Some eco-friendly menstrual products may contain harmful “forever chemicals,” according to a new study. Researchers found high levels of PFAS — toxic substances linked to health problems — in a number of reusable pads and period underwear. These chemicals may be added on purpose to help make products more leak-proof, but experts warn they can also be absorbed through the skin.

PFAS have been tied to cancer, hormone issues, fertility problems, and more. What’s especially concerning is that some products had very high levels of these chemicals, and there’s no labeling to let users know. Since many people use these products monthly for years, researchers say we need more testing, rules, and transparency.

The good news is that not all the products tested contained PFAS, which means they aren’t necessary for making reusable menstrual care items. Experts say safer options are possible — but shoppers currently have no way of knowing which ones are PFAS-free.