Selenium exists in four natural oxidation states (+6, +4, 0, and –2) and is found in several compounds capable of causing human poisoning, yet it is an essential trace element in the human diet. Table II–59 describes physical properties and toxic air concentrations or doses of common selenium compounds. Fatal acute selenium poisoning occurs most commonly from ingestion of selenious acid in gun bluing (coating) solutions. Other acute poisonings occur through the use of (often improperly formulated) dietary supplements as well as via exposure to industrial compounds. Illness caused by chronic exposure to selenium is uncommon but is seen in regions with high selenium content in food. Industries using selenium compounds include ceramics, electronics, glass, rubber, and metallurgy. Selenium dioxide is the most commonly used compound industrially. Selenium is produced largely as a by-product of copper refining.