Australian Researchers Find Zinc out of Zinc Oxide Sunscreens Penetrate Skin of Human Test Subjects
Most reviews in recent times came to the conclusion that nanoparticles in sunscreens do not penetrate the outer horny layers of skin and would therefore not reach the blood circulation. However, most of these investigations base their findings on tests with cell cultures or animal models. Australian researchers now provide first real exposure evidence that small amounts of zinc out of zinc oxide sunscreen formulations may penetrate the intact human skin and be translocated via blood and urine.
Zinc oxide is used in sunscreens to reflect and scatter sunlight. By shrinking the zinc oxide particles to the nanometer scale, the particles themselves become invisible and the sunscreen forumlations can be made transparent which is considered an advantage over conventional, white sunblocks.
While in the USA and in Australia, ZnO is approved for use as UV filter, in the EU and in Switzerland it is not allowed. In Germany, an exemption clause is in force permitting the use of ZnO as a UV filter until 2010. While production and import of new zinc oxide containing sun protection products is prohibited in Switzerland, selling products that have previously been on the market is allowed.
In 2009, the Swedish Drug Administration has banned in 2009 10 companies from marketing sunscreens containing zinc oxide particles as UV filters. As the use of zinc oxide is currently not permitted under current Swedish and EU legislation, the Drug Agency ruled that the companies are no longer permitted to sell the examined sunscreen products containing zinc oxide.
Examples of research that found no evidence that zinc oxide nanoparticles would penetrate healthy human skin include an analysis of over 400 peer reviewd literature sources and over 2000 sunscreen products conducted by a NGO called Environmental Working Group. Most recently, US FDA researchers have also confirmed that nanoscale sunscreen ingredients do not penetrate healthy skin.
Geochemist Brian Gulson, of Sydney's Macquarie University, has now provided contrary evidence showing that zinc oxide nanoparticles can be absorbed by the body and remain there for extended periods of time. The researchers used a stable isotope of zinc to trace the zinc oxide once it was dermally applied (in a sunscreen formulation) on 20 test persons.
Blood and urine samples were taken several times daily, and suncreen was applied twice daily over five days. All subjects are reported to show significant increases in the abundance of the zinc isotope six days after completion of the trial.
Some interesting points are also revealed by the study: Both sunscreens with nanoparticles and larger particles (non-nano, > 100 nm) led to very similar results, indicating that the particle size of the zinc oxide in the sunscreen formulation was not a decisive factor for the dermal absorption of zinc. The authors also make clear that it is not known yet due to limitations of the isotope methodology, whether zinc oxide (nano)particles penetrated the skin, or whether it was dissolved zinc ions.
The study does not address the impacts that zinc nanoparticles might have on the body's cells or immune system and does not say whether the amounts of zinc found would be critical.
After all, any possible effects of nanoscale sunscreen ingredients in the body need to be balanced against the efficacy of the sunscreen to prevent skin damage through UV radiation - where zinc oxide and titanium dioxide nanoparticle sunscreens are particularly promising.