Mercury is a metal that occurs naturally in the environment. Mercury can exist as a liquid, as in many thermometers. When heated, it becomes a gas. It also can be combined with many other materials.
Everyone is exposed to mercury through air, drinking water, soil and food. Concerns have been raised, for instance, about the amount of mercury building up in fish as a result of pollution. Mercury enters the air from industries that burn mercury-containing fuels. Mercury from all sources can build up in body organs.
As with most substances, the degree of harm caused by mercury in the body is related to the amount. Very low levels don't cause any ill effects. At higher levels — for instance, when workers are exposed to mercury through their jobs — mercury can cause several symptoms. These include anxiety, irritability, memory loss, headaches and fatigue.