“Don’t forget the sunscreen!” The imperative cried before a trip to the beach is an important one, with the pasty white lotion saving many a sunbather from sunburns and damage from the sun’s UV rays, which can lead to skin cancer. Many of these lotions use small, invisible nanoparticles of the minerals titanium dioxide and zinc oxide to save your skin. But when the warmth of the sun’s rays encourages us to splash in the waves on the beach, we do not usually think of where washed off sunscreen components are going. When these incredibly tiny nanoparticles enter the environment, they become pollutants and how they interact with light is cause for concern.
The ultraviolet light in sunlight has a high enough energy to break the bonds in titanium dioxide and zinc oxide that hold their electrons in place. When the light hits these nanoparticles, it activates the electrons in the material like they are playing a game of musical chairs. Just as people move and hover over the chairs as the music plays, electrons leave their “chairs” when the light shines. The “chairs” are actually called holes. Both the electrons and holes move throughout the material and when they are able to move to the surface of a nanoparticle, they react with the molecules that surround them, including water. These reactions yield products called reactive oxygen species, some of which have a great deal of energy, and can be harmful.
Wait! If these sunscreens produce damaging reactive oxygen species, should we be worried about their effects on our skin? The answer is no; the nanoparticles in sunscreen are coated by a thin layer of material that prevents the electrons and holes from reaching the particle surface. However, when the sunscreen wearers play in the ocean and their sunscreen washes off, the coatings dissolve and the reactive oxygen species are produced in the ocean water. There, the reactive oxygen species can attack and harm microorganisms in the water. Although microorganisms may seem insignificant, they are very important to ocean ecosystems and the earth as recyclers of nutrients and the base of the food web. Recently, researchers found evidence of elevated levels of these reactive oxygen species in waters off of Mediterranean beach areas, and furthermore, showed that reactive oxygen species produced by sunscreen had negative effects on phytoplankton, photosynthesizing microorganisms in the water.
So should you stop wearing sunscreen? For the sake of your health, probably not; however, the unintended consequences of this human activity and its future effects on marine and lake ecosystems remain to be seen.