Sulfur dioxide is considered the primary element of acidic precipitation. The sources of SO2 are natural resources such as volcanoes and manmade sources like power plants and industrial sources that burn fuel or coal. The burning of fossil fuels 2lbs. of SO2 is produced for each pound of sulfur present in the fuel. The lungs of humans and animals can be harmed as well as trees and plants. Sulfur dioxide is the main contributor to acid rain. Sulfur dioxide reacts with oxygen in the air to become sulfur trioxide, which then reacts with water in the air to form sulfuric acid. Acid rain can slowly kill both animal populations in rivers and lakes and other plants by damaging leaves and root systems. Sulfur dioxide deteriorates metal and stone on statues and buildings.