yAerosols play an important role in all problems connected with air pollution, ranging from very local effects and human health problems to regional problems such as acid deposition and eutrophication up to continental and global questions such as stratospheric ozone loss and climatic change. In this report, an explanation of these effects is given and an assessment is made for parts of China, based on the aerosol data given by Zhang et al. elsewhere in this volume. Epidemiological research has made clear that aerosols are a cause of enhanced mortality due to cardiopulmonary diseases (heart and lung diseases). Based on the same mortality as found in Europe and on linear extrapolation (with large uncertainties; no sufficient data are available to make better estimates), an excess mortality of 5000 to 10 000 due to acute effects and 20 000 to 50 000 due to chronic effects per year could be expected for a city like Beijing with a population of 14.5 million. A major cause of these uncertainties is problems in the determination of semivolatile compounds and elemental or black carbon in aerosols. Aerosols have a strong impact on the radiative balance of the earth, in a direct way by reflecting solar light as well as in an indirect way by cloud formation leading to clouds with higher albedo, which reflect sunlight better. The total direct effect, backscatter of sunlight, including backscatter from nitrates and organic compounds, is estimated to be approximately 2 to 3 W m(-2) for Western Europe, 2 while the indirect effect is approximately 0 to -6 W m(-2). Soot absorbs incoming solar radiation and heats the atmosphere. This process contributes 0.1 to 0.2 W m(-2) on a global scale. If PM-2.5 levels are compared with Europe, a direct effect of approximately -4 to -10 W m(-2) would be plausible for China, black carbon could contribute 2 probably about 0.5 to 2 W m(-2), and the indirect effect could be about 0 to -6 W m(-2). These effects could cause a net cooling over China (and over many developing countries in the same position) of about -4 to -15 W m(-2). This estimate is obviously based on many assumptions and hence is quite uncertain. One must be aware that measures reducing local aerosol concentrations will have a large impact on the radiative balance and could, over a few decades, have potentially at least the same impact as the build-up of greenhouse gases since the beginning of the Industrial Revolution.