Metals can be toxic in high quantities. Either ingestion or faulty metabolic pathways can lead to metal toxicity (metal poisoning). Sources of toxic metals include cadmium from tobacco, arsenic from agriculture and mercury from volcanoes and forest fires. Nature, in the form of trees and plants, is able to trap many toxins and can bring abnormally high levels back into equilibrium. Toxic metal poisoning is usually treated with some type of chelating agent. Heavy metal poisoning, such as from mercury, cadmium, or lead, is particularly pernicious.
Examples of specific types of toxic metals include:
* Copper: copper toxicity usually presents itself as a side effect of low levels of the protein ceruloplasmin, which normally is involved in copper storage. This is referred to as Wilson’s disease. Wilson's disease is an autosomal recessive genetic disorder whose mutation causes the ATPase that transports copper into bile and ultimately incorporates it into ceruloplasmin to malfunction. 
* Plutonium: ever since the nuclear age, plutonium poisoning is a potential danger, especially among nuclear reactor employees; inhalation of Pu dust is particularly dangerous due to its intense alpha particle emission. There have been very few cases of plutonium poisoning. 
* Mercury: mercury is usually ingested from agricultural sources or other environmental sources. Mercury poisoning can lead to neurological disease and kidney failure if left untreated.
* Iron: iron toxicity, iron poisoning, or iron overload is well known. Iron does test only very weakly positive for the Ames test for cancer, however, since it is such a strong catalyst and essential for the production of ATP and consequently DNA production, any excess soluble iron is toxic especially over time. Too much iron deposited in tissues or high levels in the blood stream has been successfully linked to a vast majority of human diseases from Alzheimer's to Malaria. In Botany, iron is a severe problem for the irrigation of plants like rice, maize, or wheat in Sub-Saharan Africa whose subterranean water contains excessive amounts of iron which then poisons these crops.
* Lead and cadmium: lead poisoning and cadmium poisoning can lead to gastrointestinal, kidney, and neurological dysfunction. The use of unleaded paints and gas has successfully decreased the number of cases of lead heavy metal poisoning.
* Nickel, chromium, and cadmium: via metal-DNA interactions, these metals can be carcinogenic.
* Nickel: allergies to nickel, particularly from skin to metal contact via jewelry, are common. 
* Zinc, cadmium, magnesium, chromium: metal fume fever can be caused by ingestion of the fumes of these metals and leads to flu-like symptoms.
* Beryllium: The risk of beryllium poisoning is relevant to occupational safety and health for metalworkers or ore millers who work with alloys or ores that contain beryllium in greater than trace amounts. For alloys, this means those alloys in which beryllium is intentionally featured, and for ores, it means those pursued for beryllium or those with nonnegligible beryllium co-occurrence.