The adverse health effects of air pollution became widely acknowledged after severe pollution episodes occurred in Europe and North America before the 1960s. In these areas, pollutant levels have decreased. During the last 15 years, however, consistent results, mainly from epidemiological studies, have provided evidence that current air pollutant levels have been associated with adverse long- and short-term health effects, including an increase in mortality. These effects have been better studied for ambient particle concentrations but there is also substantial evidence concerning gaseous pollutants such as ozone, NO(2) and CO. Attempts to estimate the impact of air pollution effects on health in terms of the attributable number of events indicate that the ubiquitous nature of the exposure results in a considerable public health burden from relatively weak relative risks.