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WHO Guidelines for Indoor Air Quality: Selected Pollutants. Geneva: World Health Organization; 2010.

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WHO Guidelines for Indoor Air Quality: Selected Pollutants.

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Introduction

Human beings need a regular supply of food and water and an essentially continuous supply of air. The requirements for air and water are relatively constant (10–20 m3 and 1–2 litres per day, respectively). That all people should have free access to air and water of acceptable quality is a fundamental human right. Recognizing the need of humans for clean air, in 1987 the WHO Regional Office for Europe published the first edition of Air quality guidelines for Europe (1), containing health risk assessments of 28 chemical air contaminants.

In 2000, WHO published a second edition of the guidelines (2) and a “global update” was published in 2006 (3). The second edition focused on the pollutants considered in the first edition. The global update focused on a small group of pollutants (particulate matter, ozone, nitrogen dioxide and sulfur dioxide) but also included chapters that addressed some health-related general subjects of importance to the air pollution field, including a chapter on indoor air quality. The WHO air quality guidelines have played an important role in providing information and guidance for regulatory authorities working in the air pollution field. In Europe, the guidelines are now seen as the key source on which the European Commission's directive on air quality is based.

That people are exposed to air pollutants both outdoors and indoors is obvious. Globally, people are spending an increasing amount of time indoors. There they are exposed to pollutants generated outdoors that penetrate to the indoor environment and also to pollutants produced indoors, for example as a result of space heating, cooking and other indoor activities, or emitted from products used indoors.

The first edition of the Air quality guidelines for Europe published in 1987 (1) included a chapter on radon and an annex on tobacco smoke, indoor air pollutants with significant adverse public health impacts. The second edition published in 2000 (2) provided a section on indoor air pollutants and added man-made vitreous fibres to radon and tobacco smoke.

The 2005 global update of the air quality guidelines (3) drew attention to the large impact on health of indoor air pollution in developing countries. The high concentration of particulates and gases found indoors in houses using solid fuel, including biomass, were noted and it was estimated that exposure might be responsible for nearly 1.6 million excess deaths annually and about 3% of the global burden of disease. This is a huge impact on health; indeed, far larger than that imposed by exposure to outdoor air pollutants.

Work on assessing the health effects of indoor air pollution has lagged behind that on outdoor air pollution for a number of reasons, including:

  • the fact that policy development in the air pollution field has focused on outdoor air pollution as a result of the correctly perceived need to deal with the high levels of outdoor air pollutants associated with both coal smoke and photochemical smog;
  • the ready applicability of standards to outdoor concentrations of air pollutants;
  • the feasibility of monitoring concentrations of outdoor air pollutants on a large scale;
  • the focus of epidemiologists on defining coefficients linking outdoor concentrations of air pollutants with effects on health; and
  • the fact that the science and policy communities have focused on the public health impacts of air pollution in wealthy developed countries, while often disregarding the larger burden of disease due to indoor air pollution from solid fuel burning in the developing world.

Questions such as: “how could air quality standards be enforced indoors?” have delayed work on specific indoor air quality guidelines. However, WHO has not ignored the problem of indoor exposure to air pollutants and has stressed since the publication of the first edition of the guidelines in 1987 (1) that they should be applicable to both indoor and outdoor air. This was reinforced in the global update published in 2006 (3) and the guidelines were recommended for application in all microenvironments. It should be noted that the workplace has been specifically excluded: WHO air quality guidelines have not been seen as a basis for occupational exposure standards.

Developing indoor air quality guidelines

Acknowledging that indoor air has a special role as a health determinant and that the management of indoor air quality requires approaches different from those used for outdoor air, the working group preparing the global update of the WHO air quality guidelines (3) recommended that WHO should also prepare guidelines for indoor air quality. This is in line with the recommendations of an earlier WHO working group formulating a set of statements on “The right to healthy indoor air” and in particular with 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.” (4).

