Invited commentary: heterogeneity of particulate matter health risks.

Ambient particulate matter varies in composition and toxicity; therefore, some heterogeneity in risk per unit mass can be anticipated. In this issue of the Journal (Am J Epidemiol 2007;166:880-888), Dominici et al. explore temporal and spatial effect modification of particulate matter potency with the goal of gaining insights about how differences in the mixture may affect toxicity of the particulate matter, and they propose that it may be a useful tool in assessing the net impact of regulatory activity. Their approach is fresh and creative and yields provocative results. Such assessments will, of course, be constrained by limited power to detect the expected small degree of effect modification, as well as an abundance of possible explanations for observed differences in risk per unit mass. The utility for accountability assessment is further limited because regulatory actions impacting particulate matter levels do not necessarily target the most toxic species within the mixture and may confer benefits by reducing population exposure to particles and components of lesser toxicity. Used cautiously, however, this new methodology may provide a complementary approach to more direct assessments in shedding light on the characteristics of ambient particulate matter predictive of health effects.