We’ve already seen scientists research the effects of urine and pharmaceuticals in pool water. Now they’re scrutinizing a poolside staple: sunscreen.

The U.S. Environmental Protection Agency is studying how an ingredient in sunscreen reacts when exposed to chemicals in swimming pool water.

This is just a small part of a much larger study examining the environmental impacts of engineered nanomaterials, which are used in a wide variety of consumer products. In this case, scientists are zeroing in on titanium dioxide (TiO2) — the thing that protects skin from harmful UV rays.

Here’s the thing about titanium dioxide: In the presence of water and UV radiation, it decomposes into other molecules, including a reactive oxygen species that has been shown to cause tissue damage. To prevent this, sunscreen manufacturers wisely place a protective coating around the material, called aluminum hydroxide.

EPA researchers wanted to learn how the protective layer holds up against the rigors of a chemically treated pool.

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