Concern about polluted air in our urban and industrial areas began gathering momentum shortly after World War II. At that time it seemed obvious that clean air, like clean water, clean food, and a clean body, was a worth while goal in itself, requiring no further justification. But it soon became evident that this goal is expensive to attain, and that rigid adherence to stringent standards of cleanliness diverts limited human resources away from other pressing and critical problems. Awareness of such facts has reoriented the goal to one of protecting public health. This emphasis is clearly stated in the US Clean Air Act of 1963: 'The Congress...finds that the growth in the amount and complexity of air pollution brought about by urbanization, industrial development, and the increasing use of motor vehicles, has resulted in mounting dangers to the public health and welfare'. Two decades later we ask: What do we know of these dangers and what must we do to improve our knowledge.