September 23, 2014 — In 2013, more than 200 million tourists sunbathed on Mediterranean beaches. You can bet that most of them applied sunscreen. That sunscreen may be valuable for preventing skin cancer for humans, but it can also wreak havoc on marine ecosystems.

A new study published in Environmental Science & Technology found that when sunscreen washes off a person into the ocean, certain ingredients react with sunlight to form hydrogen peroxide, which can be toxic to phytoplankton.

Researchers Antonio Tovar-Sánchez and David Sánchez-Quiles took to the waters near Majorca Island to verify the impacts of Mediterranean tourists on marine life. They sampled water under different sunlight conditions. Using a combination of data from ultraviolet radiation, tourist numbers and seawater samples, the team concluded that sunscreen alters oceanic composition and affects phytoplankton populations.

It turns out titanium dioxide — a common ingredient in LEDs, solar cells, surface coatings and sunscreen — is the chemical culprit. When titanium dioxide nanoparticles enter the water, their protective coatings dissolve. This allows the compound to react with ultraviolet light and form new compounds, such as hydrogen peroxide.

Hydrogen peroxide generates high levels of stress on phytoplankton, the cornerstone of the oceanic food web. All marine life depends on these primary producers to survive. Phytoplankton also play a key role in carbon sequestration, thus contributing to overall climate stabilization.

How can beachgoers stay healthy and maintain healthy ecosystems, too? One strategy is to cover up with long sleeves or wide-brimmed hats before resorting to sunscreen. If sunscreen is necessary, just make sure it is fully absorbed before hitting the waves. Photo by HJL (Flickr | Creative Commons)