Air pollution is an old problem. During the mid-twentieth century, concern about the effects of air pollutants on health rose to a peak and, in some countries, measures were introduced to control emissions. In the last decade, interest and concern have surged again and a great deal of new research has been undertaken. This has led to a literature explosion which resulted in problems such as rapid updating of standard accounts and difficulties for students trying to identify the leading edge of the field. The results of recent research have also posed problems for those involved in developing policies for controlling the levels of air pollutants. The application of time-series methodology to the analysis of the day-to-day effects of air pollutants and the maturing of the Harvard Six Cities cohort study have led to the assumptions about thresholds of effects and safe levels of exposure being challenged. Demands for tighter standards and continued reductions in levels of air pollution have intensified.