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] It is often thought that only heavy metals can be toxic, but lighter metals such as beryllium and lithium may also be in certain circumstances. 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 surround 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.

Structure of a metal aquo complex, a typical soluble form for many metal ions in water.

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.[3] This method uses plants to extract and lower the concentration of toxic heavy metals in the soil.[3] 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.[4] 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.

Major types of metal poisoning

Arsenic poisoning

A dominant kind of metal toxicity is arsenic poisoning. This problem mainly arises from ground water that naturally contains high concentrations of arsenic. A 2007 study found that over 137 million people indicates that more than 70 countries may be affected by arsenic poisoning from drinking water.[5]

Lead poisoning

Lead poisoning, in contrast to arsenic poisoning, is inflicted by industry. Most lead on the planet is immobilized as minerals, which are relatively harmless. Two major sources of lead poisoning are leaded gasoline and lead leached from plumbing (from Latin, plumbus for lead). Use of leaded gasoline has declined precipitously since the 1970s.[6][7] One lead-containing pigments is lead chromate (the yellow-orange of U.S. school buses), but this material is so stable and so insoluble that little evidence exists for its toxicity.

Toxicities from essential metals

Essential elements[8][9][10][11][12]
H   He
Li Be   B C N O F Ne
Na Mg   Al Si P S Cl Ar
K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe
Cs Ba * Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn
Fr Ra ** Lr Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og
 
  * La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb
  ** Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No
Legend:
  Quantity elements
  Essential trace elements
  Essentiality or function in mammals debated
  No evidence for biological action in mammals, but essential in some lower organisms.
(In the case of the lanthanides, the definition of an essential nutrient as being indispensable and irreplaceable is not completely applicable due to their extreme similarity. The stable early lanthanides La–Nd are known to stimulate the growth of various lanthanide-using organisms, and Sm–Gd show lesser effects for some such organisms. The later elements in the lanthanide series do not appear to have such effects.)[13]

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

Toxicities from nonessential metals

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

A 92-year-old Caucasian man (right) with pigmentary changes had used nose drops containing silver for many years. His skin biopsy showed silver deposits in the dermis, confirming the diagnosis of generalized argyria.[27]

Society and culture

It is difficult to differentiate the effects of low level metal poisoning from the environment with other kinds of environmental harms, including nonmetal pollution.[32] Generally, increased exposure to heavy metals in the environment increases risk of developing cancer.[33]

Without a diagnosis of metal toxicity and outside of evidence-based medicine, but perhaps because of worry about metal toxicity, some people seek chelation therapy to treat autism, cardiovascular disease, Alzheimer's disease, or any sort of neurodegeneration.[34] Chelation therapy does not improve outcomes for those diseases.[34]

Treatment for poisoning

Chelation therapy is a medical procedure that involves the administration of chelating agents to remove or deactivate heavy metals from the body. Chelating agents are molecules that form particularly stable coordination complexes with metal ions. Complexation prevents the metal ions from reacting with molecules in the body, and enable them to be dissolved in blood and eliminated in urine. It should only be used in people who have a diagnosis of metal intoxication.[35] That diagnosis should be validated with tests done in appropriate biological samples.[34]

References

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  2. "Announcement: Response to the Advisory Committee on Childhood Lead Poisoning Prevention Report, Low Level Lead Exposure Harms Children: A Renewed Call for Primary Prevention". Centers for Disease Control and Prevention. 2012-05-25. Archived from the original on 2017-04-30.
  3. 1 2 Ali, Hazrat; Khan, Ezzat; Sajad, Muhammad Anwar (2013-05-01). "Phytoremediation of heavy metals—Concepts and applications". Chemosphere. 91 (7): 869–881. Bibcode:2013Chmsp..91..869A. doi:10.1016/j.chemosphere.2013.01.075. ISSN 0045-6535. PMID 23466085.
  4. Okereafor, Uchenna; Makhatha, Mamookho; Mekuto, Lukhanyo; Uche-Okereafor, Nkemdinma; Sebola, Tendani; Mavumengwana, Vuyo (January 2020). "Toxic Metal Implications on Agricultural Soils, Plants, Animals, Aquatic life and Human Health". International Journal of Environmental Research and Public Health. 17 (7): 2204. doi:10.3390/ijerph17072204. ISSN 1660-4601. PMC 7178168. PMID 32218329.
  5. See:
  6. . doi:10.1002/14356007.a15_249. {{cite journal}}: Cite journal requires |journal= (help); Missing or empty |title= (help)
  7. O'Malley, R.; O'Malley, G. (February 2018). "Lead Poisoning (Plumbism)". Merck Manual.
  8. Ultratrace minerals. Authors: Nielsen, Forrest H. USDA, ARS Source: Modern nutrition in health and disease / editors, Maurice E. Shils ... et al. Baltimore: Williams & Wilkins, c1999., p. 283-303. Issue Date: 1999 URI:
  9. Szklarska D, Rzymski P (May 2019). "Is Lithium a Micronutrient? From Biological Activity and Epidemiological Observation to Food Fortification". Biol Trace Elem Res. 189 (1): 18–27. doi:10.1007/s12011-018-1455-2. PMC 6443601. PMID 30066063.
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  12. Zoroddu, Maria Antonietta; Aaseth, Jan; Crisponi, Guido; Medici, Serenella; Peana, Massimiliano; Nurchi, Valeria Marina (2019). "The essential metals for humans: a brief overview". Journal of Inorganic Biochemistry. 195: 120–129. doi:10.1016/j.jinorgbio.2019.03.013.
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  15. "Dietary Supplement Fact Sheet: Selenium". National Institutes of Health; Office of Dietary Supplements. Retrieved 2009-01-05.
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  25. Official government figure as of March 2001. See "Minamata Disease: The History and Measures, ch2"
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