An international team of researchers studied the concentrations of a common sunscreen ingredient called oxybenzone in popular swimming spots in the Virgin Islands and Hawaii. They found that even minuscule amounts—just 62 parts per trillion—are enough to have toxic effects on baby corals. That’s equivalent to a single drop of water in six-and-a-half Olympic-size swimming pools, said study author Craig Downs, a molecular biologist with the nonprofit Haereticus Environmental Laboratory in Clifford, Virginia.
The United States National Park Service estimates that 4,000 to 6,000 tons of sunscreen enter reef areas annually.
At Trunk Bay in the Virgin Islands, concentrations of the chemical were as high as 1.4 parts per million. In Hawaii, the scientists recorded levels between 19 and 800 parts per billion, according to the study, which was published Tuesday in the journal Archives of Environmental Contamination and Toxicology.
That matters. Coral reefs may cover less than 1 percent of the ocean’s floor, but they provide a home for more than 1 million species of fish, invertebrates, and algae.
Sunscreen could be one of reasons why the National Park Service has noticed coral reef die-offs even in protected areas, Downs said.
“We’ve lost about 90 percent of coral reefs in U.S. territorial waters,” he said. “These chemicals are having an impact on conch, shrimps, arthropods, and cuttlefish—pretty much everything in the water.”
Oxybenzone affects corals in several ways, the scientists found. First, the chemical damages the corals’ DNA, making it difficult for them to reproduce. Oxybenzone also causes the corals to bleach, which can lead to die-offs. Third, it deforms juvenile corals.
Downs said the chemical is also an endocrine disruptor, mimicking estrogen in biological systems.
“It may be that oxybenzone makes the baby corals encase themselves in their own skeletons, creating their own coffins,” he said. Typically, baby corals float around until they find a permanent home, where they start to build out their structure. But if they start creating a skeleton too early, they can’t find a place to live and so die.
This could explain why so few new coral reefs are growing in areas like Trunk Bay. “In the presence of oxybenzone, baby corals basically kill themselves,” said Downs.
That means people should avoid sunscreens containing oxybenzone and switch to zinc oxide and other alternatives.
The National Park Service has started a campaign called "Protect Yourself, Protect the Reef," which suggests people wear long-sleeved clothing and use mineral sunscreens when swimming near reefs.
Downs said people should read sunscreen ingredients carefully. Some products include neem oil, an insecticide that is not harmful to humans but can kill crustaceans and other marine arthropods.
“It makes it harder for the consumer, but a responsible consumer should always be learning,” said Downs. “The idea is to use oils in sunscreens that are not toxic to marine life.”