The WHO working group that subsequently met in Bonn in October 2006 acknowledged the applicability of the existing WHO guidelines for air quality (2,3) to indoor air and identified a number of chemical substances for which specific indoor air guidelines should be recommended (5). The working group also recommended developing guidelines for two additional categories of risk factor of particular importance for health in indoor environments: biological agents and indoor combustion. Following these recommendations, the WHO guidelines on dampness and mould were published in 2009 (6).

The working group defined the following criteria for selecting compounds for which the development of WHO guidelines for indoor air could be recommended:

  • existence of indoor sources
  • availability of toxicological and epidemiological data
  • indoor levels exceeding the levels of health concern (no observed adverse effect level (NOAEL) and/or lowest observed adverse effect level (LOAEL)).

Based on these criteria, pollutants considered were divided into two categories (Table 1). Group 1 included pollutants for which WHO guidelines for indoor air were needed and WHO was requested to plan their development. Group 2 included pollutants of potential interest, but the group concluded that further investigation would be needed before it was clear whether there was sufficient evidence to warrant their inclusion in the guidelines at present.

Table 1. Pollutants considered for inclusion in the WHO indoor air quality guidelines by the WHO working group in October 2006.

Table 1

Pollutants considered for inclusion in the WHO indoor air quality guidelines by the WHO working group in October 2006.

The group concluded that the WHO guidelines for environmental tobacco smoke (ETS) published in the second edition of Air quality guidelines for Europe (2), stating that there is no evidence for a safe exposure level, are clear and still valid. Therefore, ETS is not included in the current work. Furthermore, the guidelines for other pollutants should be developed based on the assumption that ETS is eliminated from indoor spaces.

The steering group1 assisting WHO in designing the indoor air quality guidelines concluded that there is no convincing evidence of a difference in the hazardous nature of particulate matter from indoor sources as compared with those from outdoors and that the indoor levels of PM10 and PM2.5, in the presence of indoor sources of PM, are usually higher than the outdoor PM levels. Therefore, the air quality guidelines for particulate matter recommended by the 2005 global update (3) are also applicable to indoor spaces and a new review of the evidence is not necessary at present. Consequently, the work on developing indoor air quality guidelines for selected pollutants focused on nine out of the ten compounds listed in Group 1 of Table 1, i.e. all except particulate matter. As decided at the working group meeting in 2006, the guidelines are intended to address various levels of economic development, cover all relevant population groups, and allow feasible approaches to reducing health risks from exposure to the selected pollutants in various regions of the world.

Setting indoor air quality guidelines

The general approach and terminology used in setting air quality guidelines has been presented in a previous WHO publication (2). It is based on a careful review and interpretation of globally accumulated scientific evidence linking exposure to a selected pollutant in the air with the health outcomes of that exposure, using the approaches proposed by the WHO guidelines on assessing human health risks of chemicals (7) and on the evaluation of epidemiological evidence for environmental health risk assessment (8). For each of the selected substances, a search of bibliographic databases was conducted to identify relevant studies, according to the search protocols described in each of the pollutant-specific chapters. Major reviews conducted by WHO, the International Agency for Research on Cancer (IARC) or national agencies were also considered an important source of information. The process followed in setting the guidelines is schematically presented in Fig. 1.

Fig. 1. The process followed in guidelines formulation.

Fig. 1

The process followed in guidelines formulation.

In reviewing the available information, a systematic review of the peer-reviewed publications was undertaken. This included specifically studies of the effects of indoor exposure to the compounds considered and also evidence gathered from studies of outdoor exposure. The evidence comes from epidemiological, toxicological and clinical research, examining associations between exposures to the pollutants and health as well as studying physiological mechanisms of the effects. The latter includes experiments based on controlled human exposure or using animals. Much of the available health evidence is indirect, based on exposures to mixtures of pollutants or to single pollutants in concentrations higher than usually encountered indoors. The advantages and disadvantages of various types of study used to assess health effects of air pollution are summarized in introductory chapters of the 2005 global update (3).

