Beach-goers lather on sun cream on a daily basis to protect their skin from harmful UVB rays.
As well as stopping your skin from burning, the cream or oil helps to guard against more harmful effects of the sun, such as skin cancer.
However, a study has suggested that the use of sun cream could be endangering coral reefs.
Some countries have introduced a ban on certain sunscreens that contain chemicals linked to coral damage to mitigate against the corals being more susceptible to bleaching.
Researchers from the University of Tehran and the US looked at how sunscreen could be dispersed by currents off the Iranian island of Kish — a popular destination for tourists.
Looking at three beaches on the east of Kish — Women’s Beach, Men’s Beach and Coral Beach — they calculated that it could take as long as two days or more to flush out pollutants.
If beaches are visited each day, corals may be continuously exposed to potentially harmful sunscreen chemicals.
“If people show up day after day, the total amount will begin to accumulate and get higher,” said Dr Craig Downs, executive director of the US non-profit organisation Haereticus Environmental Laboratory, and one of the paper’s authors.
“People assume the solution to pollution is dilution. That’s not always the case with these very popular mass tourism areas.”
Published online last month after being presented at a conference earlier this year, the research was not based on actual measurements of sunscreen contamination in the waters around Kish, which is about 10 miles wide.
Dr Downs said the University of Tehran researchers were currently taking such records to refine their model, which found that pollution could linger at Women’s Beach for about 30 hours, at Men’s Beach for around 17 hours and at Coral Beach for 48 hours.
“The dose makes the poison. The University of Tehran is going out to sample so they can determine if those concentrations can be dangerous,” added Dr Downs.
Sunscreen chemicals end up in the sea not just from being washed off swimmers. Substances are absorbed through the skin and end up in sewage systems, while contamination also comes from sunscreen washed off in beachside or hotel showers.
“The sunscreen pollution in near-shore coastal reefs can be very damaging,” said Dr Downs. “One of the things this sunscreen can cause is coral reef bleaching.
“A bigger effect is coral reef zombies (which look healthy but cannot reproduce). It may take them 15 to 20 years to fade away. We know they're fading away because they’re not reproductively viable.”
Very low threat
Not all academics see sunscreen as a major threat. Dr Nial Wheate, an associate professor at the University of Sydney who has researched the subject, described the threat to coral reefs from sunscreen as “very low to insignificant”.
“There is clear evidence that changes in sea temperatures, light levels and other sources of pollution [like shipping and run-off from land] are the major contributors to coral damage,” said Dr Wheate, whose other roles include being chief scientific officer of Vairea Skincare and a sunscreen agents panel member for Standards Australia.
“As organic molecules, it is unlikely that sunscreen molecules would accumulate in corals.
“It is well established that the active ingredients in sunscreens break down upon exposure to UV (sun) light and there is no reason to think that natural degradation would not occur in corals as well.”
Dr Downs, however, said concentrations of chemicals at some locations could be “orders of magnitude” higher than those known to have toxic effects on coral larvae.
He also said chemicals could work alongside climate change by making it harder for reefs to recover from bleaching events, which typically involve corals, in response to high temperatures, expelling the algae that normally live inside them and provide food.
“If you have these climate change events and this persistent sunscreen pollution, it will all die off and nothing comes back,” Dr Downs said.
Some sun creams banned
Some countries have banned sunscreens containing chemicals linked to coral damage.
A ban on sunscreens containing oxybenzone and octinoxate took effect in Hawaii in 2021, while Thailand announced a ban at the country’s marine national parks.
Dr Downs said studies had already shown that oxybenzone levels had dropped in Hawaii and, although it was too early to say if corals were recovering, algal growth had increased.
Warming and high salinity are the main threats to Gulf coral reefs, according to Prof Charles Sheppard, of the School of Life Sciences at the University of Warwick in the UK.
“All over now, the reckoning is that coral reefs as functioning systems are only going to be around for a few more decades. Already half of Gulf reefs are dead rubble,” said Prof Sheppard, who is the author of Coral Reefs: A Natural History.
Coastal development, such as shoreline construction and the building of ports and islands, has also damaged many reefs in the Gulf, he said, through sediment dredging and direct landfill on top of reefs.
“Safeguards cost a bit of money but when you don’t enforce them the cost to society is much greater in the end,” he said
Sunscreen could be “the nail in the coffin” for the Gulf’s reefs, he said, although the focus should be on cutting CO2 emissions and controlling building and waste discharges.