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Arsenic is a metal-like element that occurs naturally, usually in connection with other elements (like the arsenic trioxide pictured). Even the ancients knew that arsenic is poisonous, and that property eventually led to widespread use in pesticides and wood preservatives. Some of those uses continue. Aside from the acutely poisonous effects of large arsenic doses, long-term exposure to lower doses can cause skin cancer and may increase the incidence of certain other cancers.