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How does ethylmercury differ from elemental mercury and methylmercury?
Microorganisms convert inorganic mercury in the environment into the compound methylmercury, which aquatic creatures inadvertently consume. Methylmercury accumulates up the food chain, so apex predators such as sharks, tuna and swordfish have the highest concentrations. That’s why the Food and Drug Administration and the Environmental Protection Agency recommend weekly limits to the consumption of certain fish, particularly such fish consumed by children and pregnant people.
The mercury in thimerosal, however, is ethylmercury, and that one missing letter makes a big difference.
Both molecules include the element mercury, a metal that, in its elemental form, is silver-colored, liquid at room temperature and well known for its use in old thermometers. And both molecules are organic, which means they include carbon atoms. Specifically, ethylmercury’s chemical formula is C2H5Hg+, and methylmercury’s is CH3Hg+. The different numbers of carbon and hydrogen atoms in those molecules mean they have very different properties. To get a sense of the difference a single atom can make, consider that our bodies need—and are mostly made up of—water, or H2O, but if you add another oxygen atom to that molecule, you get H2O2. The latter is hydrogen peroxide, something we definitely should not drink.
Methylmercury is more easily absorbed into neurological tissues and bioaccumulates, or builds up in the body, Marino says. It can cross the blood-brain barrier, and too much of it can result in symptoms ranging from “forgetfulness, irritability and depression all the way to dementia,” he says. The half-life of methylmercury is about 50 to 80 days, so it can remain in the body for nearly four months. But ethylmercury is not absorbed as readily into tissues as methylmercury is. Ethylmercury’s half-life is just three to seven days, so the body removes it within about a week and a half.
“Ethylmercury does not behave in the same way as methylmercury,” Marino says. While too much ethylmercury can also cause poisoning, “the body can clear the amount that’s in vaccines very rapidly. You would have to get hundreds, if not thousands, of vaccines at once” to cause any problems, he says.
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