Research on the effects of acid rain has shown that it is damaging to aquatic life in lakes and streams, diminishes the productivity of forests and crops, accelerates the deterioration of buildings and other exposed structures, and contributes to human health hazards. It is the result of acid precipitation in the atmosphere through chemical transformation of sulphur dioxide (SO₂) and several nitrogen oxides (NO_X). In the air they become oxidized to form acid sulphate (SO₄) and acid nitrate (NO₃), respectively. These acids return to the earth in rain, snow, and hail, and as dry microscopic particles, sometimes after travelling hundreds or even thousands of miles.

Man-made sources of these air pollutants include electric generating plants, smelting of ores, industrial installations, motor vehicles, and residential and commercial establishments using fossil fuels. Globally the natural and man-made emissions are roughly equal. In North America and Europe, however, man-made sources contribute more than 90 per cent of the total. Man-made emissions of sulphur dioxide rose from about 7 million tons per year in 1860 to about 155 million tons per year in 1985, although the emissions during the period 1970- 1985 were reduced substantially in several countries as strict pollution standards were imposed.

In industrial economies, generation of electricity in power plants using fossil fuels is the largest single contributor to the emissions of sulphur dioxide (SO₂), and the transportation sector is the largest source of nitrogen oxides. Although emissions of nitrogen oxides were slightly reduced or stabilized during the period 1970-1985 in some of the developed market economies, the total amount of nitrogen released annually is still rising. Motor vehicles are the major source and are more difficult to identify and regulate than the major sources of SO₂emissions.

Acid deposition far downwind from big pollution sources greatly exceeds emissions from local sources in many regions. In Austria, eastern Canada, the Netherlands, Sweden, and Switzerland more than 60 per cent and in Norway about 75 per cent of the sulphur deposition comes from other areas. Acid rain falling over the Atlantic as far out as Bermuda and the acidity of snow in the Arctic are other examples of long-range transport of acids. Because relatively little of the harm from acid rain is suffered by the communities where it is produced, many countries have been unwilling to bear the costs of preventing it. Nevertheless, the ECE protocol and recent consultations between Canada and the United States indicate significant progress in international co-operation on preventive measures.

A variety of approaches and technologies aimed at the reduction of acid rain are available:

(a)   Selection of fuels low in sulphur and nitrogen content;

(b)   Combustion modification using limestone injection multi-stage burners, fluidized bed combustion, and post-combustion emission controls using scrubbers, which allow removal of 50 to 90 per cent of the SO₂ in coal-fired electric power plants;

(c)   Use of the catalytic converter to reduce emissions of nitrogen oxides in new automobiles that burn unleaded gasoline;

(d)   Increased fuel efficiency;

(e)    Vehicle sharing and limitation of private automobile use.