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This week, the Centers for Disease Control and Prevention’s Advisory Committee on Immunization Practices is meeting to review and vote on several vaccine recommendations. Just in the past week, however, Secretary of Health and Human Services Robert F. Kennedy, Jr., added additional items to the agenda, including a discussion of thimerosal—a mercury-containing compound that is used in some vaccines. Thimerosal has already been removed from all childhood vaccines, and detailed research has shown it does not cause neurodevelopmental disorders. The mercury in thimerosal is quickly and easily cleared by the body. Here’s how we know and why we still use the compound in some adult vaccines.

What is thimerosal, and why has it been used in vaccines?

Thimerosal is a preservative that was first added to the manufacturing of vaccines in the 1930s. Because it is a highly effective antiseptic, it can prevent the introduction of fungi or bacteria that could be harmful to inject. By weight, about 50 percent of thimerosal is ethylmercury, a compound that contains mercury. That sounds scary to some people because it’s well understood that mercury can be toxic to the brain. Many people are aware, for example, that eating too much tuna can be unsafe because of how much mercury the fish can contain.

“I think everyone is pretty familiar with the concept that mercury is toxic,” says Ryan Marino, a medical toxicologist at University Hospitals in Cleveland. “The thing that is not conveyed is that there are multiple different forms of mercury with very different toxicities.”

Mercury is ubiquitous in our environment, Marino says, and it arises from both natural and human sources. Volcanoes, forest fires and rock weathering all release mercury into the air, but the vast majority of the element comes from mining, the burning of coal and other fossil fuels and industrial waste.

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.

When and why was thimerosal removed from vaccines?

In the late 1990s, the U.S. government took measures to reduce human exposure to mercury, including the 1997 Food and Drug Administration Modernization Act. This law required the FDA to make a list of all foods and drugs containing mercury compounds and the amounts of those compounds. At the time, three childhood vaccines contained thimerosal: the diphtheria, tetanus, acellular pertussis (DTaP), hepatitis B, and Haemophilus influenzae type b (Hib) vaccines.

Although no evidence up to that point had suggested the thimerosal in vaccines was harmful, only limited research existed at the time on the differences between ethylmercury and methylmercury. The total ethylmercury in childhood vaccines that infants could have been exposed to fell below the FDA’s recommended safety limits for methylmercury: 0.4 microgram per kilogram of body weight per day (0.4 μg/kg/d). But they slightly exceeded the EPA’s recommended limit of 0.1 μg/kg/d. (Agencies have different limits depending on their data sources and purpose.) But again, those were limits for methylmercury.

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