What Is Thimerosal? Why Most Vaccines Don't Contain Mercury Anymore

[LiberalArkie]

JUNE 25, 2025

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.

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https://www.scientificamerican.com/article/what-is-thimerosal-why-most-vaccines-dont-contain-mercury-anymore/