Metal toxicity

Metal toxicity or metal poisoning is the toxic effect of certain metals in certain forms and doses on life. Some metals are toxic when they form poisonous soluble compounds. Certain metals have no biological role, i.e. are not essential minerals, or are toxic when in a certain form.^[1] In the case of lead, any measurable amount may have negative health effects.^[2] There is a popular misconception that only heavy metals can be toxic, but lighter metals such as beryllium and lithium can be toxic too.^[3] Not all heavy metals are particularly toxic, and some are essential, such as iron. The definition may also include trace elements when abnormally high doses may be toxic. An option for treatment of metal poisoning may be chelation therapy, a technique involving the administration of chelation agents to remove metals from the body.

Toxic metals sometimes imitate the action of an essential element, interfering with the metabolic processes resulting in illness. Many metals, particularly heavy metals are toxic, but some are essential, and some, such as bismuth, have a low toxicity. Metals in an oxidation state abnormal to the body may also become toxic: chromium(III) is an essential trace element, but chromium(VI) is a carcinogen.

Only soluble metal-containing compounds are toxic. Soluble metals are called 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. Usually metal complexes consist of a mixture of ligands.

Toxic metal complexes can be detoxified by conversion to insoluble derivatives or (ii) by encasing in rigid molecular environments using chelating agents. Alternatively, when very dilute, metal complexes are often innocuous.^[4] This method uses plants to extract and lower the concentration of toxic heavy metals in the soil.^[4] An aspirational method of decontamination of heavy metals is phytoremediation or bioremediation, but these approaches have solved few real world problems.

Toxic metals can bioaccumulate in the body and in the food chain.^[5] Therefore, a common characteristic of toxic metals is the chronic nature of their toxicity. This is particularly notable with radioactive heavy metals such as radium, which imitates calcium to the point of being incorporated into human bone, although similar health implications are found in lead or mercury poisoning.

Many metal ions are required for life. Even in these cases, a large excess of these ions can prove toxic.

• Cobalt poisoning
• Copper poisoning
• Iron poisoning
• Manganese poisoning was first identified in 1837 by James Couper.^[16]
• Selenium poisoning has been observed even though Se is an essential trace element. The Tolerable Upper Intake Level is 400 micrograms per day. Additional Se intake can lead to selenosis.^[17] Signs and symptoms of selenosis include a garlic odor on the breath, gastrointestinal disorders, hair loss, sloughing of nails, fatigue, irritability, and neurological damage.
• Zinc toxicity has been seen to occur at ingestion of greater than 225 mg of zinc.^[18] Excessive absorption of zinc can suppress copper and iron absorption. The free zinc ion in solution is highly toxic to bacteria, plants, invertebrates, and even vertebrate fish.^[19][20][21]

No global mechanism exists for the toxicities of these metal ions. Excessive exposure, when it occurs, typically is associated with industrial activities.

• Beryllium poisoning is attributed to the ability of Be²⁺ to replace Mg²⁺ in some enzymes.^[22] Be has been classified by one agency as a carcinogen.^[23]
• Cadmium poisoning came into focus with the discovery of the Itai-itai disease due to cadmium contaminated waters resulting from mining in the Toyama Prefecture starting around 1912.^[24] The term refers to the severe pains (Japanese: 痛い itai) people with the condition felt in the spine and joints. Cd2+ is thought to accumulate in the kidneys, where it tightly binds to the sulfur in cysteine-containing proteins.^[25]
• Lithium toxicity arises from overdose of lithium-containing drugs.^[26]
• Mercury poisoning came into sharp focus with the discovery of Minamata disease, named for the Japanese city of Minamata. In 1956, a factory in that city released of methylmercury in the industrial wastewater resulting in thousands of deaths and many other health problems.^[27] This incident alerted the world to the phenomenon of bioaccumulation. While all mercury compounds are toxic, organomercury compounds are especially dangerous because they are more mobile. Methyl mercury and related compounds are thought to bind to the sulfur of cysteinyl residues in proteins.^[28]

• Silver poisoning,^[30] like lithium poisoning, arises from misapplication of medications. A dramatic symptom of "argyria" is that the skin turns blue or bluish-grey.^[31]
• Thallium poisoning has been observed on several occasions, and it is well known that thallium compounds are highly toxic. Nonetheless, incidents of thallium poisoning are few.^[32] Tl is located on the periodic table near two other highly toxic metals, mercury and lead.
• Tin poisoning from tin metal, its oxides, and its salts are "almost unknown"; on the other hand certain organotin compounds are almost as toxic as cyanide. Such organotin compounds were once widely used as anti-fouling agents.^[33]