Small invisible particles in the urban air, especially those produced by human activities, have recently stimulated intense scrutiny, debate, regulation, and legal proceedings. The stakes are high, both with respect to health impacts and economic costs, and the methods used previously to resolve similar issues are no longer adequate. Everyone on earth inhales thousands to millions of particles in each breath, so if urban particulate air pollution particulate matter (PM) is significantly hazardous, the negative impact on health could be staggering. Yet the activities that generate PM, such as...