Demystifying the toxicity of mercury
Dear Dr. Roach: How much mercury is safe? — A.C.
The chemical element mercury exists in three forms: elemental mercury, which is the kind in thermometers; inorganic mercury, especially mercuric chloride; and organic mercury, including methyl and ethyl mercury. The safe level depends on which type it is.
Elemental mercury is dangerous primarily when it is inhaled, which occurs mainly in occupational settings. Accidental swallowing of the small amount has very low toxicity, but I still don’t recommend it. Inorganic mercury is seldom a problem, as it is almost never used these days in the U.S. and Canada. Most products containing this form of mercury have been banned.
Methyl mercury is the major concern for consumers, as this is found in fish. It accumulates in the environment, and tends to be worst in larger fish, who eat the smaller fish and over their lifetime accumulate the methyl mercury, which was formerly used as a fungicide. The Environmental Protection Agency warns an average-size person not to exceed 70 mcg per day of methyl mercury: Below this dose, toxicity is unlikely. This translates to two to three servings of fish per week, but no more than one serving of highest-risk fish for methyl mercury per week. High-risk sources include most tuna, halibut and snapper.
Ethyl mercury is significantly less toxic than methyl mercury, as it is removed from the body more quickly. There is no EPA recommendation for ethyl mercury levels. Ethyl mercury has been used as a preservative in vaccines, but it has been removed from all childhood and most adult vaccines. It is still used in some flu vaccines — the ones in multidose vials: Single-use vials do not need a preservative. The amount of ethyl mercury in a flu shot is less than the more dangerous methyl mercury found in a can of tuna.
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