We identified associations between ambient air pollution and adverse human health effects. In our analyses, we considered individual subjects and their responses to ambient pollution, as well as citywide and nationwide population health characteristics and their aggregate associations with air pollution. Our studies have included both healthy and asthmatic children at summer camps in the northeastern United States, as these children are often outdoors and active during summer air-pollution episodes. Our studies of aggregate populations considered both human mortality, i.e., numbers of deaths per day by cause, and morbidity, e.g., numbers of hospital admissions per day for respiratory causes. We found that air pollution produces consistent adverse health consequences across the various populations and locations we researched. For example, on a high-ozone, air-pollution day, New York City hospital admissions for respiratory causes rise approximately 20% above otherwise expected figures.