Acid rain results when sulphur dioxide (SO2) and nitrogen oxides (NOx) are emitted into the atmosphere and transported by wind and air currents. The SO2 and NOx react with water, oxygen and other chemicals to form sulphuric and nitric acids. These then mix with water and other materials before falling to the ground. While a small portion of the SO2 and NOx that cause acid rain is from natural sources such as volcanoes, most of it comes from the burning of fossil fuels.