be attributed to specific agents, such as house dust mites and pets.
While quantitative guidelines could not be provided for the concentrations of many biological agents, recommendations, guidance or guidelines (all referred to ‘guidelines’ in this text) have been formulated to protect health and identify health risks by defining indicators of a safe indoor environment. Thus, the guidelines define problems and the circumstances in which risk is likely to occur but do not include recommendations on acceptable levels of exposure to biological agents. The aim of these guidelines is to help public health authorities and the general public to identify the hazards and reduce the risks. They do not, however, make recommendations for specific actions to manage indoor air quality, which must be addressed at the national or local level, as has already been done in some countries.
Although the literature on biological contaminants in indoor air is vast, there is no universal agreement on the precise meaning of the terms used to describe the (micro-)environmental conditions determining their presence and proliferation. The definitions used in these guidelines are shown in Box 1.
Definitions of some terms used in the guidelines. Air-conditioning system: assembly of equipment for treating air to control simultaneously its temperature, humidity, cleanliness and distribution to meet the requirements of a conditioned space. Dampness: (more...)
The presence of many biological agents in indoor environments is attributable to dampness and inadequate ventilation. Excess moisture on almost all indoor materials leads to growth of microbes, such as mould, fungi and bacteria, which subsequently emit spores, cells, fragments and volatile organic compounds into indoor air. Moreover, dampness initiates chemical or biological degradation of materials, which also pollute indoor air. Dampness has therefore been suggested to be strong, consistent indicator of risk of asthma and respiratory symptoms (e.g. cough and wheeze). The health risks of biological contaminants of indoor air could thus be addressed by considering dampness as the risk indicator.
Several widely acknowledged global trends contribute to the conditions associated with increased exposure to dampness and mould:
energy conservation measures that are not properly implemented (tightened building envelopes, ventilation deficits, improper insulation);
urbanization (migration, building type and density, urban degradation, housing availability and social inequity);
climate change (increasing frequency of extreme weather conditions, shifting of climate zones); and
the quality and globalization of building materials and components, construction concepts and techniques.
These conditions increase the risks of adverse health effects due to biological contaminants of indoor air.
1.2. Scope of the review
One of the recommendations of the WHO working group that met in October 2006 was to identify the main health risks due to dampness, the associated microbial growth and contamination of indoor spaces and to formulate relevant guidelines for the protection of public health. As separate guidelines are to be prepared specifically for allergens from pets and from house-dust mites, these aspects are mentioned only briefly in these guidelines and only if moisture and dampness determine their presence.
The review covers not only mould but also covers to some extent other biological agents, such as bacteria associated with excess moisture in indoor environments. As it was not feasible to include all the exposure factors in the title, dampness and mould were accepted as a compromise between brevity and precision concerning the content of the guidelines.
The need for definitions of terms such as dampness and moisture was discussed extensively during the review. The working group agreed, however, that while it is important to use terms consistently, explicit definitions have limitations. In the scientific studies reviewed, various definitions were used, and even native language speakers could not recognize subtle differences in the different definitions. Therefore, the terms used had to be evaluated separately in each context.
The complex causal chain of events that constitute a health hazard links sources of water through excessive moisture to biological growth and physical and chemical degradation and further to emission of hazardous biological and chemical agents (). The review of evidence follows this chain, summarizing the issues related to building dampness and its impacts on indoor exposure to biological and non-biological pollutants in Chapter 2, which also presents approaches to exposure assessment.
Pathways linking sources of dampness with health. Note:
a MVOC, methane VOC;
b VOC, Volatile organic compounds.
An important determinant of dampness and biological growth in indoor spaces is ventilation, which is discussed in Chapter 3. The working group acknowledged that ventilation plays a major role in building operation and occupants’ health and that these aspects should be covered in another volume of the Indoor air quality guidelines (e.g. in guidelines on indoor combustion). Moisture control in building structures is the second important issue summarized in Chapter 3. Unfavourable combinations of air humidity and temperature differences in building structures may lead to condensation of water on surfaces and consequent decomposition of building materials and microbial growth.
The evidence for the health effects of indoor exposure to dampness and mould is presented in Chapter 4, which is based on reviews of epidemiological studies and clinical and toxicological research.
1.3. Preparation of the guidelines
The planning meeting in 2006 recommended that a steering group2 be established to guide preparation of the guidelines. The group recommended potential authors and reviewers for the first draft of the background material, which was submitted to WHO in August 2007 and, after compilation, distributed for external review. The drafts and the reviewers’ comments provided the background for the discussions of the WHO working group meeting in Bonn, 17–18 October 2007, which evaluated both the scientific evidence and the comments made in the review. The working group then summarized their evaluations of the health risks and recommended the guidelines presented in Chapter 5 of this document. The comments collected before and during the working group meeting were used by the primary authors of the background material to prepare the second drafts of Chapters 1–4. These drafts were reviewed by the working group members. The final versions of the chapters incorporated the comments of all reviewers and were accepted by the steering group.
It is anticipated that the recommendations in these guidelines will remain valid until 2018. The Department of Public Health and Environment at WHO headquarters in Geneva will be responsible for initiating a review of these guidelines at that time.
1.4. Guidelines and indoor air quality management
The guidelines are intended for worldwide use, to protect public health under various environmental, social and economic conditions and to support the achievement of optimal indoor air quality. They focus on building characteristics that prevent the occurrence of adverse health effects associated with dampness or mould. The guidelines are applicable to various levels of economic development and different climates, cover all relevant population groups, and propose feasible approaches to reducing health risks due to dampness and microbial contamination. Chapters 2–4 summarize the scientific evidence on the relationships between dampness, mould and health effects. Both private and public buildings (e.g. offices and nursing homes) are covered, as dampness and mould are risks everywhere. Settings with particular production processes and hospitals with high-risk patients or sources of exposure to pathogens are not, however, considered.
While the guidelines provide objectives for indoor air quality management, they do not give instructions for achieving those objectives. The necessary action and indicators depend on local technical conditions, the level of development, human capacities and resources. The guidelines recommended by WHO acknowledge this heterogeneity. In formulating policy targets, governments should consider their local circumstances and select actions that will ensure achievement of their health objectives most effectively.
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Steering group members: Ross Anderson, Aaron Cohen, Séverine Kirchner, Lars Mølhave, Aino Nevalainen, Bernd Seifert, Kirk Smith and John Spengler
