heavy metal

Arsenic, beryllium, cadmium, chromium, lead, manganese, mercury, nickel, and selenium are some of the metals (called 'heavy' because of their high relative atomic mass) which persist in nature and can cause damage or death in animals, humans, and plants even at very low concentrations (1 or 2 micrograms in some cases). Used in industrial processes, they are carried by air and water when discharged in the environment. Since heavy metals have a propensity to accumulate in selective body organs (such as brain and liver) their prescribed average safety levels in food or water are often misleadingly high.