Zinc Oxide, widely used in broad-spectrum sunscreens, can be safely applied in repeated doses without the risk of local skin toxicity, according to the findings of a recent study.1
Recently, the belief that zinc oxide was toxic to humans has grown among consumers as public advocacy groups raised questions on its safety following animal studies that suggested higher absorptions of the nanoparticles compared with human studies.
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“There were concerns that these zinc oxide nanoparticles could be absorbed into the epidermis, with toxic consequences, including DNA damage,” said corresponding author Michael Roberts, PhD, chair of Therapeutics and Pharmaceutical Science at the University of South Australia and professor of Clinical Pharmacology and Therapeutics at the University of Queensland.2
He and his colleagues assessed local skin toxicity among 5 healthy volunteers. The volunteers applied sunscreen every 4 to 6 hours for 5 days. Skin penetration of intact zinc oxide and zinc ions and viable epidermal metabolic changes was assessed using multiphoton tomography with fluorescence lifetime imaging. In addition, the fate of zinc oxide was characterized using excised human skin in vitro.
While zinc oxide accumulated on the skin’s surface and within skin furrows, it did not enter or cause cellular toxicity in the viable epidermis. However, zinc ion concentrations were slightly elevated in the viable epidermis of excised human skin.
“In conclusion, repeated application of zinc oxide to the skin, as used in global sunscreen products, appears to be safe, with no evidence of zinc oxide penetration into the viable epidermis nor toxicity in the underlying viable epidermis,” the researchers concluded.
“We hope that these findings help improve consumer confidence in these products and in turn lead to better sun protection,” Dr Roger added. “The terrible consequences of skin cancer and skin damage caused by prolonged sun exposure are much greater than any toxicity posed by approved sunscreens.”
1. Mohammed YH, Holmes A, Haridass, et al. Support for the safe use of zinc oxide nanoparticle sunscreens: lack of skin penetration or cellular toxicity after repeated application in volunteers [published online November 15, 2018]. J Invest Dermatol. https://doi.org/10.1016/j.jid.2018.08.024
2. Keep slapping on that sunscreen and ignore toxic claims [press release]. Adelaide, Australia: University of South Australia; November 21, 2018. http://www.unisa.edu.au/Media-Centre/Releases/2018/keep-slapping-on-that-sunscreen-and-ignore-toxic-claims/#.XAFMrRNKhTY. Accessed November 30, 2018.