Though some heavy metals serve as essential trace nutrients, these can have toxic effects at higher concentrations, while most metalloids and heavy metals have no known biological function. Arsenic causes pigmentation and keratosis of skin, cancer, and other afflictions. Cadmium and hexavalent chromium are known carcinogens. Cadmium was responsible for the itai-itai disease that caused osteomalacia and renal dysfunction among Japanese people living downstream of a zinc mine. Though copper is an essential trace element, it can cause cellular damage resulting in Wilson’s disease. Lead is used in lead-acid batteries and other items, and tetraethyl lead was used till recently as a gasoline additive in most countries. The most pronounced effects of lead are on the nervous system. Dental amalgam is the major source of elemental mercury, while the most infamous instance of poisoning by methylmercury – which is produced by microbial methylation of inorganic mercury – is from Minamata in Japan. Nickel in drinking water causes skin afflictions, while zinc, an essential nutrient for all organisms, can have several toxic effects. Heavy metals released from black shale and acid sulfate soils, and mining-affected and industrial areas can also pose risks to human health.