Although most beachgoers reach for sunscreen to shield their skin from dangerous ultraviolet radiation, scientists know little about the environmental impact when the lotions wash off into nearshore waters. A new study finds that titanium dioxide nanoparticles in some sunscreens can generate hydrogen peroxide in coastal waters at levels high enough to stress algae (Environ. Sci. Technol. 2014, DOI:10.1021/es5020696).

Earlier studies have analyzed the environmental toxicity of certain chemicals in sunscreens, including linking benzophenone to coral bleaching, says David Sánchez-Quiles, a marine chemist at the Mediterranean Institute for Advanced Studies, in Spain. But he realized that there hadn’t been studies of how sunscreens affect algal communities in seawater. These species are important because the entire aquatic food chain rests on energy fixed by algae.

Sánchez-Quiles and colleague Antonio Tovar-Sánchez wanted to track one of the most common UV filters in sunscreens, titanium dioxide nanoparticles, because the metal oxide generates toxic hydrogen peroxide in the presence of light. Silica or alumina coatings around the particles protect people’s skin from the H₂O₂, but these coatings dissolve in water, releasing the naked TiO₂.

The duo hit popular Palmira beach on the Spanish island of Majorca in August 2013 to collect water samples in the bathing area throughout the day. The scientists measured H₂O₂ levels using a chemiluminescent assay and monitored algal biomass by measuring concentrations of chlorophyll a with a fluorometer.

Peroxide concentrations at the beach were less than 100 nM at dawn but peaked around noon at 278 nM, a level known to cause oxidative stress in algae (Mar. Pollut. Bull. 2014, DOI: 10.1016/j.marpolbul.2007.10.017). Previous studies have pegged hydrogen peroxide concentrations in the open Mediterranean Sea at 91 nM.