Metal toxicity

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.

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.

Occupational exposure

Gold mining

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

Mercury is commonly used in small scale gold mining. To do this, large amounts of mercury are usually mixed with gold-containing materials to create a gold-mercury alloy called amalgam. To separate the gold, the amalgam is heated in a furnace causing the mercury to vaporize. During this process, miners are directly exposed to mercury vapors, and surrounding communities may be indirectly exposed through contaminated air, water, and soil.

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. Prolonged exposure can lead to kidney damage, respiratory failure, and even death. Ingestion of mercury through contaminated water, food, or soil pose great risk to pregnant women and their developing fetuses. When born, this can impair the infants' cognitive functions, memory, language development, and fine motor skills.

Despite its widespread use across countries, mercury exposure in artisanal small-scale gold mining is preventable. Mercury-free techniques like direct smelting result in gold recovery without the need of mercury resulting in the elimination of mercury. 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. This not only results in improved worker and community health but also lower in cost and eco-friendly.

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.

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. These fumes often contain metals like manganese, chromium, nickel, and lead, depending on the materials being welded. Breathing in these particles over time can lead to different health issues, including neurological symptoms associated with manganeseand lung irritation from the fumes. Stainless steel welding can also create hexavalent chromium, which is a known carcinogen. Exposures to metals are regulated in most countries. In the US, the Occupational Safety and Health Administration (OSHA) sets permissible exposure limits for metals found in welding fumes, while the National Institute for Occupational Safety and Health (NIOSH) identifies recommended exposure limits. 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. 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

Main article: 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.

Lead poisoning

Main article: Lead poisoning

Lead poisoning, in contrast to arsenic poisoning, is caused by industrial materials, such as leaded gasoline and lead leached from plumbing.

Toxicities from metals

Some metal elements are required for life, although they may be toxic in high exposure amounts. selenium, and zinc. 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.

Toxicities from nonessential metals

No global mechanism has been identified 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. Be has been classified by one agency as a carcinogen.
• 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. The term refers to the severe pains () 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.
• Lithium toxicity arises from overdose of lithium-containing drugs.
• Mercury poisoning came into sharp focus with the discovery of Minamata disease, named for the Japanese city of Minamata. In 1956, a factory in the city released methylmercury in the industrial wastewater resulting in thousands of deaths and many other health problems. 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.

• Silver poisoning, like lithium poisoning, arises from misapplication of medications. A dramatic symptom of "argyria" is that the skin turns blue or bluish-grey.
• 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. 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.

Treatment for poisoning

Chelation therapy

Main article: Chelation

Chelation therapy is a medical procedure that involves the administration of chelating agents to remove or deactivate heavy metals from the body.

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. Generally, increased exposure to heavy metals in the environment increases the risks for several diseases. 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.

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. 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. 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, and at least one child has died due to errors in administration of chelation therapy for this purpose.

References

References

1. (January 2025). "Heavy metals: toxicity and human health effects". Archives of Toxicology.
2. (27 February 2024). "Heavy metals". StatPearls, US National Library of Medicine.
3. (23 March 2023). "Heavy metal toxicity". StatPearls, US National Library of Medicine.
4. (2024). "Dictionary of Toxicology". Springer.
5. (10 March 2025). "Metals". Causal Analysis and Diagnosis Decision Information System (CADDIS), US Environmental Protection Agency.
6. (24 October 2024). "Mercury".
7. (2022). "Advances in Neurotoxicology". Academic Press.
8. (26 August 2015). "How People are Exposed to Mercury".
9. US EPA, OCSPP. (2015-09-03). "Health Effects of Exposures to Mercury".
10. US EPA, OITA. (2015-01-22). "Artisanal and Small-Scale Gold Mining Without Mercury".
11. Stoffersen, Birgitte. (September 2018). "Introduction of Mercury-Free Gold Extraction to Small-Scale Miners in the Cabo Delgado Province in Mozambique". Journal of Health & Pollution.
12. (2021). "Arsenic exposure among agricultural communities: Sources, health effects, and prevention". Environmental Research.
13. "Wayback Machine".
14. CDC. (2025-06-02). "Welding Fumes and Manganese".
15. (February 2018). "Lead Poisoning (Plumbism)".
16. (15 April 2024). "Dietary Supplement Fact Sheet: Selenium". Office of Dietary Supplements, US National Institutes of Health.
17. (1990). "Zinc toxicity". The American Journal of Clinical Nutrition.
18. (2009). "Sustainable Agriculture".
19. {{Greenwood&Earnshaw2nd
20. (1993). "IARC Monograph, Volume 58". International Agency for Research on Cancer.
21. (1998). "Preventative Measures Against Water Pollution".
22. {{Greenwood&Earnshaw2nd
23. (2019). "StatPearls". StatPearls Publishing.
24. Official government figure as of March 2001. See [https://www.env.go.jp/en/chemi/hs/minamata2002/ch2.html "Minamata Disease: The History and Measures, ch2"]
25. {{Greenwood&Earnshaw2nd
26. Fred, Herbert. (2008). "Images of Memorable Cases: 50 Years at the Bedside". Long Tail Press/Rice University Press.
27. (2006). "Andrews' diseases of the skin: clinical dermatology". Saunders Elsevier.
28. Verena Isak. (January–April 2019). "A Rare Case of Localized Argyria on the Face". Case Reports in Dermatology.
29. (2000). "Ullmann's Encyclopedia of Industrial Chemistry".
30. (2000). "Ullmann's Encyclopedia of Industrial Chemistry". Wiley.
31. (February 2013). "Five Things Physicians and Patients Should Question". American College of Medical Toxicology and American Academy of Clinical Toxicology.
32. (20 September 2010). "Nonstandard uses of chelation therapy". The Medical Letter on Drugs and Therapeutics.
33. (2010). "Chelation for Heavy Metals (Arsenic, Lead, and Mercury): Protective or Perilous?". Clinical Pharmacology & Therapeutics.
34. (1 October 2010). "Prenatal and Infant Exposure to Thimerosal From Vaccines and Immunoglobulins and Risk of Autism". Pediatrics.
35. (1 September 2003). "Thimerosal and the Occurrence of Autism: Negative Ecological Evidence From Danish Population-Based Data". Pediatrics.
36. (1 January 2008). "Continuing Increases in Autism Reported to California's Developmental Services System: Mercury in Retrograde". Archives of General Psychiatry.
37. Fombonne, Eric. (1 January 2008). "Thimerosal Disappears but Autism Remains". Archives of General Psychiatry.
38. (7 October 2006). "Chelation therapy and autism". BMJ.
39. Brent, Jeffrey. (1 December 2013). "Commentary on the Abuse of Metal Chelation Therapy in Patients with Autism Spectrum Disorders". Journal of Medical Toxicology.
40. (25 August 2005). "Boy with autism dies after chelation therapy".
41. (January 2008). "Pediatric fatality secondary to EDTA chelation". Clinical Toxicology.
42. Offit, Paul. (31 August 2005). "Death of an Autistic Child From Chelation Therapy - Op-ed".