Metal toxicity

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Structure of a metal aquo complex, a typical soluble form for many metal ions in water M(H2O)6 cation.png
Structure of a metal aquo complex, a typical soluble form for many metal ions in water

Metal toxicity or metal poisoning is the toxic effect of certain metals that accumulate damage ecosystems, plants and animals, including human health. [1] [2] [3] Environmental pollution with heavy metals can result in contamination of drinking water, air, and waterways, accumulating in plants, crops, seafood, and meat. [3] Such pollution may indirectly affect humans via the food chain and through occupational or domestic exposure by inhalation, ingestion, or contact with the skin. [1] [3]

Contents

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. [2] Other heavy metals having no biological roles in animals, but with potential for toxicity include arsenic, cadmium, lead, mercury, and thallium. [1] [2] [4]

Some metals are toxic when they form poisonous soluble compounds which interfere with enzyme systems, such as superoxide dismutase, catalase, or glutathione peroxidase. [1] Only soluble metal-containing compounds are toxic by forming coordination complexes, which consist of a metal ion surrounded by ligands. [1] 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. [3]

Sources and site evidence

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. [1] [5]

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. [5] Commonly, there is no visible evidence of metals pollution in soil or water. [5]

When metal toxicity in the environment is suspected, pathologies in fish, clams, and insects may serve as signals for contamination and toxicities. [5] Physiological mechanisms of metal toxicity may have a spectrum of effects, ranging from changes in behavior to death of small animal species. [5]

Occupational exposure

Gold mining

Artisanal small-scale gold miners are at high risk to exposure of metal toxicity. [6] While there is a wide array of hard metals that are toxic, mercury poses the greatest risk from inhalation and ingestion from environmental contamination. [7]

Countless countries use mercury to extract gold in gold mining. [8] To do this, large amounts of mercury are usually mixed with gold-containing materials to create a gold-mercury alloy called, amalgam. [9] To separate the gold, the amalgam is heated in a furnace causing the mercury to vaporize. [9] During this process, miners are directly exposed to mercury vapors, and surrounding communities may be indirectly exposed through contaminated air, water, and soil. [6]

Continuous high levels of mercury vapor inhalation can cause a variety of health effects. Inhalation may result in tremors, mood swings, muscle weakness, memory loss, or headaches. [7] [9] Prolonged exposure can lead to kidney damage, respiratory failure, and even death. [10] Ingestion of mercury through contaminated water, food, or soil pose great risk to pregnant women and their developing fetuses. [6] When born, this can impair the infants’ cognitive functions, memory, language development, and fine motor skills. [10]

Despite its widespread use across countries, mercury exposure in artisanal small-scale gold mining is preventative. [7] Mercury-free techniques like direct smelting result in gold recovery without the need of mercury. [11] In this method, borax is used to decrease the viscosity and melting temperature of non-gold minerals so they can be easily separated from the gold. [11] This not only results in improved worker and community health but also lower in cost and eco-friendly. [12] [6]

Agricultural workers

Arsenic exposure remains a major concern in agricultural communities that rely on untreated groundwater for crop irrigation and drinking. Studies have found higher urinary arsenic levels among farm workers exposed to contaminated well water and pesticides. [13]

Welders

People who work as welders can be exposed to metal fumes because welding uses extremely high heat, which turns the metal into very small airborne particles [14] .These fumes often contain metals like manganese, chromium, nickel, and lead, depending on the materials being welded [15] .Breathing in these particles over time can lead to different health issues, including neurological symptoms associated with manganese [15] and lung irritation from the fumes. Stainless steel welding can also create hexavalent chromium [15] , which is a known carcinogen [15] . OSHA and NIOSH have both set exposure limits for metals found in welding fumes [14] [15] . Using some industrial hygiene controls such as local exhaust ventilation, fume extraction systems, respirators, and routine air monitoring can help protect welders from harmful metal exposure [14] . Because welding is widely used in construction and manufacturing and many more occupations, controlling metal fumes is essential for maintaining worker health and safety.

