[1][clarification needed] Methylmercury (derived biologically from dimethylmercury) is the causative agent of the infamous Minamata disease.

Thiols are generated when the amino acid cysteine and the peptide glutathione form strong complexes with methylmercury:[3] Methylmercury is formed from inorganic mercury by the action of microbes that live in aquatic systems including lakes, rivers, wetlands, sediments, soils and the open ocean.

[13] Direct measurements of methylmercury production using stable mercury isotopes have also been observed in marine waters,[14][15] but the microbes involved are still unknown.

Increased methylmercury concentrations in water and fish have been detected after flooding of soils associated with reservoir creation (e.g. for hydroelectric power generation) and in thermokarst wetlands that form after permafrost thaw.

Natural sources of mercury released to the atmosphere include volcanoes, forest fires, volatilization from the ocean[19] and weathering of mercury-bearing rocks.

[21] In the past, methylmercury was produced directly and indirectly as part of several industrial processes such as the manufacture of acetaldehyde.

[31] Methylmercury exposure in adults has also been linked to increased risk of cardiovascular disease including heart attack.

[35] There is some evidence suggesting a possible connection between post-natal mercury exposure and autism; however, it is not clear whether methylmercury intake in particular is linked in a similar way.

Recent evidence suggests that the developmental and cardiovascular toxicity of methylmercury may be mitigated by co-exposures to omega-3 fatty acids and perhaps selenium, both found in fish and elsewhere.

These episodes resulted in neurological symptoms including paresthesias, loss of physical coordination, difficulty in speech, narrowing of the visual field, hearing impairment, blindness, and death.

Children who had been exposed in utero through their mothers' ingestion were also affected with a range of symptoms including motor difficulties, sensory problems and intellectual disability.

Many governmental agencies, the most notable ones being the United States Environmental Protection Agency (EPA), the United States Food and Drug Administration (FDA), Health Canada, and the European Union Health and Consumer Protection Directorate-General, as well as the World Health Organization (WHO) and the United Nations Food and Agriculture Organization (FAO), have issued guidance for fish consumers that is designed to limit methylmercury exposure from fish consumption.

Two reviews[24][47] document numerous studies of diminished reproductive success of fish, fish-eating birds, and mammals due to methylmercury contamination in aquatic ecosystems.

[48] Furthermore, proposed limits on mercury emissions have the potential to add costly pollution controls on coal-fired utility boilers.

Nevertheless, substantial benefits can be achieved globally by introducing mercury emission reduction measures because they reduce human and wildlife exposure to methylmercury.