For everyday toxicological purposes, there are three forms of mercury that we need to think about: metallic (liquid) mercury, methyl mercury, and ethyl mercury. Liquid mercury essentially isn't absorbed from skin contact or even swallowing, though it does evaporate, especially when hot, and emits dangerous fumes. The real threat posed by metallic mercury comes when it unites with other chemicals to form compounds. For example, mercury vapor in the atmosphere (which mostly comes from burning coal or processing ore) combines with atmospheric pollutants to form inorganic compounds. When these are washed into the sea, micro-organisms in the seabed convert the compounds into methyl mercury, which is ingested by fish and becomes dangerous indeed when it enters the human food chain.

Although the toxic nature of mercury had long been known (the Mad Hatter in Alice in Wonderland was a victim of mercury poisoning), it was not until the 1950s that we began to understand the magnitude of the problem. The trigger was an ecological disaster in Japan in which large-scale industrial contamination of the waters of Minamata Bay by mercury compounds started a chain of neurological disease and death that affected thousands of children and pregnant women. Mercury and its compounds, dumped into the bay, were converted into extremely toxic methyl mercury, which was taken up and concentrated by the fish, which were, in turn, eaten by the residents of Minamata.

The third toxicologically significant form of mercury is ethyl mercury. It lacks the extreme neurological toxicity of methyl mercury because it doesn't penetrate the central nervous system. I stress this difference because thimerosal, the preservative formerly used in vaccine production, is ultimately converted in the body into ethyl mercury (and not methyl mercury).