Sulfur dioxide (also sulphur dioxide) is the chemical compound with the formula . At standard atmosphere, it is a toxic gas with a pungent, irritating, and rotten smell. The triple point is 197.69 K and 1.67kPa. It is released naturally by volcanic activity. Sulfur dioxide was used by the Romans in winemaking when they discovered that burning sulfur candles inside empty wine vessels kept them fresh and free from vinegar smell.