Mercury
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Mercury is a heavy metal which can exist in a number of forms, which includes as a liquid (known as quick silver), a vapour, as inorganic salts or organomercury compounds.
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Exposure to the vapours of mercuric nitrate by the felters in hat making led to the use of the phrase “mad as a hatter” during the 1800s.
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The resultant symptoms will depend on whether exposure has been to elemental mercury, organomercury compounds or inorganic mercury salts.
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Inorganic mercury salts will affect the kidneys and the GI tract. They do not readily cross the blood brain barrier and therefore do not affect the nervous system (unless exposure is high or continuous)
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Elemental mercury is poorly absorbed across the skin or via the GI tract. Its potential to cause toxicity arises if its vapours are inhaled.
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Organomercury compounds cause the greatest concern as they can be extremely toxic. In the environment concerns are primarily focused on methyl mercury due to its ability to bioaccumulate.
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Known incidents involving mercury include the exposures in Minamata Bay in Japan (1960), and poisoning due to methyl mercury-treated grain in Iraq (1970).