Scientists have now discovered that sunscreen that includes zinc oxide, a common ingredient, loses much of its effectiveness and becomes toxic after two hours of exposure to ultraviolet radiation.
Scientists sought to answer important but largely neglected questions regarding the massive global sunscreen market, predicted to be worth more than $24 billion by the end of the decade. Sunscreens containing inorganic compounds like zinc oxide or titanium dioxide, that block UV rays, are being marketed more and more heavily as safe alternatives to the organic small-molecule compounds that absorb the rays. The questions: How stable, safe, and effective are sunscreen ingredients in combination rather than as individual compounds — which is how they are considered for Food and Drug Administration approval — and what about the safety of any chemical products that result from reactions caused by exposure to sunlight?
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 at the lower end of the commercially recommended amount.
The researchers then exposed the mixtures to ultraviolet radiation for two hours and used spectroscopy to check their photostability — i.e., 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. Zebrafish, share a remarkable similarity to humans at the molecular, genetic and cellular levels, meaning many zebrafish studies are immediately relevant to people.
The researchers found that the UV-exposed mixture without zinc oxide did not cause any significant changes in the fish. But scientists saw big differences in photostability and phototoxicity when zinc oxide particles were added — either nanoparticles or the larger microparticles…
“With either size of particle, zinc oxide degraded the organic mixture and caused a greater than 80% loss in organic filter protection against ultraviolet-A rays, which make up 95% of the UV radiation that reaches the Earth. Also, the zinc-oxide-induced photodegradation products caused significant increases in defects to the zebrafish we used to test toxicity. That suggests zinc oxide particles are leading to degradants whose introduction to aquatic ecosystems is environmentally hazardous.”
-Claudia Santillan, researchers and graduate fellow
Journal Reference: Aurora L. Ginzburg, Richard S. Blackburn, Claudia Santillan, Lisa Truong, Robyn L. Tanguay, James E. Hutchison. Zinc oxide-induced changes to sunscreen ingredient efficacy and toxicity under UV irradiation. Photochemical & Photobiological Sciences, 2021; 20 (10): 1273 DOI: 10.1007/s43630-021-00101-2