Sunscreens are beneficial for humans, helping prevent skin cancer, but they're not good for the world's coral reef systems, as a common chemical found in the lotions threatens their health, researchers report.

That chemical found in sunscreen preparations and in other kinds of cosmetic products can, in even tiny amounts, harm coral and coral reef systems, scientists say.

"The chemical, oxybenzone (benzophenone-3), is found in more than 3,500 sunscreen products worldwide," says researcher Omri Bronstein in the Department of Zoology at Tel Aviv University.

"It pollutes coral reefs via swimmers who wear sunscreen or wastewater discharges from municipal sewage outfalls and coastal septic systems," he explains.

While the chemical pollution occurs mostly in areas where large numbers of swimmers take to the water, reefs 5 to 20 miles beyond a coastline can be affected by submarine seeps of freshwater that have been contaminated by sewage, the researchers say.

Around 14,000 tons of sunscreen lotions make their way onto coral reefs around the globe every year, they add.

Baby or juvenile corals are most susceptible to harm from the chemical, they say, which acts as an "endocrine disruptor" that causes young corals to encase themselves in their own skeletons, leading to their death.

Oxybenzone makes corals more vulnerable to bleaching at lower water temperatures so they're less able to cope with the warmer ocean temperatures caused by climate change. The chemical also damages coral DNA, interfering with coral's ability to effectively reproduce, the researchers report.

The damaging effect on corals can be the result of even miniscule amounts of the chemical, they found.

"We found the lowest concentration to see a toxicity effect was 62 parts per trillion—equivalent to a drop of water in six and a half Olympic-sized swimming pools," Bronstein says.

Recent studies in the U.S. Virgin Islands found oxybenzone concentrations 23 times the minimum amount seen as toxic for corals.

The average beachgoer might use 2 to 4 ounces of sunblock before going for a swim; multiply that by a thousand or more swimmers taking to the water and the risk to coral reefs becomes apparent, Bronstein says.

A significant ecological threat is posed by current concentrations of oxybenzone in coral reef areas, he says.

"Although the use of sunscreen is recognized as important for protection from the harmful effects of sunlight, there are alternatives—including other chemical sunscreens, as well as wearing sun clothing on the beach and in the water," Bronstein advises.

Brands of sunscreen made without oxybenzone exist, and can be found on a website on sunscreens provided by the nonprofit Environmental Working Group.

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