There are two forms of mercury: inorganic mercury and organic methylmercury (MeHg). When inorganic mercury evaporates, it is biotransformed into methylmercury through microbial methylation in saltwater and freshwater sediments. Methylmercury is a persistent environmental contaminant and it bioaccumulates once it enters the food chain, building up in larger organisms that eat smaller, contaminated organisms. This is why large predatory fish such as tuna and shark have notoriously high mercury levels. The process, known as Biomagnification, occurs with many hydrophobic toxins.

Mercury causes nervous system defects in children and impedes fetal development. Numerous poisonings have highlighted the harmful effects of mercury use and mercury pollution.

The neuropsychiatric effects of mercury (and lead) exposure are discussed by Mark Filidei, of the San Francisco Preventive Medical Group, in an article, Toxic Metals and Mental Health.