Setting indoor air quality guidelines

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. 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

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

Fig. 1. The process followed in guidelines formulation

Systematic search of the literature: – epidemiological studies – controlled exposure studies – occupational studies Methods outlined in pollutant-specific chapters Identification of previous reviews by WHO, IARC and national agencies

Analysis of the reasoning of the reviews and relevance for indoor air exposures

Identification of the most relevant studies and data on effects of low-level exposures considering: – strength of evidence – vulnerable populations – relevance for indoor air exposure

Kinetics and metabolism Current outdoor and indoor air levels

Consensus-based formulation of the guidelines as acceptable levels of population exposure (concentration, averaging time) by expert working group considering strength of evidence and: – critical health outcomes – no observed adverse effects level – mechanism of effect

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 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 categories are used for classifying the evidence in this review, in the WHO guidelines for indoor air quality: dampness and mould (6) and in that of the Institute of Medicine (9).

Sufficient evidence of a causal relationship

The evidence is sufficient to conclude that a causal relationship exists between the agent and the outcome. That is, the evidence fulfils the criteria for “sufficient evidence of an association” and, in addition, satisfies the following evaluation criteria: strength of association, biological gradient, consistency of association, biological plausibility and coherence and temporally correct association.

The finding of sufficient evidence of a causal relationship between an exposure and a health outcome does not mean that the exposure inevitably leads to that outcome. Rather, it means that the exposure can cause the outcome, at least in some people under some circumstances.

Sufficient evidence of an association

The evidence is sufficient to conclude that there is an association. That is, an association between the agent and the outcome has been observed in studies in which chance, bias and confounding could be ruled out with reasonable confidence. For example, if several small studies that are free from bias and confounding show an association that is consistent in magnitude and direction, there may be sufficient evidence of an association.

Limited or suggestive evidence of an association

The evidence is suggestive of an association between the agent and the outcome but is limited because chance, bias and confounding could not be ruled out with confidence. For example, at least one high-quality study shows a positive association, but the results of other studies are inconsistent.

Inadequate or insufficient evidence to determine whether an association exists

The available studies are of insufficient quality, consistency or statistical power to permit a conclusion regarding the presence or absence of an association. Alternatively, no studies of the association exist.

Limited or suggestive evidence of no association

Several adequate studies are consistent in not showing an association between the agent and the outcome. A conclusion of “no association” is inevitably limited to the conditions, magnitude of exposure and length of observation covered by the studies available.

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