The review of the evidence focuses on the papers considered to be most relevant for development of the guidelines, and in particular on the studies providing quantitative links between health outcomes and the exposures (as determined by the concentrations of pollutants and the duration of exposure) encountered in indoor environments. The strength of evidence for a link between exposure and health outcome was classified according to the criteria used in the WHO guidelines for indoor air quality: dampness and mould (6), based on the approach developed by the Institute of Medicine (9) and presented in Box 1. The evidence was classified according to the professional judgement of the experts of the clarity of the reported findings with consideration of the strength, quality, diversity and number of studies. Understanding of biological mechanisms responsible for associations observed in epidemiological studies, and described in the “kinetics and metabolism” sections of each pollutant-specific chapter, strengthened the conclusions reached.

Box Icon

BOX 1

Classifying the strength of evidence. The categories in this box refer to the association between exposure to an agent and a health outcome and not to the likelihood that any individual's health problem is associated with or caused by the exposure. These (more...)

In estimating the health risks of exposure, it was not possible to apply the techniques of formal meta-analysis to the evidence base; marked differences in study design and, in some cases, the very limited number of studies available made this impossible.

The evaluation of health risks, which follows the presentation of the most important studies, sets out the conclusions of the experts based on the accumulated evidence. It includes risk characterization (i.e. a summary, integration and evaluation of the major scientific evidence) and considers the relevance to health of indoor exposures encountered in various non-occupational settings as well as the conclusions of other reviews.

Ideally, guideline values should represent concentrations of chemical compounds in air that would not pose any hazard to the human population. Realistic assessment of hazards to human health, however, necessitates a distinction between absolute safety and acceptable risk. To produce a guideline with a high probability of offering absolute safety, one would need a detailed knowledge of dose–response relationships in individuals in relation to all sources of exposure, the types of toxicological effect elicited by specific pollutants or their mixtures, the existence or non-existence of “thresholds” for specified toxicological effects, the significance of interactions, and the variation in sensitivity and exposure levels within the human population. Such comprehensive and conclusive data on indoor contaminants are generally unavailable. Very often, the relevant data are scarce and the quantitative relationships uncertain. The professional judgement of the scientists evaluating the evidence and consensus therefore play an important role in establishing guidance that can be used to indicate acceptable levels of population exposure. Value judgements are needed and the use of subjective terms such as “adverse effects” is unavoidable.

Distinction between adverse and non-adverse effects is difficult. For example, changes in a physiological variable such as an index of lung function that might be regarded as minor and reversible could imply a significant short- or perhaps long-term effect on health. In developing these guidelines, concerns were often expressed about the possible long-term effects of repeated insults that individually produce only small changes in physiological end-points. It was also noted that physiological changes that had previously been seen as indicative of only minor effects (such as a small increase in carboxyhaemoglobin concentration) might not explain all the effects of exposure to a pollutant.

Although it might be accepted that a certain risk can be tolerated, the risks to individuals within a population may not be equally distributed: there may be subpopulations that are at considerably increased risk. Therefore, groups at special risk in the general population must be taken specifically into account in the risk management process. Even if knowledge about groups with specific sensitivity is available, unknown factors may exist that change the risk in an unpredictable manner. During the preparation of these guidelines, attention was paid to defining specific sensitive subgroups in the population.

Preparation of the guidelines

As recommended by the working group that met in 2006 to plan the development of the guidelines, a steering group was established to advise on the scientific issues concerning their development. This group recommended potential authors, who would be invited to review the evidence and develop the first draft of the background material during the summer and autumn of 2008. The steering group also recommended other experts to act as reviewers of the background material. All invited experts, including the members of the steering group, are internationally recognized scientists conducting research in academic or public health institutions and active in the assessment of health risks related to exposure to chemicals. Their expertise includes exposure assessment, toxicology, epidemiology and risk assessment. All experts were requested to disclose any circumstances that could give rise to a potential conflict of interests, i.e. any interests that might affect or might reasonably be perceived to affect the experts' objectivity and independence. A standard “Declaration of interests for WHO experts” form was used, and any positive responses to a set of questions in the Declaration were evaluated by the WHO Legal Office and the representatives of the WHO Guidelines Review Committee. Only experts with no declared conflicts of interests, or for whom the declared activities were not considered to create such conflicts, participated in the formulation of the guidelines.

