Environmental epidemiological studies of the health effects of air pollution have been major contributors to the understanding of such effects. The chronic effects of atmospheric pollutants have been studied, but, except for the known respiratory effects of particulate matter (PM), they have not been studied conclusively. There are ongoing studies of the chronic effects of certain pollutant classes, such as ozone, acid rain, airborne toxics, and the chemical form of PM (including diesel exhaust). Acute effects on humans due to outdoor and indoor exposures to several gases/fumes and PM have been demonstrated in epidemiological studies. However, the effects of these environmental factors on susceptible individuals are not known conclusively. These acute effects are especially important because they increase the human burden of minor illnesses, increase disability, and are thought to decrease productivity. They may be related to the increased likelihood of chronic disease as well. Further research is needed in this latter area, to determine the contributions of the time-related activities of individuals in different microenvironments (outdoors, in homes, in transit). Key elements of further studies are the assessment of total exposure to the different pollutants (occurring from indoor and outdoor source) and the interactive effects of pollutants. Major research areas include determination of the contributions of indoor sources and of vehicle emissions to total exposure, how to measure such exposures, and how to measure human susceptibility and responses (including those at the cellular and molecular level). Biomarkers of exposures, doses and responses, including immunochemicals, biochemicals and deoxyribonucleic acid (DNA) adducts, are beginning to promote some basic knowledge of exposure-response, especially the mechanisms. These will be extremely useful additions to standard physiological, immunological, and clinical instruments, and the understanding of biological plausibility. The outcomes of all this work will be the management of risks and the prevention of respiratory diseases related to air pollution.