Human health effects Ingested methylmercury is readily and completely absorbed by the
gastrointestinal tract. It is mostly found complexed with free cysteine and with proteins and
peptides containing that amino acid. The methylmercuric-cysteinyl complex is recognized by amino acids transporting proteins in the body as
methionine, another
essential amino acid. Because of this mimicry, it is transported freely throughout the body including across the
blood–brain barrier and across the
placenta, where it is absorbed by the developing
fetus. Also for this reason as well as its strong binding to proteins, methylmercury is not readily eliminated. Methylmercury has an
elimination half-life in human
blood of about 50 days, varying by age, sex, and body weight. Methylmercury exposure in adults has also been linked to increased risk of
cardiovascular disease including
heart attack. Some evidence also suggests that methylmercury can cause
autoimmune effects in sensitive individuals. 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. Although there is no doubt that methylmercury is toxic in several respects, including through exposure of the developing fetus, there is still some controversy as to the levels of methylmercury in the diet that can result in adverse effects. 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. There have been several episodes in which large numbers of people were severely poisoned by food contaminated with high levels of methylmercury, notably the dumping of
industrial waste that resulted in the
pollution and subsequent mass poisoning in
Minamata and
Niigata,
Japan and the situation in
Iraq in the 1960s and 1970s in which wheat treated with methylmercury as a preservative and intended as seed grain was fed to animals and directly consumed by people (see
Basra poison grain disaster). 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. At present, exposures of this magnitude are rarely seen and are confined to isolated incidents. Accordingly, concern over methylmercury pollution is currently focused on more subtle effects that may be linked to levels of exposure presently seen in populations with high to moderate levels of dietary fish consumption. These effects are not necessarily identifiable on an individual level or may not be uniquely recognizable as due to methylmercury. However, such effects may be detected by comparing populations with different levels of exposure. There are isolated reports of various clinical health effects in individuals who consume large amounts of fish; however, the specific health effects and exposure patterns have not been verified with larger, controlled studies. 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. At present, most of this guidance is based on protection of the developing fetus; future guidance, however, may also address cardiovascular risk. In general, fish consumption advice attempts to convey the message that fish is a good source of nutrition and has significant health benefits, but that consumers, in particular pregnant women, women of child-bearing age, nursing mothers, and young children, should avoid fish with high levels of methylmercury, limit their intake of fish with moderate levels of methylmercury, and consume fish with low levels of methylmercury no more than twice a week.
Effects on fish and wildlife '' after one month in normal water for the first batch, and in water containing 0.6PPB and 1.26PPB and 2.5PPB (parts per billion) of methylmercury for the three bottles at right. MeHg is a highly toxic compound to wild fish, mammals and birds, and hazardous contaminant that affects the environmental habitat where it is encountered, making it hard to achieve good environmental status as defined by the Water Framework Directive (WFD). In recent years, there has been increasing recognition that methylmercury affects fish and wildlife health, both in acutely polluted ecosystems and ecosystems with modest methylmercury levels. Two reviews document numerous studies of diminished reproductive success of fish, fish-eating birds, and mammals due to methylmercury contamination in aquatic ecosystems.
In public policy Reported methylmercury levels in fish, along with fish consumption advisories, have the potential to disrupt people's eating habits, fishing traditions, and the livelihoods of the people involved in the capture, distribution, and preparation of fish as a foodstuff for humans. 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. About 30% of the distributed mercury depositional input is from current anthropogenic sources, and 70% is from natural sources. The natural sources category includes re-emission of mercury previously deposited from anthropogenic sources. According to one study, based on modeled concentrations, pre-
Anthropocene tissue-bound levels in freshwater fish may not have differed markedly from current levels in some individual watersheds. However, based on a comprehensive set of global measurements, the ocean contains about 60,000 to 80,000 tons of mercury from pollution, and mercury levels in the upper ocean have tripled since the beginning of the industrial revolution. Higher mercury levels in shallower ocean waters could increase the amount of the toxicant accumulating in food fish, exposing people to a greater risk of mercury poisoning. == Removal ==