Summer is the time for beach trips and swimming, and often a sunburn or two. Lathering up with sunscreen is one way to protect against the sun’s scorching rays, but it also has an unintended consequence we are only now learning about. When swimmers jump into the ocean, rivers, and lakes, some of the sunscreen they’ve applied to their skin washes off into the water. It is estimated that roughly 14,000 tons of sunscreen enters waterways across the globe every year from both sunscreen applications and wastewater runoff. In shallow areas that are popular with swimmers, significant amounts of sunscreen can accumulate in the water. Now, evidence shows it could be harming ocean life. 

The key to sunscreen’s sun blocking power is a list of molecules that either absorb or repel the ultraviolet (UV) rays. Most sunscreens contain molecules that protect against the skin cancer causing UVA rays and the sunburn causing UVB rays. The sun-blocking capabilities of these molecules were discovered and used in sunscreen starting in the 1950s as a replacement to the thick petroleum-based sunscreens developed the decade before.  Both titanium dioxide, a UVB blocker, and zinc oxide, a UVA blocker, are naturally occurring minerals that have sun blocking capabilities, and both were first used in these second-generation sunscreen products. But one draw-back to mineral based sunscreens is that they are also a naturally white pigment, and application causes an undesirable chalky skin color. Sunscreen companies were already well on their way testing lab-based molecules that could absorb UV rays without the chalkiness, and by the 1980s new molecules began appearing in sunscreens. Among others, avobenzone became one of the go-to UVA absorbers, while sister molecules oxybenzone and octinoxate were added to protect against the UVB molecules. 

In recent years, coral scientists began to notice heightened levels of sunscreen molecules in popular swimming and diving areas, which are often near coral reefs. Climate change, overfishing, and pollution are already causing severe damage to coral reefs around the world, and the added stressor concerned many scientists in the field. The first study indicating sunscreen causes bleaching in corals was published in 2008. Corals were taken from the environment and exposed to several store-bought sunscreen lotions in seawater tanks. The purpose of this initial study was to simply determine if sunscreen at high concentrations caused corals to bleach, also called a toxicity study. All the corals tested bleached.  Critics were quick to point out the conditions were a far cry from what happens in the environment. Regardless, people began to seriously question whether any amount of sunscreen near coral reefs could be harmful to the delicate ecosystem. 

Since that first study, scientists continue to build evidence that some sunscreens may harm corals, and even other marine animals. In particular, studies continue to show that oxybenzone and octinoxate are detrimental to coral health. Research shows octinoxate degrades into a chemical called benzophenone, which is a known cancer-causing agent and hormone disruptor. A notable 2016 study found that oxybenzone can cause deformities in both coral larvae and baby corals, damage to coral DNA, and abnormal skeletal growth. A later 2022 study of anemones, the coral’s close relative, determined one reason why. When anemones encounter oxybenzone in the presence of sunlight they attempt to rid themselves of the chemical by slightly altering its chemical makeup, but in the process, they end up making a lethal toxin. The new molecule builds in both the anemone tissue and in the anemone’s symbiotic algae where it then wreaks havoc and bleaches the anemone.  

 
Again, these studies were conducted in a lab. The big question still remains—are corals in the environment experiencing similar levels of distress from sunscreen washing off swimmer’s bodies? A 2022 National Academies review of sunscreen in the environment found that between 1 and 10 μg/L of sunscreen chemicals can be found in a popular tourist site, which is a concentration comparable to 2 grains of sugar in a standard bathtub at the upper amount. That same review noted that toxic levels of sunscreen molecules, both in acute exposure and over time (chronic), are significantly higher than what is observed in the environment. In some cases, studies claiming oxybenzone or octinoxate kill corals use sunscreen concentrations higher than what is observed in the environment. It is possible that the small amounts of sunscreen that wash off are tolerated by the corals. Mass coral bleachings clearly occur during marine heat waves, the result of climate change, and there is no evidence that sunscreens play a significant role in these events. However, more subtle impacts are harder to rule out. 
 
For concerned consumers the best course of action is to cover up during the midday hours and use an oxybenzone and octinoxate free sunscreen when needed. Mineral sunscreens that use titanium dioxide or zinc oxide can be a good choice, but know that nothing is truly “reef safe” as scientists are continually expanding our understanding of sunscreen toxicity. Even mineral sunscreens have additives and coatings, and many now use nano sized mineral particles to decrease the white hue left on the skin when applied. Any of these changes may alter the toxicity of a mineral sunscreen. Of course, the best chance coral reefs have of survival is to slow the progress of climate change, so choosing a local travel destination over a long, gas-guzzling international flight might just be the most reef friendly choice.