The authors have performed a structured expert judgement study of the population mortality effects of fine particulate matter (PM2.5) air pollution. The, opinions of six European air pollution experts were elicited. The ability of each expert to probabilistically characterize uncertainty was evaluated using 12 calibration questions-relevant variables whose true values were unknown at the time of elicitation, but available at the time of analysis. The elicited opinions exhibited both uncertainty and disagreement. It emerged that there were significant differences in expert performance. Two combinations of the experts' judgements were computed and evaluated-one in which each expert's views received equal weight; the other in which the expert's judgements were weighted by their performance on the calibration variables. When the performance of these combinations was evaluated the equal-weight combination exhibited acceptable performance, but was nonetheless inferior to the performance-based combination. In general, the experts agreed with published studies for the best estimate of all-cause mortality from PM2.5; however, as would be expected, they gave confidence intervals that were several times broader than the statistical confidence intervals taken directly from the most frequently cited published studies. The experts were rather comfortable with applying epidemiological results from one geographic region to another. However, there was more uncertainty and disagreement about issues of timing of the effect and about the relative toxicity of different constituents of PM2.5. Even so, the experts were in fairly good agreement that an appreciable fraction of the long-term health effects occurs within a few months after the exposure and that combustion-derived particles are more toxic than PM2.5 on average, while secondary sulphates, nitrates and/or crustal materials may be less toxic. These assessments bring very valuable and relevant information to air pollution risk assessment. (c) 2007 Elsevier Ltd. All rights reserved.