Metals with a high density that are toxic or poisonous at low concentrations.
Metals, such as mercury, arsenic, lead, nickel, and zinc, that may enter the human food chain from industrial pollution slowly accumulate in the body and adversely affect the nervous system.
Metals such as mercury, lead, chromium, cadmium, and arsenic that have known toxic effects on internal organs, such as the kidneys, brain, bone, or retina.
An element among the group of 23 chemical elements characterized by a specific gravity (a measure of density) at least five times greater than that of water.