Sunscreens with zinc oxide may become toxic over time, study finds

Photo (c) Carol Yepes - Getty Images

Experts say the products are less effective and potentially dangerous after only a few hours

A new study conducted by researchers from Oregon State University explored some of the risks associated with certain kinds of sunscreens

Their work showed that sunscreens that contain zinc oxide may not be consumers’ best bet for long days in the sun. They learned that after two hours, these sunscreens may start losing effectiveness and become toxic. 

“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 researcher Robyn Tanguay. “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.” 

Toxic after only two hours

To better understand the impact of zinc oxide-based sunscreens, the researchers conducted an experiment on zebrafish. The team created five mixtures that contained the active ingredients in sunscreen and other creams that contain varying amounts of zinc oxide. Then they exposed the mixtures to UV light and observed them for changes. 

The researchers observed the biggest differences in sun protection and overall safety when the zebrafish were given sunscreens that contained zinc oxide in any amount. The researchers found that the ingredient was harmful after only two hours of sun exposure. When sunscreen becomes toxic to the skin, it makes consumers more vulnerable to light exposure. This can ultimately increase the risk of sunburn, blisters, or sun poisoning. 

“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,” said Claudia Santillan. 

While many consumers have started to use mineral-based sunscreens because of the ingredient lists, the researchers explained that any type of sunscreen that contains zinc oxide can pose a risk to consumers’ skin health. 

“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,” Tanguay said. “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.” 

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