Acid rain is precipitation (rain, snow, or fog) that has become unusually acidic due to air pollution, particularly sulphur dioxide and nitrogen oxides released by burning fossil fuels, industrial processes, and vehicle emissions. When these pollutants mix with water in the atmosphere, they form acids that fall to earth, damaging forests, lakes, buildings, and wildlife. Acid rain was a major environmental issue in Canada during the 1980s, particularly in Ontario and Quebec, where it was damaging lakes and forests. Canada and the United States signed the Air Quality Agreement in 1991 to reduce emissions, and significant progress has been made, though the problem hasn't been completely eliminated.