Do you apply sunscreen before swimming in the sea? You’re harming the oceans

Dermatologists warn not to go out in the sun without protection. Unfortunately, what’s good for humans doesn’t always benefit nature. A new study by a team of scientists from the University of Stirling in Scotland shows that sunscreen entering seawater increases the harmful effects of plastic already circulating in the oceans.

The problematic sunscreen ingredient

The study, published on July 3 in the scientific journal Journal of Hazardous Materials, highlights the issue of hundreds of millions of beachgoers enjoying the sea. To protect themselves from skin cancer, they eagerly apply sunscreens, most of which contain Ethylhexyl Methoxycinnamate (EHMC), a compound that absorbs UVB radiation. As a hydrophobic compound, EHMC does not dissolve in water but, like oils, spreads easily over the surface.

Sunscreen applied to the skin enters the sea, and its water-insoluble components easily settle on plastic waste. This enriches the “plastisphere” and affects the rate of bio- and photodegradation of plastics.

What is the plastisphere?

About 0.5 percent of the plastic waste we produce each year ends up in the oceans. That may seem like little, but it amounts to 1.7 million tons of plastic. In total, more than 5 trillion pieces of plastic are already floating on the surface of the world’s oceans, killing fish, water birds, and marine mammals, and entering the human food chain.

On the surface of this floating debris, a so-called plastisphere develops over time – a layer of microorganisms including actinobacteria, ciliates, algae, bacteria, viruses, and fungi. Remarkably, some bacteria, such as those in the genera Marinomonas and Pseudomonas, can break down plastic, thus helping reduce the burden on the oceans. However, sunscreen might be interfering with this process.

How sunscreen harms the oceans

The Scottish researchers’ study shows that EHMC in sunscreens settles on the surface of plastic waste and alters the functioning of the plastisphere, harming aerobic organisms while favoring those that do not require oxygen. Not only does it significantly reduce the population of Marinomonas bacteria, which can degrade hydrocarbons, but it also shields other microorganisms from UVB radiation, thereby hindering photodegradation – the natural breakdown of compounds under the influence of light.

This effect, combined with the growth of potentially pathogenic bacteria, raises serious concerns about ecosystem stability and human health, especially in coastal regions with high tourism and heavy plastic pollution, emphasized Dr. Sabine Matallana-Surget, who led the study at the University of Stirling.

The proliferation of bacteria on plastic waste is a problem in itself, as some of them can opportunistically act as pathogens. According to the authors, EHMC’s protective influence on Pseudomonas bacteria could turn directly against humans. Certain strains are capable of causing serious illnesses that require antibiotic treatment.

To apply or not to apply?

Given these findings, the ideal would be to apply sunscreen only after coming out of the sea. However, spending long periods in the water without SPF protection poses a serious risk to skin health, especially during peak sunlight hours. To minimize risk, it’s worth consciously choosing sunscreens by reading labels and avoiding products containing the controversial EHMC.

From an aquatic ecosystem perspective, the safest sunscreens are those with mineral filters containing zinc oxide or titanium dioxide. These create a physical protective layer on the skin and block UV rays without harming marine ecosystems.