Mercury is a rare, heavy, silver, metal which is found in liquid form at room temperature. It has many uses, including thermometers, cosmetics, batteries and dental amalgam. It is harmless in insoluble form, such as mercuric sulphide, but poisonous in soluble forms such as mercuric chloride. It can be absorbed through the skin and mucous membranes, as well as through inhalation. It affects the nervous system. Low level exposure can cause fatigue, memory loss, depression and tremors.

Mercury can build up in the body over a long period. Low levels in sea-water, for example, can build up significantly in fish.