Only miniscule amounts are absorbed

Indigo Sharp and Borg Partington
Indigo Sharp and Borg Partington

Brian Gulson, of Nambucca Heads, his colleagues at CSIRO in Sydney and Environment Working Group (based in Washington DC) would like to sincerely thank lifesavers at the Nambucca Heads SLSC for volunteering their services for science, and especially Jayne Morrison.

Charlotte Williams

Charlotte Williams

“I have been part of a team that has written a chapter in a book on ‘Consumer use of nanoparticles in sunscreens’ and we needed an up-to-date photo showing the difference between the older (white) zinc cream and sunscreen containing nanoparticles, which appears clear,” Prof. Gulson said.

“So our local lifesavers agreed to be our models … we are very much obliged.”

Professor Brian Gulson

Professor Brian Gulson

Prof. Gulson said the team looked into what levels of metal oxides (such as zinc and titanium) used in sunscreen, can be absorbed through the skin.

What they found was that small amounts of zinc oxide particles in sunscreens are absorbed through human skin, but the quantities were miniscule (* see below).

“This was in contrast to previous studies that said no traces penetrated the outer skin layer. We found the amounts entering the body were around 1/1000th of zinc concentration already in the volunteers’ bloodstream,” Prof. Gulson said.

“This is about 1/1000th of the amount of zinc recommended in a person’s daily diet. The overwhelming majority of the applied zinc was not absorbed.”

He said the nanoparticles in zinc offered excellent protection from both UVA and UVB rays and that given the tiny amounts absorbed, there was no need for concern.

Further, he noted the body keeps a very tight control on its zinc levels and any excess is passed as waste.

“We need to do more research around this subject, including on titanium dioxide. Until we know more, I suggest that Slip, Slop, Slap is still the best way to go – along with shade and sunglasses and avoiding sun exposure between 10am and 3pm.”

* Although previous research has concluded that nanoparticles do not penetrate healthy skin, it remains contentious whether this conclusion holds under normal conditions of sunscreen use.

Twenty human volunteers were exposed to sunscreens containing zinc oxide (ZnO) particles to determine if Zn from the particles was absorbed through skin over five consecutive days under outdoor conditions at North Curl Curl beach in Sydney.

Two sunscreens were tested—‘‘nano sunscreen’’ containing 19-nm nanoparticles and ‘‘bulk sunscreen’’ containing > 100-nm particles.

Venous blood and urine samples were collected 8 days before exposure, twice daily during the trial, and 6 days post-exposure. As the first application in nanotechnology studies using stable isotope tracing,  ZnO, enriched to > 99% with the stable isotope 68Zn, allowed dermally absorbed zinc to be distinguished from naturally occurring zinc.

The overwhelming majority of applied 68Zn was not absorbed, although blood and urine samples from all subjects exhibited small increases in levels of tracer 68Zn.

Prof. Brian Gulson, Maxine McCall, Michael Korsch, Laura Gomez, Philip Casey, Yalchin Oytam, Alan Taylor, Malcolm McCulloch, Julie Trotter, Leslie Kinsley, and Gavin Greenoakkj

(Graduate School of the Environment, Macquarie University, CSIRO Earth Science and Resource Engineering and others)