Sulphur dioxide (SO₂) is a colourless, reactive gas. At higher concentrations it can have a strong odour. It is produced during the combustion of sulphur-containing fuels and industrial operations involving sulphur-containing materials. Major sources of SO₂ in BC include the upstream oil and gas industry, metal smelting facilities, pulp / paper mills and marine operations (in the Lower Mainland area). Once released, one possibility is that SO₂ can react with other compounds in the air to form fine particulate matter (small solid or liquid particles suspended in air).