Mercury and its compounds are highly poisonous. They are released into the environment by such activities as the burning of fossil fuels and the use of certain pesticides, and by such natural processes as volcanic eruptions. Mercury especially affects the brain and kidneys, and may cause tremors, paralysis, blindness, or kidney failure. Most cases of mercury poisoning are caused by inhalation of mercury vapor. Other cases are caused by eating food contaminated with mercury and by absorbing mercury through the skin. Mercury poisoning is treated with chelating agents, substances that combine chemically with poisonous metals to form compounds that the body can readily excrete.

Some mercury occurs in nature uncombined with other elements. Most mercury is obtained by roasting (heating) cinnabar, or red mercuric sulfide. Spain has the largest reserves of cinnabar—about two-thirds of the world total—and is the leading exporter of mercury. Most mercury produced in the United States comes from ores mined in Nevada.

Mercury is used in thermometers and barometers. Mercury conducts electricity and is used in such electrical devices as switches and rectifiers. Mercury vapor is used in lamps to produce ultraviolet light and also blue or yellow-green visible light. Many metals will combine with mercury to form substances known as amalgams.

Mercury forms a large number of compounds. Mercury fulminate (Hg[CNO]2) is explosive and used to detonate high explosives. Mercuric chloride, or bichloride of mercury (HgCl2), is used in photography, dry batteries, and the electroplating of aluminum. It is also used as a pesticide and, in diluted form, as an antiseptic. Mercurous chloride, or calomel (Hg2Cl2), is sometimes used as a medicine. Mercuric oxide is used in paint and batteries. Vermilion (HgS) is used as a pigment.

Symbol: Hg (from hydrargyrum, the Latin word for liquid silver). Atomic number: 80. Atomic weight: 200.59. Specific gravity: 13.5. Melting point: -38.02 F. (-38.9 C.). Boiling point: 673.88 F. (356.6 C.). Mercury has seven stable isotopes: Hg-196, 198, 199, 200, 201, 202, and 204. Mercury belongs to Group IIB of the Periodic Table and has a valence of + 1 or + 2.