A common class of chemicals that’s been linked to cancer, fertility problems, and thyroid dysfunction has now been tied to another major health issue: According to a new study in PLOS Medicine, women who have high levels of perfluoroalkyl substances (PFAS) in their blood tend to gain back more unwanted weight after dieting.

The new study included both men and women who’d been enrolled in a two-year clinical trial and who lost weight by following a heart-healthy diet. But when researchers factored in the levels of PFAS in participants’ blood at the start of the study, they found that people with high levels tended to gain more of that weight back after initially losing it.

The association was found almost exclusively in women, and the researchers say that PFAS’ effects on estrogen in the body may be one reason why. But the study also found that people with high PFAS concentrations had lower resting metabolic rates; in other words, their metabolism was slower and they burned fewer calories doing daily activities.

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The researchers concluded that PFAS may play a role in body weight regulation, and therefore in the country’s current obesity epidemic. “We all know it’s feasible to lose weight through diet or physical activity; however the challenging part is that almost no one can maintain that weight loss,” says senior study Qi Sun, assistant professor in the department of nutrition at Harvard T.H. Chan School of Public Health. “Now we’ve shown that PFAS level may actually determine how much weight people regain.”

But what exactly are these chemicals, and why are they in our bodies to begin with? Here’s what you need to know, and how you can reduce your exposure.

Ditch fast food and microwave popcorn

PFAS chemicals have water- and oil-repellant properties, which makes them valuable to the fast-food industry and for packaged foods like microwave popcorn. In a 2017 study published in Environmental Science & Technology Letters, researchers found that about half of the 400 food wrappers and containers they analyzed contained fluorine, an indicator of PFAS.

Previous studies have found that PFAS have the potential to leach into food—and that once PFAS enter the body, they stay there for years. That’s reason enough to avoid exposure whenever possible, says Laurel Schaider, PhD, an environmental chemist at the Silent Spring Institute and lead author of the food-wrapper study.

“I think we all already have some reasons to reduce how much fast food we consume, and this may be another one,” Schaider told Health in 2017. “If you’re going to eat it, you could try to get the food out of the wrapper as quickly as possible—that might help a little bit.”

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Think twice about stain- or water-resistant products

Another common use for PFAS is making clothing, carpets, upholstery, and other textiles stain- or water-resistant. (Think of advertisements where spilled wine on a sofa beads up and wipes right off.) And while some older PFAS have been phased out of textile production because of associated health and environmental risks, some newer ones have taken their place, says Tom Brutton, PhD, a fellow and PFAS researcher at the Green Science Policy Institute—and their health effects are not yet known.

To be safe, Brutton recommends avoiding stain-, water-, soil-, or grease-repellant products whenever they’re not necessary. And when they are—in the case of a raincoat, for example—look for gear labeled PFAS-free or fluoro-free. “You’re starting to be able to find rain jackets and outdoor gear without these chemicals,” he says, “and I think there will be many more options in as little as two or three years.”

If you already own fabrics with PFAS, don’t panic. “The harm that’s going to happen to one person from the exposure of wearing a raincoat or sitting on a stain-resistant carpet is probably quite minimal,” says Brutton. “What we’re really concerned about are the chemicals released when these products are manufactured and also when they’re disposed of and end up in a landfill.” If consumers can make smarter choices so there are fewer of these products in circulation, he says, it will be better for our health, and for the environment as a whole.

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Don’t buy another nonstick pan

The same advice goes for nonstick cookware: If you already own pots and pans with these chemicals, you don’t have to stop using them or throw them away—at least not until they’re scratched or damaged. But don’t buy a new set either. “The exposure to you from your use of that pan isn’t going to be so huge that it represents a significant health threat,” says Brutton. “But when it’s time to buy a new one, perhaps look for one that doesn’t contain PFAS.” Many experts recommend stainless steel, ceramic, or cast-iron cookware, or you can look for brands that advertise being PFAS-free.

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Be smart about seafood

Because they’re so prevalent in the environment, PFAS can also accumulate in the tissue of animals that humans then consume for food. The chemicals have been found in contaminated seafood, for example, and Brutton says that buying organic won’t necessarily reduce your exposure.

What will help, however, is choosing fish that are lower on the food chain. You may already be doing that if you’re concerned about mercury and other heavy metals in seafood, says Brutton. Following those same rules will also help you avoid PFAS. “Instead of buying swordfish, for example, choose salmon,” he says.

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Check on your water supply

PFAS released during industrial and manufacturing processes can also accumulate in water supplies, especially near industrial sites, wastewater treatment plants, and military fire-training areas. A 2016 study found that drinking water supplies for at least 6 million Americans may exceed the Environmental Protection Agency’s health advisory limit for lifetime exposure to certain PFAS from drinking water.

Unfortunately, there may not be an easy way to know if your community’s drinking water is contaminated with PFAS, since the EPA does not currently require municipalities to notify residents about these chemicals. But if you’re concerned, it’s worth asking your local supplier.

“In a lot of cases when water utilities find that their levels are high, they’ve taken action and installed filters and alerted consumers, although there’s no guarantee,” Brutton says. Consumers who are concerned about contaminant levels can also install activated carbon filters in their homes. “These products do a fairly good job at removing a lot of these chemicals from drinking water,” he adds.