Metal toxicity or metal poisoning is the toxic effect of certain metals that accumulate in the environment and damage ecosystems, plants and animals, including human health. Environmental pollution with heavy metals can result in contamination of drinking water, air, and waterways, accumulating in plants, crops, seafood, and meat. Such pollution may indirectly affect humans via the food chain and through occupational or domestic exposure by inhalation, ingestion, or contact with the skin.

At low concentrations, heavy metals such as copper, iron, manganese, and zinc are essential nutrients obtained through the diet supporting health, but have toxicity at high exposure concentrations. Other heavy metals having no biological roles in animals, but with potential for toxicity include arsenic, cadmium, lead, mercury, and thallium.

Some metals are toxic when they form poisonous soluble compounds which interfere with enzyme systems, such as superoxide dismutase, catalase, or glutathione peroxidase. Only soluble metal-containing compounds are toxic by forming coordination complexes, which consist of a metal ion surrounded by ligands. Ligands can range from water in metal aquo complexes to methyl groups, as in tetraethyl lead.

Toxic metal complexes can be detoxified by conversion to insoluble derivatives or by binding them in rigid molecular environments using chelating agents. An option for treatment of metal poisoning may be chelation therapy, which involves the administration of chelation agents to remove metals from the body.

Heavy metals are found throughout natural ecosystems, including rocks, soils, and water, and originate from diverse sources, such as natural weathering, erosion, mining, industrial and urban runoff, sewage, pesticides on crops, metal pipes carrying potable water, traffic pollution, coal-burning emissions, and various other industrial and urban outputs.

Toxic metal particles in ecosystems may remain for hundreds or even thousands of years, with potentially millions of people exposed to high concentrations at some point in their lives. Commonly, there is no visible evidence of metals pollution in soil or water.

When metal toxicity in the environment is suspected, pathologies in fish, clams, and insects may serve as signals for contamination and toxicities. Physiological mechanisms of metal toxicity may have a spectrum of effects, ranging from changes in behavior to death of small animal species.

A dominant kind of metal toxicity is arsenic poisoning, which mainly arises from ground water naturally containing high concentrations of arsenic in the supply of drinking water.