Heavy metals are natural constituents of the Earth's crust.
Human activities have drastically altered the biochemical and geochemical cycles and balance of some heavy metals. Heavy metals are stable and persistent environmental contaminants since they cannot be degraded or destroyed. Therefore, they tend to accumulate in the soils and sediments. Excessive levels of metals in the marine environment can affect marine biota and pose risk to human consumers of seafood. Metals and their compounds, both inorganic and organic, are released to the environment as a result of a variety of human activities.