Sunscreen is an important part of a complete sun protection strategy in decrease the risk of skin cancers and skin precancers. The active ingredients in sunscreen help prevent the sun’s UV radiation from reaching the skin.
Zinc oxide (ZnO) sunscreens work by scattering the sun’s rays, preventing potentially damaging ultraviolet radiation from getting to the skin. However, while the U.S. Food and Drug Administration includes zinc oxide and titanium dioxide (TiO2) in sunscreens as safe, and both ingredients meet generally recognized as safe and effective (GRASE) status, a recent study demonstrates that sunscreen that includes zinc oxide loses much of its effectiveness and becomes toxic after two hours of exposure to ultraviolet radiation.
The study, funded by the National Science Foundation and the National Institutes of Health supported this research, was conducted by scientists from Oregon State University and the University of Leeds and the findings were published in Photochemical & Photobiological Sciences.
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.
The research team, which included College of Agriculture Sciences faculty Robyn Leigh Tanguay, Ph.D. and Lisa Truong, Ph.D, and graduate fellow Claudia Santillan, sought to answer important but largely neglected questions regarding the massive global sunscreen market, predicted by market data firm Statista to be worth more than US $ 24 billion* by the end of the decade.
The scientists wondered 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?
“Sunscreens are important consumer products that help to reduce UV exposures and thus skin cancer, but we do not know if the use of some sunscreen formulations may have unintended toxicity because of interactions between some ingredients and UV light,” said Tanguay, an OSU distinguished professor and an international expert in toxicology.
“What the public thinks about sunscreen safety has caused manufacturers, often based on limited data, to use more of some ingredients while limiting others,” Tanguay further said.
For example, oxybenzone has effectively been discontinued because of concerns that it harms coral reefs. This discontinuation was not based on public perception but resulted in the State of Hawaii prohibiting the sale and distribution of sunscreen formulas that contain either oxybenzone or octinoxate. 
“And 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,” Tanguay noted.
But while inorganic sunscreens containing TiO2 and ZnO are touted as safer alternatives to small-molecule (“chemical”) sunscreens, the claims of product safety appear to ignore the fact the hazards that can result from UV irradiation of metal oxides in these products, including well-documented generation of reactive oxygen species (ROS) and the degradation of organic compounds. 
Testing UV filters
Scientists including the University of Oregon’s James Hutchinson and Aurora Ginzburg and the University of Leeds’ Richard Blackburn 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 researchers 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 – the specifically intended setting for their use – so it’s pretty surprising how little toxicity testing has been done on the photodegradation products,” Truong explained.
“Our findings suggest that commercially available small-molecule-based formulas, which were the basis for the formulas we studied, can be combined in different ingredient ratios that minimize photodegradation,” Truong added.
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,” Santillan noted.
“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.”
Tanguay said that she was surprised that all five small-molecule mixtures were generally photostable but not surprised that adding zinc oxide particles led to toxicity upon UV irradiation.
“As a team at Oregon State that specializes in studying nanoparticle toxicity, these results were not a shock,” Tanguay said.
“The findings would surprise many consumers who are misled by ‘nano free’ labels on mineral-based sunscreens that imply the sunscreens are safe just because they don’t contain those smaller particles. Any size of metal oxide particle can have reactive surface sites, whether it is less than 100 nanometers or not. More important than size is the metal identity, its crystal structure and any surface coatings,” she concluded.
* In 2020, the global sun care market generated a revenue of 9.3 billion U.S. dollars, a decrease of around 600 million dollar. This was most likely due to the Covid-19 pandemic and the corresponding restrictions. The Statista Consumer Market Outlook however estimates that revenues will again begin to grow starting in 2021 and by 2025 the world wide revenue of the sun care market will amount to 12.5 billion U.S. dollars.
 Ginzburg, A.L., Blackburn, R.S., Santillan, C. et al. Zinc oxide-induced changes to sunscreen ingredient efficacy and toxicity under UV irradiation. Photochem Photobiol Sci 20, 1273–1285 (2021). https://doi.org/10.1007/s43630-021-00101-2
 Bever, L. (2018, July 6). Hawaii just banned your favorite sunscreen to protect its coral reefs. Washington Post. Online. Last accessed on October 13, 2021.
 State of Hawaii. (2018). Relating to Water Pollution. Senate Bill No. 2571, Twenty-Ninth Legislature. Onine. Last accessed on October 13, 2021
 LaMotte, S. (2019, May 6). Sunscreen enters bloodstream after just one day of use, study says. CNN. Online. Last accessed on October 13, 2021.
 Environmental Working Group. (2020). EWG’s 14th annual guide to sunscreens. Online. Last accessed on Octobers 13, 2021.
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