Sulphur dioxide / SO₂

Sulphur dioxide, once also known as sulphurous anhydride, is a colourless gas with a strong, pungent and suffocating odour. Detectable from 0.45 ppm, its presence can be identified before reaching the occupational exposure limit (2 ppm) and the short-term exposure limit (5 ppm).
However, odour alone cannot be considered an appropriate or trustworthy warning sign of dangerous exposure levels, a measuring instrument must be used to identify the product and measure the concentration.
Furthermore, sense of smell varies form person to person. It is released into Earth's atmosphere by volcanoes and by many industrial processes, as well as when certain coals, oils and natural gases that are not sulphur free are burned. The oxidisation of sulphur dioxide, most often in the presence of catalysts such as nitrogen dioxide (NO₂), produces sulphur trioxide (SO₃) and sulphuric acid (H₂SO₄), which is how acid rain forms. It can cause inflammation of the respiratory tract.
It is a colourless gas or a liquefied compressed gas with a pungent odour.