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Yet beach crowds aren’t the only people who add to the demise of the coral reefs found just off shore. Athletes who slather sunscreen on before a run, mothers who coat their children before outdoor play and people trying to catch some rays in the park all come home and wash it off.

Cities such as Ocean City, Md., and Fort Lauderdale, Fla., have built sewer outfalls that jettison tainted wastewater away from public beaches, sending personal care products with a cocktail of chemicals into the ocean. On top of that, sewer overflows during heavy rains spew millions of tons of waste mixed with stormwater into rivers and streams. Like sunscreen lotions, products like birth-control pills contain chemicals that are endocrine disruptors and alter the way organisms grow. Those are among the main suspects in an investigation into why male fish such as bass are developing female organs.

Research for the new study was conducted only on the two islands. But across the world each year, up to 14,000 tons of sunscreen lotions are discharged into coral reef, and much of it “contains between 1 and 10 per cent oxybenzone,” the authors said. They estimate that places at least 10 per cent of reefs at risk of high exposure, judging from how reefs are located in popular tourism areas.

“The most direct evidence we have is from beaches with a large amount of people in the water,” said John Fauth, an associate professor of biology at the University of Central Florida in Orlando. “But another way is through the wastewater streams. People come inside and step into the shower. People forget it goes somewhere.”