Many sunscreens contain zinc oxide and titanium dioxide as these compounds absorb UV radiation and provide excellent protection against sunburn. Early sunscreens containing zinc oxide particles were applied as a white paste to the skin, but modern sunscreens containing nanoparticles are transparent on the skin and are more attractive. However, due to their much smaller size compared with particles used previously, concerns have been raised over the potential for nanoparticles to penetrate the skin.
To investigate dermal absorption from sunscreens containing zinc oxide, we have performed studies on human volunteers1,2 and hairless mice. To determine if there is a dependence on particle size, we used two sunscreens with one containing nanoparticles (20nm) and the other larger particles (>100nm). The sunscreens were made with zinc oxide enriched to >99% with a stable, non-radioactive isotope of zinc, 68Zn. This enabled discrimination between zinc from the sunscreens and normal endogenous zinc. Dermal absorption of zinc from either sunscreen was indicated by changes in ratios of zinc isotopes in biological samples, as measured by multi-collector inductively-coupled plasma mass spectrometry.
The study to assess dermal absorption in humans was performed at a Sydney beach, with volunteers undergoing normal activities including exposure to the sun. The sunscreens were applied twice daily for five consecutive days. Numerous blood and urine samples were obtained before, during, and after the 5-day period at the beach.
The study to assess dermal absorption in immune-competent, hairless mice was conducted using the same sunscreens. Here, the presence of zinc from sunscreen was measured in blood and internal organs. As mouse skin is more permeable than human skin, this study represents a “worst–case” scenario for human skin and provides information on what may happen when sunscreen containing zinc oxide particles is applied to compromised skin.
Results from both studies will be presented.