The goal of this report is to provide a broad framework from the latest available data, mostly since 1990, in an attempt to estimate the specific agents within the air pollution mixture of acid rain that are related to adverse effects on human health. Direct and indirect health effects of sulphur dioxide and its derivatives, sulphate, particulate matter, and acid aerosols are reviewed separately. Information is included on the distribution of sulphur oxides across Canada and on epidemiological, clinical, and toxicological studies related to the direct health effects. In addition, indirect health effects such as changes in visibility and climate, and leaching of metals into water supplies, are also reviewed.