"Acid rain" is the popular term for all forms of precipitation turned acidic by atmospheric pollutants. Pollutants such as sulphur dioxide (SO2) and nitrogen oxides (NOx) are converted in the atmosphere into sulphuric and nitric acid. In Canada, the largest sources of SO2 and NOx are the smelting of sulphur-bearing metal ores and the burning of fossil fuels for energy. Once released into the atmosphere, acidic pollutants may be transported great distances. For example, more than 50% of the acid rain in eastern Canada comes from U.S. sources.

The geology of eastern Canada, in particular the Canadian Shield, makes it highly sensitive to acid rain. High levels of acid rain can result in the acidification of lakes, rivers, and streams and cause metals to leach from surrounding soils into the water system. Both of these effects can seriously impair the ability of water bodies to support aquatic life.

There is a concern about the impact of decades of acid rain on soils, particularly forest soils. Changes in soil chemistry caused by acid rain may be responsible for a decline in soil nutrients in certain forest ecosystems, resulting in slower growth rates and increased death rates in trees.

Canada has made progress in combating acid rain. Sulphur dioxide emissions in eastern Canada fell 56% from 1980 to 1994. These reductions are the result of proactive efforts by governments and industry to change industrial processes, install emission-reducing technology, and switch to less harmful fuel sources. Canada's goal of reducing total SO2 emissions to 3.2 tonnes annually by the year 2000 was achieved by 1993 and surpassed by 1994 (Figure 12).

However, acid rain remains a problem, particularly in eastern Canada (Figure 13). Despite SO2 emission reductions, recovery of acidified lakes has been slow and incomplete (Figure 14). Significant progress has occurred near Sudbury, where SO2 emission reductions have been greatest. However, 11% of the lakes in eastern Canada continue to acidify. Scientists predict that, even after full implementation of Canadian and U.S. SO2 emission controls, large areas of southern and central Ontario, southern Quebec, New Brunswick, and Nova Scotia will continue to receive more acid rain than aquatic ecosystems can tolerate. Implications for the health and productivity of both soft and hardwood forests in Ontario, Quebec, and New Brunswick are potentially serious. Recent research indicates that further SO2 emission reductions of 75% in targeted regions of eastern Canada are necessary to protect ecosystems.

Emissions of NOx, the other main pollutant causing acid rain, have not changed much since 1980, and this may undermine some of the benefits resulting from the control of SO2 emissions.