Vigorous outdoor exercise in smoggy conditions can lead to painful or difficult breathing. Children and individuals with respiratory problems are most vulnerable. Acid air, acid rain Acid rain occurs when common air pollutants nitrogen oxides and sulphur dioxide combine with water in the atmosphere to form droplets of very weak acid. Nitrogen oxides, which also contribute to smog, are released by our motor vehicles, power plants and factories. Most sulphur dioxide is produced by coal- burning smelters, power plants and factories.
Sulphur dioxide emissions form toxic compounds such as acid aerosols.
Some of this acid air pollution falls to the ground along with dust, rain, fog and snow where it damages our lakes, forests, crops and buildings. Some remains suspended in the air. Air currents can transport this mixture hundreds and thousands of kilometres from the sources of pollution. Small enough to enter deeply into the lungs, acid air pollution may contribute to respiratory illness and reduced lung function. Governments and industry are working together to reduce emissions of sulphur dioxide and other air pollutants.