Based on a new study published in the journal Environment International, there’s a pretty good chance your tampons might contain lead.

The study was conducted on 30 unique tampons from 14 different “top selling” brands in the U.S. and Europe. The researchers assessed the products for 16 different metals—including lead, arsenic, nickel, mercury, and zinc—and alarmingly, they reported finding measurable concentrations of all 16 metals “in at least one sampled tampon.”

Unfortunately, the paper’s corresponding author, Jenni Shearston, told Fast Company that the names of the brands in the study could not be shared.

Most notably, every tampon that was tested contained some amount of lead. According to the authors, lead has no safe exposure level in the body, and could cause negative outcomes if it were to leach out of a tampon and into the circulatory system.

“Even low-level exposure to [lead] can result in neurobehavioral impacts in adults and children, including decreased cognitive function such as impaired attention, memory, and learning ability,” the authors wrote.

Compared to the concentrations of other metals, researchers found high levels of calcium and zinc in the products. They also identified differences between organic and nonorganic brands, finding that organic tampons tend to contain less lead but more arsenic.

The parent companies of popular brands including Playtex, Kotex, Tampax, and Always, as well as the organic brands This is L, O.B., and Rael, did not immediately respond to Fast Company’s request for comment on the study.

However, Shearston did emphasize to Fast Company that “we do not yet know if metals can leach out of the tampon and whether they are absorbed by the body. We therefore cannot yet assess to what extent (if any) metals in tampons contribute to any health problems.”

“We definitely need more research on this severely understudied area, especially because millions of people could be affected,” Shearston said.

How are tampons tested in the U.S.?

In the U.S., tampons are regulated by the FDA as a Class II medical device. The study’s authors note that this classification doesn’t include a requirement to test for chemical contaminants, and that the FDA “only recommends that tampons not contain two dioxin compounds or pesticide residues.” Based on their findings, the researchers are calling for regulations requiring the testing of metals in tampons by manufacturers in both the U.S. and Europe.

To understand how metals in tampons might impact menstruators—the majority of whom use tampons—the researchers suggest that future studies test how metals might leach out of tampons and into the circulatory system. As of yet, they wrote, they were unaware of any existing research assessing the risk of vaginal exposure to metals. The murky scientific landscape of menstrual products is part of a larger historical pattern of female health often going understudied and underrepresented in the medical world.

“To our knowledge, our study is the first to assess concentrations of metals in tampons, despite the potential for substantial vaginal absorption of metals and the widespread and frequent use of tampons among menstruators,” the study’s authors wrote.