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Sulfur dioxide
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<p>Sulfur dioxide (<a href="/facts/IUPAC/2AV6myCK">IUPAC</a>-recommended spelling) or sulphur dioxide (traditional <a href="/facts/Commonwealth_English/FfHnv1CT">Commonwealth English</a>) is the <a href="/facts/Chemical_compound/37FXqqtv">chemical compound</a> with the formula <a href="/facts/Sulfur/IK0amTfM">S</a><a href="/facts/Oxygen/acyn6o8r">O</a>2. It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is released naturally by <a href="/facts/Volcanic_activity/c2M5upml">volcanic activity</a> and is produced as a by-product of metals refining and the burning of <a href="/facts/Sour_gas/vFVnV45T">sulfur</a>-<a href="/facts/Sour_crude_oil/0kMQ9LxA">bearing</a> fossil fuels. </p><p>Sulfur dioxide is somewhat toxic to humans, although only when inhaled in relatively large quantities for a period of several minutes or more. It was known to medieval <a href="/facts/Alchemy/CEx0U1Em">alchemists</a> as "volatile spirit of sulfur". </p>