Combined exposures

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.

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 coexposure 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 combus-

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

Working group meeting on development of WHO guidelines on indoor air quality, 2006 Selection of pollutants to be included

Steering Group created

Identification of structure for WHO guidelines Recommendation of authors and reviewers

Recruitment of authors/reviewers, clearance of Declaration of Interests

Bibliographic search and analysis of evidence First draft of evidence description

Peer review of first drafts

Second drafts of evidence description Health risk evaluation: first draft

Working group meeting, Bonn, 2–6 November 2009 1. Finalization of health risk evaluation 2. Comments on second draft of evidence description 3. Formulation of guidelines by small groups 4. Review of guidelines by plenary (steps 3 and 4 repeated 3 times) 5. Agreement on guidelines text by plenary consensus

Third drafts of full chapters

Peer review of full chapters by working group members

Fourth drafts of full text (including introduction) available for comments by working group

Language editing, layout WHO review and clearance Publication

tion 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 work-