A toxic heavy metal is a common but misleading term for a metal noted for its potential toxicity.[4] Not all heavy metals are toxic and some toxic metals are not heavy.[5] Elements often discussed as toxic include cadmium, mercury and lead,[6] all of which appear in the World Health Organization's list of 10 chemicals of major public concern.[7] Other examples include chromium and nickel,[8] thallium, bismuth, arsenic, antimony and tin.[4]
These toxic elements are found naturally in the earth. They become concentrated as a result of human caused activities and can enter plant and animal (including human) tissues via inhalation, diet, and manual handling. Then, they can bind to and interfere with the functioning of vital cellular components. The toxic effects of arsenic, mercury, and lead were known to the ancients, but methodical studies of the toxicity of some heavy metals appear to date from only 1868. In humans, heavy metal poisoning is generally treated by the administration of chelating agents. Some elements otherwise regarded as toxic heavy metals are essential, in small quantities, for human health.