Sunscreen that includes zinc oxide, a common ingredient, loses much of its effectiveness and becomes toxic after two hours of exposure to ultraviolet radiation, according to a collaboration that included scientists from the Oregon State University and Leeds University.
The toxicity analysis involved zebrafish, which share a remarkable similarity to humans at the molecular, genetic and cellular levels, meaning many zebrafish studies are immediately relevant to people. Findings were published Thursday in Photochemical & Photobiological Sciences.
Scientists made five mixtures containing the UV filters -- the active ingredients in sunscreens -- from different products available in the United States and Europe. They also made additional mixtures with the same ingredients, plus zinc oxide.
The researchers then exposed the mixtures to ultraviolet radiation for two hours and used spectroscopy to check their photostability--what did sunlight do to the compounds in the mixtures and their UV-protective capabilities?
The scientists also looked at whether the UV radiation had caused any of the mixtures to become toxic to zebrafish, a widely used model organism that goes from egg to swimming in five days, and found that the UV-exposed mixture without zinc oxide did not cause any significant changes in the fish.
"There have been several studies that showed sunscreens can quickly react under UV exposure. So it's pretty surprising how little toxicity testing has been done on the photodegradation products. Our findings suggest that there are big differences in photostability and phototoxicity when zinc oxide particles were added -- either nanoparticles or the larger microparticles," said Robyn Tanguay, international toxicity expert and professor at the University of Oregon.
"With either size of particle, zinc oxide degraded the organic mixture and caused a greater than 80 percent loss in organic filter protection against ultraviolet-A rays, which make up to 95 percent of the UV radiation that reaches the Earth," he added.