The background material on each of the nine pollutants reviewed contained sections on:

  • general description of the compound;
  • indoor sources and pathways of exposure;
  • current indoor levels and relationship with outdoor levels;
  • kinetics and metabolism (including experimental evidence on pathogenic mechanisms from animal and in vitro studies); and
  • health effects.

The authors submitted the drafts to WHO in November/December 2008. The complete drafts on each of the compounds were distributed to the reviewers with a request that they evaluate the completeness of the scientific evidence used to prepare the manuscript, the scientific reliability of the evidence review and the clarity of the conclusions of the review. The comments received from the reviewers and collated by the WHO secretariat, were used by the authors to prepare the second drafts, including, in addition to the sections listed above, a first draft of the “Health risk evaluation” section. These second drafts were made available to the WHO working group in advance of its meeting in Bonn on 2–6 November 2009.

The working group meeting was convened to agree on the risk evaluation for each of the pollutants and to formulate WHO guidelines for protecting public health from these risks. Existing national and international guidelines, experience in indoor air quality regulation and the results of completed international reviews supported the discussion and its conclusions. The meeting brought together 47 experts from 15 countries, who had reviewed the evidence and prepared the background papers, as well as members of the steering group. It also involved three observers from national agencies potentially interested in using WHO indoor air quality guidelines in shaping policies and actions addressing health risks of indoor air pollutants, as well as five scientists from WHO headquarters, the WHO Regional Office for Europe and IARC. Robert Maynard and Bernd Seifert chaired the meeting.

Several consecutive drafts of the guidelines section for each pollutant were prepared by small groups of experts and discussed in plenary. The final text was reviewed and approved by consensus at the plenary session. Besides agreeing on the health risk evaluations of the pollutants and formulating the guidelines sections, the experts provided comments on the final text of the background material. These comments were used by the authors to finalize the background sections summarizing the evidence supporting the guidelines in the two months following the meeting. The complete unedited draft was made available to all the working group members for final comments. Final changes to the background sections (but not in the guidelines), as well as to the boxes summarizing the main decisions leading to setting the guidelines, were made following language editing. The edited text was then reviewed by the WHO Guidelines Review Committee. Fig. 2 illustrates the major steps in the process leading to the publication of the guidelines.

Fig. 2. Major stages in preparation of the WHO guidelines on indoor air quality: selected pollutants.

Fig. 2

Major stages in preparation of the WHO guidelines on indoor air quality: selected pollutants.

Combined exposures

This volume contains an evaluation of the health effects of specific chemicals. However, exposure to combinations of air pollutants is inevitable. Data dealing with the effects of co-exposure to air pollutants are very limited and, in most cases, it is not possible to recommend guidelines for such combinations. Notable exceptions are guidelines on particulate matter, assessed on the basis of mass concentration of particles of a broad range of physical and chemical properties. Of course, measures taken to control air pollution frequently lead to the reduction in concentrations of more than one pollutant. This is often achieved by controlling sources of pollutants rather than by focusing on individual pollutants. This is especially important in the indoor environment. In developing countries, the use of solid fuel, often including biomass, in poorly ventilated buildings leads to exposure to a mixture of air pollutants. Combinations of pollutants can lead to additive or synergistic effects: the combinations of exposure to tobacco smoke and radon and asbestos fibres provide examples of synergistic effects. These are well-known effects. Less well-known is, for example, the possibility that cataract formation may be linked to exposure to the mixture of pollutants generated by burning biomass indoors. Whether this effect is due to a single pollutant or to co-exposure to a group of pollutants is unknown.

