Methylmercury is an organic form of mercury that can cause environmental problems because it is readily absorbed and hard to eliminate. It can be produced directly through some industrial processes, and also by natural reactions in the environment. Concerns about methylmercury poisoning are the basis of recommendations to restrict pregnant women from the consumption of certain fish. Environmental agencies in regions where this compound may be a concern perform periodic tests to check for signs of dangerously high levels so they can warn the general population.
Chemically, this compound consists of a methyl group attached to an atom of mercury, forming a positively charged ion. It attaches readily to proteins, and does not easily release, which makes it a cause for concern in the natural environment. When an organism absorbs methylmercury, it cannot eliminate the cation. If that organism is in turn eaten by a larger organism, it gets a load of toxic mercury along with its meal. This creates a bioaccumulative effect, where concentrations of the compound can become high in animals far up the food chain, like sharks.
Historically, some industrial processes produced methylmercury and companies released the compound directly into the environment, creating pollution. It was also used for activities like treating grain to prevent insect infestation, which occasionally led to incidents when animals ate the grain and then were eaten by humans. Environmental regulations have cracked down on this practice, reducing the amount produced directly by humans. Elemental mercury continues to be produced, however, mostly through the burning of fossil fuels.
The entrance of elemental mercury into the environment doesn’t necessarily mean methylmercury will form. It needs to fall in locations with specialized anaerobic organisms that methylate it, adding the necessary methyl group. In addition, the demethylation process where the compound breaks down needs to be slow enough to allow it to build up. Lakes and streams are common sites for this transformation, which can affect animals relying on them as a water and food source.
Humans may consume small amounts of methylmercury without significant problems, but if it starts to build up, they can develop neurological symptoms. It is especially damaging for developing fetuses and young infants, who have rapidly growing nervous systems. Neurological impairments can develop because of exposure to methylmercury in the diet or through the placenta, in the case of a fetus. These are irreversible and can vary in severity depending on the timing and size of the dosage.