Arsenic is a versatile metal, forming various compounds, either inorganic or organic. Inorganic arsenic is widely distributed in nature, usually in the trivalent form (As3+) but also as pentavalent arsenic (As5+). Most rocks contain 1-5ppm of arsenic. The trivalent forms include arsenic trioxide, sodium arsenite, and arsenic trichloride. Organic arsenic, much less toxic than inorganic arsenic, is produced in a biomethylation process by many organisms including humans and shellfish. Though arsenic occurs naturally in rocks and soil, the majority of arsenic released into the environment is from industrial smelting.