A good example of difficulty in attributing health effects to one of the components of indoor air pollution mixture is provided by particulate matter. The measures to assess or control particle mass concentration are rarely effective in respect to very small particles, often referred to as ultrafine (< 0.1 μm), and most commonly measured as number concentration. Operation of combustion sources always results in the emission of ultrafine particles as well as many other pollutants, and particularly as discussed in this document, benzene, carbon monoxide, nitrogen dioxide and polycyclic aromatic hydrocarbons. Possibly synergistic effects of exposure to these pollutants and ultrafine particles are not known at this point. It is also not known whether some of the pollutants act as surrogates for ultrafine particles (or vice versa).

Some of these problems will be addressed by the guidelines on indoor combustion, to be developed following the recommendations of the WHO working group from 2006 (5). Nevertheless, it is important to emphasize the need to consider the health risks of all pollutants for which guidelines are available in activities to improve indoor air quality. This, to some extent, is addressing the health hazard of combined exposure. Furthermore, situations where following the guideline for one pollutant adversely affects other aspects of indoor air quality should be avoided.

When strategies to protect public health are under consideration, the air quality guidelines on selected substances need to be placed in the perspective of total chemical exposure. The interaction of humans and the biosphere is complex. Individuals can be exposed briefly or throughout their lifetimes to chemicals in air, water and food; exposures may be environmental or occupational. In addition, individuals vary widely in their response to exposure to chemicals; each person has a pre-existing status (defined by, for example, age, sex, pregnancy, pulmonary disease, cardiovascular disease, genetic make-up) and a lifestyle, in which such factors as exercise and nutrition play key roles. All these different elements may influence a person's susceptibility to chemicals.

Use of the indoor air quality guidelines in protecting public health

The primary aim of these guidelines is to provide a uniform basis for the protection of public health from adverse effects of indoor exposure to air pollution, and to eliminate or reduce to a minimum exposure to those pollutants that are known or are likely to be hazardous. The guidelines are targeted at the public health professionals involved in preventing health risks of environmental exposures, as well as the specialists and authorities involved in the design and use of buildings, indoor materials and products. The guidelines are based on the scientific knowledge available at the time of their development. They have the character of recommendations, and it is not intended or advocated that they be adopted as standards. Nevertheless, countries may wish to transform the recommended guidelines into legally enforceable standards.

In the process of moving from a “guideline” or a “guideline value” to a “standard”, a number of factors beyond the exposure–response relationship need to be taken into account. These factors include current concentrations of pollutants and exposure levels of a population, the specific mixture of air pollutants and the specific social, economic and cultural conditions encountered. In addition, the standard-setting procedure may be influenced by the likelihood of implementing the standard. Broader discussion of these considerations has been presented in an earlier edition of the WHO air quality guidelines (2).

Establishing legally binding concentration-based standards of indoor air quality requires, inter alia, determination of the methods for enforcement of the regulation, including compliance testing. This poses difficulties for the use of standards as a means of reducing the effects on health of indoor exposure to air pollutants. Routine monitoring in people's homes is unlikely to be widespread, and the application of measures to enforce standards is often not feasible or, at least, difficult.

The concentrations of pollutants in both indoor and outdoor air can be reduced by controlling the primary factor that determines their presence in the air: their sources. In the outdoor environment, this is all that can be done because the secondary factors that control concentrations – dispersion and dilution – are not generally under control as they depend largely on meteorological conditions. In the indoor environment, dispersion (to and from the outdoor environment) can be influenced by controlling the ventilation of the indoor space. This provides an additional means of changing indoor concentrations of pollutants. It also creates a difficulty: a source that might be entirely acceptable as regards output of pollutants in a well-ventilated space might be unacceptable in a poorly ventilated space. Of course, significant indoor sources should not be allowed to release pollutants into the indoor space: conducting pollutants to the outside space by flues and chimneys is clearly desirable. It should be noted, however, that indoor sources can make a substantial contribution to outdoor concentrations, especially in places where there are large, widespread sources of indoor pollution. This should be avoided by control of emission from indoor sources.

