Heavy metal poisoning is the toxic accumulation of heavy metals in the soft tissues of the body. Heavy metals are chemical elements that have a specific gravity (a measure of density) at least five times that of water. The heavy metals most often implicated in accidental human poisoning are lead, mercury, arsenic, and cadmium. More recently, thallium has gained some attention in the media as the poison used in several murder cases in the 1990s. Some heavy metals, such as zinc, copper, chromium, iron, and manganese, are required by the body in small amounts, but these same elements can be toxic in larger quantities.

The chelation process can only halt further effects of the poisoning; it cannot reverse neurological damage already sustained.

Depending on the metal, there may be blue-black lines in the gum tissues. In severe cases, patients exhibit obvious impairment of cognitive, motor, and language skills. The expression “mad as a hatter” comes from the mercury poisoning prevalent in 17th-century France among hatmakers who soaked animal hides in a solution of mercuric nitrate to soften the hair.