Major types of metal poisoning

Arsenic poisoning

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. [1] [2]

Lead poisoning

Lead poisoning, in contrast to arsenic poisoning, is caused by industrial materials, such as leaded gasoline and lead leached from plumbing. [1] [2] [3] Use of leaded gasoline has declined precipitously since the 1970s. [16] [17]

Toxicities from metals

Essential elements [18] [19] [20]
H He
LiBe BCNOFNe
NaMg AlSiPSClAr
KCaScTiVCrMnFeCoNiCuZnGaGeAsSeBrKr
RbSrYZrNbMoTcRuRhPdAgCdInSnSbTeIXe
CsBa*LuHfTaWReOsIrPtAuHgTlPbBiPoAtRn
FrRa**LrRfDbSgBhHsMtDsRgCnNhFlMcLvTsOg
 
 *LaCePrNdPmSmEuGdTbDyHoErTmYb
 **AcThPaUNpPuAmCmBkCfEsFmMdNo
Legend:
  The six basic organic elements
  Quantity elements
  Essential trace elements
  Essentiality or function in mammals debated
  No evidence for biological action in mammals, but essential or beneficial in some 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 chemical 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. [21]

Some metal elements are required for life, although they may be toxic in high exposure amounts. [1] [2] [3] Included are cobalt, copper, iron, manganese, [22] selenium, [23] and zinc. [24] Excessive absorption of zinc can suppress copper and iron absorption. The free zinc ion in solution is highly toxic to bacteria, plants, invertebrates, and fish. [25]

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. Argyria 2a.jpg
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.

Treatment for poisoning

Chelation therapy

Chelation therapy is a medical procedure that involves the administration of chelating agents to remove or deactivate heavy metals from the body. [3] Chelating agents are molecules that form particularly stable coordination complexes with metal ions. [3] Complexation prevents the metal ions from reacting with molecules in the body, and enable them to be dissolved in blood and eliminated in urine. [3] [38] [39] [40]

Other conditions

It is difficult to differentiate the effects of low level metal poisoning from the environment with other kinds of environmental harms, including nonmetal pollution. [1] Generally, increased exposure to heavy metals in the environment increases the risks for several diseases. [1] Despite a lack of evidence to support its use, some people seek chelation therapy to treat a wide variety of conditions such as autism, cardiovascular disease, Alzheimer's disease, or any sort of neurodegeneration. [38]

Treatment of autism by chelation therapy has been promoted by alternative medicine practitioners based on an unsupported hypothesis that autism is a result of heavy metal poisoning. This hypothesis likely emerged from the more specific claim that autism was caused by the preservative thiomersal, which in the past has been used in multi-dose vials of vaccines. Despite extensive study, no connection has been found between vaccines and autism diagnosis rates. [41] [42] Despite this lack of evidence, thimerosal was removed from vaccines out of an abundance of caution by 2001; autism diagnosis rates did not decrease in response to the exclusion of thimerosal, disproving the association. [43] [44] Regardless of the removal of thimerosal and the evidence that it never influenced autism in the first place, the idea of heavy metal exposure causing autism has persisted, and thus has the use of chelation therapy as treatment. Systematic reviews of available evidence do not support the use of chelation therapy for autism, [45] [46] and at least one child has died due to errors in administration of chelation therapy for this purpose. [47] [48] [49]

