After two hours, sunscreen that includes zinc oxide loses effectiveness, becomes toxic: study

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According to scientists from Oregon State University, sunscreens containing zinc oxide, a common component, can lose much of their effectiveness and become toxic after being exposed to ultraviolet radiation for two hours.

The toxic analysis was done on zebrafish. Zebrafish share remarkable similarities to humans at all levels (molecular, genetic, and cellular). This means that many studies using zebrafish are directly applicable to people.

Today's findings were published in Photochemical & Photobiological Sciences.

Robyn Tanguay, a professor in College of Agriculture Sciences, and Lisa Truong, a graduate fellow, were part of the research team. They sought to answer crucial but often overlooked questions about the huge global sunscreen market. Statista had predicted that it would be worth more than $24billion by the end of this decade.

These are the questions: Are sunscreen ingredients safe, effective, and stable when used in combination? This is how they are approved by Food and Drug Administration. What about safety of chemical products that are a result of sunlight-induced reactions?

"Sunscreens are essential consumer products that reduce UV exposures and therefore skin cancer. However, we don't know if some sunscreen formulations can have unintended toxic effects due to interactions between certain ingredients and UV light," stated Tanguay, a distinguished OSU professor and international expert on toxicology.

She said that manufacturers have used a lot of sunscreen ingredients, while keeping others to a minimum, based on what the public believes about sun safety. Concerns that oxybenzone could harm coral reefs led to oxybenzone being effectively withdrawn.

Tanguay stated that sunscreens with inorganic compounds, such as zinc oxide and titanium dioxide, which block UV rays are being promoted more heavily than the organic small-molecule compound compounds that absorb them.

Researchers from the University of Oregon, Aurora Ginzburg, and James Hutchinson of the University of Leeds created five combinations containing UV filters-active ingredients in sunscreensfrom different products in Europe and the United States. They also created additional mixtures containing the same ingredients plus zinc oxide at a lower level than the recommended commercial amount.

The researchers exposed the mixtures for two hours to ultraviolet radiation and then used spectroscopy (spectroscopy) to verify their photostability. This is how sunlight affected the compounds and their UV-protective abilities.

Scientists also examined whether UV radiation caused the mixtures to be toxic to Zebrafish. This is a commonly used model organism that can go from egg to swimming in just five days. The scientists found that the UV-exposed mix without zinc oxide didn't cause any significant changes in the fish.

Truong stated that there have been numerous studies showing sunscreens react quickly to UV exposure. This is in line with their intended use. Truong also said that toxicity testing on photodegradation products has not been done. Our findings show that commercially available small-molecule formulas can be combined with other ingredients to minimize photodegradation.

Scientists found that there were significant differences in phototoxicity and photostability when the nanoparticles of zinc oxide particles were combined with the larger microparticles.

Santillan stated that zinc oxide, regardless of its size, degraded organic mixtures and caused more than 80% loss of organic filter protection against UV-A rays. These rays make up 95% the UV radiation that reaches Earth. The zinc-oxide-induced photosensitization products also caused significant defects in the zebrafish that we used to test for toxicity. This suggests that zinc oxide particles may be a major cause of aquatic ecosystems being exposed to toxic substances.

Tanguay stated that she was not surprised by the photostability of all five small-molecule combinations, but that it did not surprise her that zinc oxide particles caused toxicity when exposed to UV radiation.

She said that the results were not surprising because of the expertise of an Oregon State team in nanoparticle toxicology research. The findings will surprise many consumers who have been misled by the 'nano-free' labels on mineral sunscreens, which imply that sunscreens are safe because they don’t contain smaller particles. No matter how small the metal oxide particles are, they can still have reactive surface locations. The metal identity, crystal structure and any coatings are more important than the size.

Learn more FDA takes new actions to improve the quality and efficacy sunscreens

More information: Aurora L. Ginzburg et. al., Zinc oxide-induced modifications to sunscreen ingredient efficacy & toxicity under ultraviolet irradiation. Photochemical & Photobiological Sciences (2021). Aurora L. Ginzburg et al., Zinc oxide-induced modifications to sunscreen ingredient efficacy toxicity under UV irradiation. (2021). DOI: 10.1007/s43630-021-00101-2

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