Chlorine may cause the protective coating on certain sunscreens to degrade, leaving you vulnerable to skin damage.

Chlorine in swimming pools does more than just turn your hair green. A new study published in the Chemical Engineering Journal has found that it could also make your sunscreen less safe and effective, particularly if it contains nanoparticles of the mineral titanium dioxide.

Whether used in sunscreens, cosmetics, or even pharmaceuticals, nanoparticles are often coated in some other material to prevent them from reacting with other things they may come in contact with, such as human tissue. In sunscreens specifically, aluminum hydroxide is used to prevent titanium dioxide nanoparticles from clumping together, which causes the sunscreen to impart a white sheen on your skin; the whole goal in using nanoparticles is so they disappear when applied.

The scientists in this study wanted to see how well aluminum hydroxide held up to the elements, because, left uncoated, titanium dioxide can react with sunlight to form harmful free radicals that may lead to skin cancer. They took a sample of a commercial sunscreen that contained coated nanoparticles and exposed it to various concentrations of chlorine that you might encounter in a typical swimming pool.

Their tests showed that aluminum hydroxide does break down when exposed to chlorine, and the higher the chlorine concentrations, the quicker the breakdown. "This could lead to unwanted and even dangerous photocatalytic activities on human tissues," the authors write.

Skin cancer is the most common form of cancer in the U.S., so anything that raises their risk of it should send up red flags for consumers. But this study shouldn't cause you to toss your sunscreen just yet, says Chuan-Jian Zhong, PhD, professor of materials chemistry and nanotechnology at State University of New York, Binghamton. "There's lots of research going on with nanoparticle coatings," he says, and results like this help companies develop safer, more stable alternatives.

Still, if you don't want to take your chances, opt for hats, long sleeves, and other ways of covering up over slathering with sunscreen. Even chemical sunscreens that contain no nanoparticles have been found to react with sunlight and form free radicals that can lead to skin cancer. For our suggestions, check out our Sun Protection Clothing Guide then stock up for next summer. 

More on nanoparticles from Rodale.com:
• The Risky Technology Creeping into Your Food & Makeup
• Is Your Makeup a Dangerous Science Experiment?
• How to Spot a Nanoparticle