Women have been using tampons for thousands of years to absorb menstrual blood, and if they saw the cotton-based feminine hygiene product glow in the dark, they’d probably freak out. Well, it turns out that tampons that shimmer and shine after being immersed in river water are serious cause for concern.

In a study published in the Water and Environment Journal, researchers at the University of Sheffield in England reveal that they’ve been using the bits of cotton to soak up H20 in streams and rivers to check for pollution.

The cotton shirt sitting in your drawer can’t be used to test the water because clothing manufacturers commonly use optical brighteners to make the colors of clothing pop, and they make your shirts dazzling white too. Those brighteners are what show up if you walk into a bar or club that’s lit by UV light.

Tampons are made from cotton that hasn’t been treated with chemicals—inserting a chemical-laced tampon into a vagina could lead to some nasty health effects. That means an all-natural cotton tampon is perfect for testing for pollution from optical brighteners.

When the researchers dangled the feminine hygiene product into rivers and then exposed it to UV light, they could easily tell if it had been exposed to chemical contamination: If there was no glow, that meant the waterway was free from runoff that contains optical brighteners like the laundry detergent waste that flows out of your washer. But if the tampon had an otherworldly gleam when exposed to the light, that was a sure sign the water was polluted. The researchers could then work backward, dipping tampons along the way, to locate the source of the problem.

There’s a real need for simple and cost-efficient ways to check waterways for contaminants. Thanks to aging sewage infrastructure, pipe leaks and breaks often result in untreated water being released into rivers. According to the study, the pipes in more than 1 million homes in the U.K. are incorrectly connected to municipal water treatment plants. That means all the water and other fluids that go down those households’ drains are being discharged directly into rivers.

“Unfortunately, it’s very difficult to detect where this is happening, as the discharge is intermittent, can’t always be seen with the naked eye, and existing tests are complex and expensive,” David Lerner, the lead author of the study, said in a statement.

That’s where the tampons come in.

“Our new method may be unconventional—but it’s cheap and it works,” Lerner said. The research team now hopes to work with local governments to expand this use of tampons past the testing phase. Sounds great—just as long as the cotton products don’t glow when ladies use them.