By now, you’ve probably been taught to gird your sun-starved skin for battle with cancer-causing cosmic rays every time you go outside. Choose a spray, choose a lotion, but by heavens, choose something! Legions of doctors, parents, and YouTube beauty influencers are unanimous on this point. But with sunscreen application evolving from a week or two at the beach every year to a constant daily slather, US health regulators want to know more about how all those photoprotective chemicals interact with people’s skin.

If they sink into tissues and get absorbed into the bloodstream, that could be a problem. Then, like other over-the-counter drugs the Food and Drug Administration oversees, sunscreens should be studied to make sure they don’t mess up people’s hormones, affect their reproductive systems, or cause cancer. Such safety testing has never been done on the active ingredients in sunscreen, because those chemicals were approved decades ago, before anyone suspected they could be absorbed into the body. Now we know it’s more than just a suspicion.

Today, researchers at the FDA revealed the results of a small clinical trial designed to test how four of the most common sun-filtering molecules on the market behave after they’ve been sprayed on and rubbed in. The results, published in the journal JAMA, show that contrary to what sunscreen manufacturers have been saying, UV-blocking chemicals do seep into circulation. Now, don’t panic and toss your tubes. There’s no evidence yet that they’re doing anything harmful inside the body. But the revelation will have serious impacts on sunscreen manufacturers going forward, and may change what options you’ll find on drugstore shelves before the year is out.

“Everyone had always thought that because these are intended to work on the surface of the skin that they wouldn’t be absorbed, but they are,” says Theresa Michele, director of the FDA’s division of nonprescription drug products, and coauthor on the FDA-funded study. Her team found that it took only a few hours after the application of sunscreen for the photoprotective chemicals to infiltrate the bloodstream and shoot up to concentrations above the FDA’s toxicology threshold that triggers further safety testing.

The researchers saw the same patterns in all 24 of the volunteers they recruited—12 men and 12 women, who were randomly assigned to apply one of four commercially available sunscreens: two sprays, a lotion, and a cream. The participants applied their potions according to recommended labeling; four times a day for four days to 75 percent of their bodies, roughly the amount of skin you’d be showing in a bathing suit. For those four days, and three days after, the researchers collected blood every few hours to be analyzed for the presence of avobenzone, oxybenzone, octocrylene, and ecamsule, 30 samples in all. They discovered that while it took only a few hours for the UV-blocking chemicals to spike over the target, for three of the four formulations, those levels remained elevated through the end of the study—three days after participants had ceased spraying and smearing. Only the cream users saw their chemical concentrations tail off sooner.

The fact that these sun-filtering molecules do penetrate into the circulatory system does not on its own mean that such ingredients are unsafe. “There might be nothing, and that would be great,” says Kanade Shinkai, a dermatologist at UC San Francisco and editor in chief of JAMA Dermatology. “But the problem is that we just don’t know.” The bottom line, she says, is that although the evidence is irrefutable that the sun causes skin cancer, scientists know a lot less about sunscreen chemicals’ relative risks and benefits.