Sulphur dioxide (SO2) is a colourless, extremely toxic gas with a strong odor, invisible and has a nasty, sharp smell. It reacts easily with other substances to form harmful compounds, such as Sulphuric acid, Sulphurous acid and sulfate particles. Common applications of Sulphur dioxide include Sulphuric acid production, pulp and paper mills, chemical processing, food and beverage, coking operations and petroleum refineries.

About 99% of the Sulphur dioxide in air comes from human sources. The main source of Sulphur dioxide in the air is industrial activity that processes materials that contain Sulphur, eg the generation of electricity from coal, oil or gas that contains Sulphur. Some mineral ores also contain Sulphur, and Sulphur dioxide is released when they are processed. In addition, industrial activities that burn fossil fuels containing Sulphur can be important sources of Sulphur dioxide.