References

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  2. 1 2 3 4 5 6 Fisher RM, Gupta V (27 February 2024). "Heavy metals". StatPearls, US National Library of Medicine. PMID   32491738 . Retrieved 17 April 2025.
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  5. 1 2 3 4 5 "Metals". Causal Analysis and Diagnosis Decision Information System (CADDIS), US Environmental Protection Agency. 10 March 2025. Retrieved 17 April 2025.
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  8. ↑ Soe PS, Kyaw WT, Arizono K, et al. (22 May 2022). "Mercury Pollution from Artisanal and Small-Scale Gold Mining in Myanmar and Other Southeast Asian Countries". International Journal of Environmental Research and Public Health. 19 (10): 6290. doi: 10.3390/ijerph19106290 . ISSN   1660-4601. PMC   9142007 . PMID   35627826.
  9. 1 2 3 "How People are Exposed to Mercury". EPA.gov . 26 August 2015. Retrieved 10 November 2025.
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  11. 1 2 US EPA O (22 January 2015). "Artisanal and Small-Scale Gold Mining Without Mercury". www.epa.gov. Retrieved 18 November 2025.
  12. ↑ Stoffersen B, Appel PW, Na-Oy LD, et al. (September 2018). "Introduction of Mercury-Free Gold Extraction to Small-Scale Miners in the Cabo Delgado Province in Mozambique". Journal of Health & Pollution. 8 (19): 180909. doi:10.5696/2156-9614-8.19.180909. ISSN   2156-9614. PMC   6257171 . PMID   30524868.{{cite journal}}: CS1 maint: article number as page number (link)
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  14. 1 2 3 "Wayback Machine" (PDF). www.osha.gov. Archived from the original (PDF) on 2 September 2025. Retrieved 18 November 2025.
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  23. ↑ "Dietary Supplement Fact Sheet: Selenium". Office of Dietary Supplements, US National Institutes of Health. 15 April 2024. Retrieved 17 April 2025.
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  25. ↑ Rout GR, Das P (2009). "Effect of Metal Toxicity on Plant Growth and Metabolism: I. Zinc". In Lichtfouse E, Navarrete M, Debaeke P, Véronique S, Alberola C (eds.). Sustainable Agriculture . pp.  873–84. doi:10.1007/978-90-481-2666-8_53. ISBN   978-90-481-2666-8. S2CID   84595949. INIST   14709198.
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  31. ↑ Official government figure as of March 2001. See "Minamata Disease: The History and Measures, ch2"
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  35. ↑ Verena Isak, Tobias Beerli, Antonio Cozzio, et al. (January–April 2019). "A Rare Case of Localized Argyria on the Face". Case Reports in Dermatology. 11 (1): 23–27. doi: 10.1159/000494610 . PMC   6477469 . PMID   31043936.
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  41. ↑ Price CS, Thompson WW, Goodson B, et al. (1 October 2010). "Prenatal and Infant Exposure to Thimerosal From Vaccines and Immunoglobulins and Risk of Autism" . Pediatrics. 126 (4): 656–664. doi:10.1542/peds.2010-0309. ISSN   0031-4005. PMID   20837594.
  42. ↑ Madsen KM, Lauritsen MB, Pedersen CB, et al. (1 September 2003). "Thimerosal and the Occurrence of Autism: Negative Ecological Evidence From Danish Population-Based Data" . Pediatrics. 112 (3): 604–606. doi:10.1542/peds.112.3.604. ISSN   0031-4005. PMID   12949291.
  43. ↑ Schechter R, Grether JK (1 January 2008). "Continuing Increases in Autism Reported to California's Developmental Services System: Mercury in Retrograde" . Archives of General Psychiatry. 65 (1): 19–24. doi:10.1001/archgenpsychiatry.2007.1. ISSN   0003-990X. PMID   18180424.
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  45. ↑ Sinha Y, Silove N, Williams K (7 October 2006). "Chelation therapy and autism". BMJ. 333 (7571): 756.1. doi:10.1136/bmj.333.7571.756. ISSN   0959-8138. PMC   1592402 . PMID   17023484.
  46. ↑ Brent J (1 December 2013). "Commentary on the Abuse of Metal Chelation Therapy in Patients with Autism Spectrum Disorders". Journal of Medical Toxicology. 9 (4): 370–372. doi:10.1007/s13181-013-0345-4. ISSN   1937-6995. PMC   3846967 . PMID   24113859.
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  48. ↑ Baxter AJ, Krenzelok EP (January 2008). "Pediatric fatality secondary to EDTA chelation" . Clinical Toxicology. 46 (10): 1083–1084. doi:10.1080/15563650701261488. ISSN   1556-3650. PMID   18949650.
  49. ↑ Offit P (31 August 2005). "Death of an Autistic Child From Chelation Therapy - Op-ed". Children's Hospital of Philadelphia. Retrieved 16 April 2025.
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