Sulphur dioxide (SO2) is a colourless gas, belonging to the family of gases called sulphur oxides (SOx). It reacts on the surface of a variety of airborne solid particles, is soluble in water and can be oxidised within airborne water droplets.
Natural sources of sulphur dioxide include releases from volcanoes, oceans, biological decay and forest fires. The most important man-made sources of sulphur dioxide are fossil fuel combustion, smelting, manufacture of sulphuric acid, conversion of wood pulp to paper, incineration of refuse and production of elemental sulphur. Coal burning is the single largest man-made source of sulphur dioxide accounting for about 50% of annual global emissions, with oil burning accounting for a further 25 to 30%.
The major health concerns associated with exposure to high concentrations of sulphur dioxide include effects on breathing, respiratory illness, alterations in pulmonary defenses, and aggravation of existing cardiovascular disease. In the atmosphere, sulphur dioxide mixes with water vapour producing sulphuric acid. This acidic pollution can be transported by wind over many hundreds of miles, and deposited as acid rain.