Zinc oxide in sunscreens may react with other ultraviolet (UV)-blocking agents, reducing their effectiveness and potentially forming harmful compounds.
This is the conclusion of a recent study published in Photochemical & Photobiological Sciences (Oct. 14, 2021; 20:1273–1285).
“On its own, zinc oxide is an effective and harmless UV blocker. Our research raises concerns about how the individual formulation ingredients react with each other during use and this isn’t currently tested by the industry,” said study author Richard S. Blackburn, PhD, in a press release.
Dr. Blackburn is Professor of Sustainable Materials at the University of Leeds School of Design in Leeds, UK.
During tests on several sunscreen formulations, the researchers examined how zinc oxide reacted to other ingredients when it was exposed to sunlight. They also calculated how much UVA the sunscreen blocked to determine the UVA protection factor for each mixture.
“During tests, we found that zinc oxide causes degradation of other UV absorbers, and the protection provided by the sunscreens was reduced significantly over a relatively short time, particularly in the UVA region,” said study co-lead author James Hutchison, PhD, in the release. Dr. Hutchison is the Lokey-Harrington Chair in Chemistry at the University of Oregon’s Phil and Penny Knight Campus for Accelerating Scientific Impact in Eugene, Ore.
The researchers discovered that the UVA protection factor was reduced by between 84.3% and 91.8% after two hours of UV exposure in sunscreens mixed with zinc oxide particles. In comparison, sunscreen without zinc oxide showed only a 15.8% loss in UVA protection factor with the same UV exposure.
“The degradation of the UV filters not only decreased the effectiveness of the sunscreen, it also led to increased formula toxicity,” said Dr. Hutchison.
“So not only is the lack of effective UV protection an issue, the product itself may be causing harm during use in the sun,” he said.
The authors also note that zinc oxide-containing sunscreens can also come in contact with other UV-protective agents when layered with sun-protective cosmetics.
They suggest that their findings are a call to action for the development of new UV-protective agents.