Thus, acceptable indoor air quality can be achieved through source control and pollutant dispersion, and in particular through:

  • application of low-emission materials and products;
  • proper selection of the devices and fuels used for combustion indoors;
  • the venting of products to the outdoor air; and
  • ventilation control.

In many countries, these means of control are encapsulated in product standards and building standards or regulations. In practice, both sets of standards are derived from calculations or experiments and are implemented without routine monitoring of pollutant levels indoors. Surveys of indoor concentrations of pollutants act as a means of checking that the standards are appropriate or, more often, that they are being appropriately applied. Here there is another difference between the approaches taken indoors and outdoors: monitoring outdoor concentrations of air pollutants is standard practice in many countries but routine monitoring of indoor concentrations hardly exists.

The development of product and building standards requires targets for acceptable indoor concentrations of air pollutants: here guidelines can play an important role. This is not very different from the approach adopted outdoors. Even in periods of high outdoor concentrations of air pollution, reduction of emissions is seldom more than an ad hoc solution, although reducing vehicle usage or industrial emissions at such times has been tried in a number of cities. Much more important than this are calculations based on inventories of sources and atmospheric dispersion modelling that form the basis of “product standards” as applied, for example, to motor vehicle engines and to the fuels they burn, i.e. to the permanent characteristics of the sources. Such calculations are easier when applied to outdoor air; the wide range of types of dwelling and their levels of ventilation makes calculations more difficult for the indoor environment. Nevertheless, adoption of the guidelines presented in this volume as benchmarks for such models and actions is a useful option for reducing the adverse effects of indoor air pollution on health.

References

1.
Air quality guidelines for Europe. Copenhagen: WHO Regional Office for Europe; 1987. (WHO Regional Publications, European Series, No. 23)
2.
Air quality guidelines for Europe. 2nd ed. Copenhagen: WHO Regional Office for Europe; 2000. [4 August 2010]. (WHO Regional Publications, European Series, No. 91; http://www​.euro.who.int/document/e71922​.pdf.
3.
Air quality guidelines. Global update 2005. Particulate matter, ozone, nitrogen dioxide and sulfur dioxide. Copenhagen: WHO Regional Office for Europe; 2006. [4 August 2010]. http://www​.euro.who.int​/__data/assets/pdf_file​/0005/78638/E90038.pdf.
4.
The right to healthy indoor air. Report on a WHO meeting, Bilthoven, Netherlands, 15–17 May 2000. Copenhagen: WHO Regional Office for Europe; 2000.
5.
Development of WHO guidelines for indoor air quality. Report on a working group meeting, Bonn, Germany, 23–24 October 2006. Copenhagen: WHO Regional Office for Europe; 2006. [4 August 2010]. http://www​.euro.who.int​/__data/assets/pdf_file​/0007/78613/AIQIAQ​_mtgrep_Bonn_Oct06.pdf.
6.
WHO guidelines for indoor air quality: dampness and mould. Copenhagen: WHO Regional Office for Europe; 2009. [4 August 2010]. http://www​.euro.who.int​/__data/assets/pdf_file​/0017/43325/E92645.pdf.
7.
Principles for the assessment of risks to human health from exposure to chemicals. Geneva: World Health Organization; 1999. (Environmental Health Criteria, No. 210)
8.
Evaluation and use of epidemiological evidence for environmental health risk assessment: WHO guideline document. Environmental Health Perspectives. 2000;108:997–1002. [PMC free article: PMC1240136] [PubMed: 11049823]
9.
Institute of Medicine. Damp indoor spaces and health. Washington, DC: National Academies Press; 2004.

Footnotes

1

Steering group members: Ross Anderson, Aaron Cohen, Severine Kirchner, Erik Lebret, Lars Mølhave, Aino Nevalainen, Bernd Seifert and Kirk Smith.

Copyright © 2010, 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).

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