Sunscreen saves humans at the expense of ocean health

In 2013, more than 202 million international tourists found their way to the beautiful, warm beaches of the Mediterranean Sea. Spain alone was host to more than 60 million visitors that year. In many cases, tourism is a boon to local wildlife, from manta rays to whale sharks to seals. But that isn’t always the case.

Those 202 million tourists visiting Mediterranean beaches each year come along with gallons and gallons of sunscreen applied to their bodies. Sunscreen is the most effective way of reducing the skin’s exposure to ultraviolet radiation. The problem is that two common ingredients in sunblock, titanium dioxide and zinc oxide, react with ultraviolet light from the sun and form new compounds, like hydrogen peroxide. In sunscreen, they’re coated in silica or alumina to protect human skin, but in seawater the coatings wash off, leaving the underlying microparticles to react with sunlight. And high amounts of hydrogen peroxide are toxic for phytoplankton, microscopic plants that rest at the bottom of the food chain, feeding on everything from the smallest of shrimps to the most massive of whales.

Seawater sampling suggested that there was a tremendous increase in the level of hydrogen peroxide in coastal waters each summer, which would seem to implicate tourism. But Spanish researchers Antonio Tovar-Sanchez and David Sánchez-Quiles wanted to be sure. To see whether enough sunscreen that washes off of beachgoers’ skin was a hazard for marine life, the pair headed for the Spanish island of Majorca. They focused on Palmira Beach, a site that sees 10,000 beachgoers each day in the summer tourist season.

The first thing they did was drop a gram of sunblock into a liter of filtered seawater and subject it to ultraviolet light. As expected, the titanium dioxide reacted with the light and produced hydrogen peroxide. In a second experiment, the researchers then gathered up some more coastal seawater and mixed in some sunblock to see how the phytoplankton would respond. As the microparticles in the sunscreen reacted with sunlight to produce hydrogen peroxide, the microscopic algae began to decline.


Combining the results from those experiments with tourism data, seawater sampling, and some back-of-the-envelope math, the researchers became convinced that it’s the sunscreen that’s responsible for the summertime spike in hydrogen peroxide, and the subsequent decline in phytoplankton. The 10,000 beachgoers on an average day at Palmira beach wash 4 kilograms of titanium dioxide into the water each day, which produces an additional 270 nM of hydrogen peroxide, above the natural levels of 100 nM. And that’s assuming that each adult only uses half of the recommended dosage of sunblock. Scary stuff.

By 2030, the World Tourism Organization expects the Mediterranean to welcome 264 million tourists each year, or an additional 3 million people each year between now and then. “Our results suggest that the increased tourist activities will have important ecological consequences in the coastal areas as a consequence of the increased input of sunscreen products, particularly in Mediterranean beaches,” the researchers say, “where tourism is more intense” and where beachgoers stay in the water for longer. What will that mean for phytoplankton? When such a critical link in the food chain becomes imperiled, the entire ecosystem can suffer. By trying to avoid skin cancer, we may just be poisoning our oceans. – Jason G. Goldman | 27 August 2014

Source: Sánchez-Quiles D. & Tovar-Sanchez A. (2014). Sunscreens as a Source of Hydrogen Peroxide Production in Coastal Waters, Environmental Science & Technology , 48 (16) 9037-9042. DOI:

Header image:; Body image via Sanches-Quiles & Tovar-Sanchez (2014).