monomethylmercury

Monomethylmercury, also called methylmercury (MeHg), is an organomercury cation with the formula CH3Hg+. It is the most common organic mercury species in the environment and the form that readily accumulates in aquatic organisms, especially in fish.

MeHg is produced primarily by the biological methylation of inorganic mercury (Hg2+) in anoxic environments such as sediments and wetlands. It can enter water bodies via natural deposition and human activities, including fossil fuel combustion and mining. Once formed, MeHg is persistent and tends to bind to organic matter; it bioaccumulates in organisms and biomagnifies up the food chain.

MeHg is highly toxic to the nervous system, especially during development. It readily crosses the placenta and the blood-brain barrier, binds to sulfhydryl groups in proteins, and can disrupt cellular function and development. The primary route of human exposure is through the consumption of contaminated seafood, with higher trophic level fish containing higher concentrations.

MeHg is relatively persistent in aquatic systems, but it can be demethylated by microbial and photochemical processes, which provides some natural attenuation. In public health terms, exposure assessment typically focuses on dietary intake from fish and shellfish. Health agencies have issued exposure guidelines; for example, the U.S. Environmental Protection Agency uses a reference dose of about 0.1 micrograms per kilogram of body weight per day for methylmercury. Public health recommendations commonly emphasize limiting consumption of high-mercury fish, especially for pregnant women and young children.