Car exhaust fumes and smoke from factories and power stations release chemicals, including sulphur dioxide and nitrogen oxides, into the air. The chemicals react with water vapour to form specks of sulphuric and nitric acid. These acids return to the ground in raindrops, snowflakes and other types of precipitation.
The acid kills fish and other animals in lakes and rivers. It damages the soil by destroying the bacteria in it and it harms plants. Acid rain is a serious problem in North America and in central and north-western Europe. For example in the mid-1980s, West German experts estimated that one-third of their trees were dying because of acid rain. The governments of many industrial nations are now trying to find ways of reducing this form of pollution.