There are several definitions for "heavy metals". For Tracking, a heavy metal is defined as metallic chemical elements that has a relatively high density and is toxic at low concentrations. Heavy metals are natural components of the earths crust. Exposure to heavy metals may in through diet, the environment, medications, or specific occupations. As trace elements, some heavy metals (e.g. iron, copper, zinc) are essential to the human body. However, even these can cause harm at higher levels.