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Repeat application of zinc oxide sunscreen does not result in deep penetration, cell toxicity

Zinc oxide nanoparticles in sunscreens do not appear to penetrate deeply into the skin or cause cellular toxicity even with repeat application, findings which may help assuage public concerns about health risks from sunscreen use.

The findings come from a paper published online ahead of print in Journal of Investigative Dermatology (Nov. 15, 2018).

In a press release, the lead author on the paper, Professor Michael Roberts, said public concerns about sunscreen toxicity began to grow after previous animal studies found higher skin absorption of zinc-containing sunscreen than indicated by human studies.

“There were concerns that these zinc oxide nanoparticles could be absorbed into the epidermis, with toxic consequences, including DNA damage,” said Dr. Roberts, who is a professor of clinical pharmacology and therapeutics at the University of Queensland in Brisbane, Australia, and a professor of therapeutics and pharmaceutical science at the University of South Australia in Adelaide.

Data from an Australian Cancer Council 2017 National Sun Protection Survey showed a drop in the number of people who believe it was safe to use sunscreens every day.

To investigate the safety of repeat applications of these zinc nanoparticles, Dr. Roberts and his colleagues had five volunteers, aged 20 to 30 years, apply the material to their skin every hour for six hours, for five consecutive days.

The team used multiphoton tomography with fluorescence lifetime imaging microscopy to directly visualize skin penetration of the nanoparticles, as well as and viable epidermal metabolic changes in the volunteers. The fate of the zinc oxide particles was also characterized in excised human skin in vitro.

“We established that the nanoparticles remained within the superficial layers of the skin and did not cause any cellular damage,” Dr. Roberts said.

“We hope that these findings help improve consumer confidence in these products and in turn lead to better sun protection. The terrible consequences of skin cancer and skin damage caused by prolonged sun exposure are much greater than any toxicity posed by approved sunscreens.”

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