The sweet and salty aroma of sunscreen and seawater signals a relaxing trip to the shore. But scientists are now reporting that the idyllic beach vacation comes with an environmental hitch. When certain sunblock ingredients wash off skin and into the sea, they can become toxic to some of the ocean's tiniest inhabitants, which are the main course for many other marine animals. Their study appears in the ACS journal Environmental Science and Technology.

Antonio Tovar-Sanchez and David Sanchez-Quiles point out that other than staying indoors, slathering on sunscreen is currently the best way to protect skin from the sun's harmful rays. But when sunbathers splash into the ocean to cool off, some of their lotions and creams get rinsed into the water.

The problem is that titanium dioxide and zinc oxide nanoparticles, which are common ingredients in sunblock, can react with ultraviolet light from the sun and form new compounds, such as hydrogen peroxide, that could be toxic.

High amounts of hydrogen peroxide can harm phytoplankton, the microscopic algae that feed everything from small fish to shrimp to whales. The scientists wanted to figure out just how serious of an impact beachgoers could be having on life in coastal waters.

To investigate the matter, they hit the beach. They went to Majorca Island's Palmira beach on the Mediterranean along with about 10,000 beachgoers, a small portion of the more than 200 million tourists that flock to Mediterranean shores every year.

Based on lab tests, seawater sampling and tourism data, the researchers concluded that titanium dioxide from sunblock was largely responsible for a dramatic summertime spike in hydrogen peroxide levels in coastal waters - with potentially dangerous consequences for aquatic life.