Preparation of the guidelines

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 steer-

ing 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