The sunscreens on coral reef-exploring tourists often contain a component that may speed up the death of the endangered ecosystems.

New findings in the journal Science indicate how this happens. The work could help guide the development and marketing of effective, coral-safe sunscreens.

“It would be a sad irony if ecotourism aimed at protecting coral reefs were actually exacerbating their decline,” says lead author Djordje Vuckovic, a PhD student in civil and environmental engineering at Stanford University. “My hope is that our research will help lead the way to developing coral-safe sunscreens.”

Up to 6,000 tons of sunscreen—more than the weight of 50 blue whales—wash through United States reef areas every year, according to the National Park Service. Scientists have known for some time that oxybenzone, an organic compound found in many sunscreens, can damage corals. As a result, sunscreens with this compound have been banned in the US Virgin Islands and Hawaii, the island nation of Palau, and Bonaire, an island municipality of the Netherlands, among other places.

However, the mechanisms by which oxybenzone does harm have largely remained a mystery, making it difficult to ensure that sunscreen components proposed as alternatives are truly safer for corals.

William Mitch, a professor of civil and environmental engineering at Stanford, became interested in the issue several years ago when he heard about Hawaii’s then-pending ban. He and John Pringle, a professor of genetics in the Stanford School of Medicine, began work to characterize the chemical and biological mechanisms by which oxybenzone harms corals.

In their new study, Mitch, Pringle, Vuckovic, and other researchers used anemones as surrogates for corals, which are harder to experiment with, as well as mushroom corals. Exposed to oxybenzone in artificial seawater under simulated sunshine, the anemones all died within 17 days, whereas anemones exposed to oxybenzone in the absence of simulated sunlight remained viable.

“It was strange to see that oxybenzone made sunlight toxic for corals—the opposite of what it is supposed to do,” says Mitch. “The compound is good at absorbing light within the waveband we tested, which is why it’s so common in sunscreens.”

Read the full article about the dangers of suncreen on coral reefs by Rob Jordan at Futurity.