Examples of heavy metals include mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), thallium (Tl), and lead (Pb). Heavy metals are natural components of the Earth’s crust. They cannot be degraded or destroyed. To a small extent, they enter our bodies via food, drinking water and air.
As trace elements, some heavy metals (e.g. copper, selenium, zinc) are essential to maintaining the metabolism of the human body. However, at higher concentrations, they can lead to poisoning. Heavy metal poisoning could result, for instance, from drinking-water contamination(e.g. lead pipes, older brass taps which have higher levels of lead in the brass), high ambient air concentrations near emission sources, or intake via the food chain.
Heavy metals are dangerous because they tend to bioaccumulate. Bioaccumulation means an increase in the concentration of a chemical in a biological organism over time, compared to the chemical’s concentration in the environment. Compounds accumulate in living things anytime they are taken up and stored faster than they are broken down (metabolized) or excreted.
Heavy metals can enter a water supply by industrial and consumer waste, or even from acidic rain breaking down soils and releasing heavy metals into streams, lakes, rivers, and groundwater.