Despite progress in recent years, acid rain remains a significant environmental and economic concern for many regions of Canada. This section explains what acid rain is, where it comes from, and what its effects are.

Acid deposition - commonly called acid rain - is caused by emissions of sulphur dioxide and nitrogen oxides. Although natural sources of sulphur oxides and nitrogen oxides do exist, more than 90% of the sulphur and 95% of the nitrogen emissions occurring in eastern North America are of human origin. These primary air pollutants arise from the use of coal in the production of electricity, from base-metal smelting, and from fuel combustion in vehicles. Once released into the atmosphere, they can be converted chemically into such secondary pollutants as nitric acid and sulfuric acid, both of which dissolve easily in water. The resulting acidic water droplets can be carried long distances by prevailing winds, returning to Earth as acid rain, snow, or fog.

No. The acids can be transformed chemically into sulphur dioxide gas or into sulphur and nitrogen salts. In this form they are deposited 'dry', causing the same damage as when they land dissolved in rain or snow. In this form they can also do internal damage to plants as they are taken up from the soil.

About 40% of nitrogen oxides come from transportation (cars, trucks, buses, trains), about 25% from thermoelectric generating stations, and the balance from other industrial, commercial, and residential combustion processes.

Yes. Water solutions vary in their degree of acidity. If pure water is defined as neutral, baking soda solutions are basic (alkaline) and household ammonia is very basic (very alkaline). On the other side of this scale there are ascending degrees of acidity; milk is slightly acidic, tomato juice is slightly more acidic, vinegar is medium acidic, lemon juice is still more acidic, and battery acid is extremely acidic.

If there were no pollution at all, normal rainwater would fall on the acid side of this scale, not the alkaline side. Normal rainwater is less acidic than tomato juice, but more acidic than milk. What pollution does is cause the acidity of rain to increase. In some areas of Canada, rain can be as acidic as vinegar or lemon juice.

The problem is one of balance; nature depends upon balance. Normal precipitation reacts with alkaline chemicals - derived from the region's bedrock and found in the air, soils, lakes, and streams - and is thereby neutralized. However, if precipitation is more highly acidic, then acid-buffering chemicals can eventually become depleted. In this case, the buffering effect will no longer occur, and nature's ability to maintain balance will have been destroyed.

The pH scale: Acidity is measured in terms of pH, on a scale that runs from zero, the most acidic, to 14, the most alkaline. A change of one unit on the pH scale represents a 10-fold change in acidity. Organisms generally thrive near pH 7, the neutral point, and function less successfully toward either end of the scale.

No. Different types of bedrock contain variable amounts of contain variable amounts of alkaline chemicals. Regions with bedrock containing less alkali have a lower capacity for reducing acidity, and thus are more sensitive to acid deposition.

When the environment cannot neutralize acid rain, damage occurs to forests, crops, lakes, and fish. Toxic metals such as copper and lead can also be leached from water pipes into drinking water.

The interactions between living organisms and the chemistry of their aquatic habitats are extremely complex. If the number of one species or group of species changes in response to acidification, then the ecosystem of the entire water body is likely to be affected through the predator-prey relationships of the food web. At first, the effects of acid deposition may be almost imperceptible, but as acidity increases, more and more species of plants and animals decline or disappear.

• As the water pH approaches 6.0, crustaceans, insects, and some plankton species begin to disappear.
• As pH approaches 5.0, major changes in the makeup of the plankton community occur, less desirable species of mosses and plankton may begin to invade, and the progressive loss of some fish populations is likely, with the more highly valued species being generally the least tolerant of acidity.
• Below pH of 5.0, the water is largely devoid of fish, the bottom is covered with undecayed material, and the areas close to shore may be dominated by mosses.
• Terrestrial animals dependent on aquatic ecosystems are also affected. Waterfowl, for example, depend on aquatic organisms for nourishment and nutrients. As these food sources are reduced or eliminated, the quality of habitat declines and the reproductive success of the birds is affected.

Both natural vegetation and crops can be affected.

• It can alter the protective waxy surface of leaves, lowering disease resistance.
• It may inhibit plant germination and reproduction.
• It accelerates soil weathering and removal of nutrients.
• It makes some toxic elements, such as aluminum, more soluble. High aluminum concentrations in soil can prevent the uptake and use of nutrients by plants.

The effects on terrestrial wildlife are hard to assess. As a result of pollution-induced alteration of habitat or food resources, acid deposition may cause population decline through stress (because of decreases in available resources) and lower reproductive success.

• Lower productivity in fisheries, forestry, and agriculture translates to lower profits and fewer jobs for some of Canada's important industries.
• Acid deposition causes accelerated corrosion, fracturing, and discoloration of buildings, structures, and monuments.

• We eat food, drink water, and breathe air that has come in contact with acid deposition.
• Canadian and U.S. studies indicate that there is a link between this pollution and respirator problems in sensitive populations such as children and asthmatics.
• Acid deposition can increase the levels of toxic metals such as aluminum, copper, and mercury in untreated drinking water supplies.

It has been estimated that acid rain causes $1 billion worth of damage in Canada every year. Thousands of lakes have been damaged; a large part of the salmon habitat in the Maritimes has been lost; a significant proportion of eastern Canada's forests has been affected; and considerable damage to buildings and monuments has been documented.

The Canadian Council of Resource and Environment Ministers in established 20 kg/hectare per year as the target for Canadian sulphur dioxide loading. In eastern Canada, 96% of the land with high capability for forestry is subject to acidic deposition in excess of 20 kg/ha per year. In recent years, important instances of dieback and declines in growth rate have been noted in sugar maple groves in parts of Canada that receive high levels of these and other air pollutants, such as ozone. Significant growth declines in northern Ontario forests, most notable over the past 30 years, coincide with a period of rapidly increasing industrialization and urbanization across much of the province.

More than 80% of all Canadians live in areas with high acid rain-related pollution levels.

No. Sulphur emissions tend to be concentrated in relatively few locations, while the sources of nitrogen emissions are widely distributed; however, where they are deposited depends on more than just where they are produced. Airborne acidic pollutants are often transported by large scale weather systems thousands of kilometers from their point of origin before being deposited. In eastern North America, weather systems generally travel from southwest to northeast. Thus, pollutants emitted from sources in the industrial heartland of the midwestern states and central Canada regularly fall on the more rural and comparatively pristine areas of the northeastern U.S. and southeastern Canada.

The challenge is to reduce sulphur and nitrogen emissions. The two principal ways individuals can help are:

• by reducing the amount of energy used in the home (energy efficiency)
• by reducing the stress your driving habits put on the environment.

It has been estimated that about 50% of the sulphate deposited in Canada is derived from sources in the U.S.