Frequently asked questions (FAQs) on sunscreens

What research is CSIRO doing into sunscreens?

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Our experiments on nanoparticles in sunscreens are designed to answer the following questions:

  • do zinc oxide nanoparticles in sunscreen penetrate skin when used under normal conditions?
  • what are the effects on health from long-term use of sunscreens containing metal-oxide nanoparticles?
  • what is the long-term fate of metal-oxide nanoparticles in the Australian environment and are there risks to biota from waste disposal, from spills or general dispersion in the environment?

Skin penetration by zinc oxide particles

A study on skin penetration by zinc oxide particles has been conducted in collaboration with Macquarie University, Sydney, Australia.

The results have been published in a peer-reviewed journal. Gulson B et al. 2010. Small amounts of zinc from zinc oxide particles in sunscreens applied outdoors are absorbed through human skin (abstract). Toxicological Sciences. 118(1): 140-149.

Two special sunscreens were made especially for this study. One sunscreen contained nanoparticles of zinc oxide and the other contained larger (non-nano) particles, to see if there was a dependence on particle size for dermal absorption.

The study was performed on human volunteers at a Sydney beach in March 2009.

Sunscreen was applied twice daily for five days. Numerous blood and urine samples were obtained before, during, and after the period at the beach.

"The study found that zinc from zinc oxide particles in sunscreens can penetrate human skin, and appear in blood and urine."

To distinguish between zinc in blood and urine that may have come from the sunscreens and that which is in blood and urine from natural sources, the special sunscreens were made with zinc oxide enriched to 99 per cent with a stable, non-radioactive isotope of zinc.

Skin penetration of zinc from either sunscreen is indicated by changes in ratios of zinc isotopes in the blood and urine samples.

The study found that zinc from zinc oxide particles in sunscreens can penetrate human skin, and appear in blood and urine.

Importantly, the form of zinc that entered the body has not yet been identified.

The zinc could have penetrated as dissolved zinc ions, or as nanoparticles of zinc oxide, or both.

Further studies to investigate this are under way.

The total amounts of traceable zinc from the sunscreens detected in blood and urine were very small when compared with amounts of natural zinc normally present in the human body.

The study also found that blood and urine samples from females receiving the sunscreen with nanoparticles had slightly higher levels of traceable zinc than those from females receiving the sunscreen with larger particles, and males receiving either sunscreen.

A similar study has been carried out using the same sunscreens with traceable zinc applied to hairless mice. This animal study provided information on the distribution of zinc from the sunscreens to internal organs.

The results have been published in a peer-reviewed journal. Osmond-McLeod et al 2013. Dermal absorption and short-term biological impact in hairless mice from sunscreens containing zinc oxide nano- or larger particles. Nanotoxicology. doi:10.3109/17435390.2013.855832.

Higher concentrations of traceable zinc were found in all organs of mice receiving the sunscreen with nanoparticles compared with mice receiving the sunscreen with larger particles.

Importantly, there were no changes in concentrations of total zinc (a combination of zinc from the sunscreen and zinc which is naturally present, for example from the diet). This indicates that mechanisms to maintain appropriate levels of zinc in mice were effective.

There was a mild inflammatory response in some mice receiving either of the two sunscreens and also the control group which received the sunscreen base without zinc oxide particles. This indicates that the inflammatory response was caused by an ingredient in the sunscreen base, and not the zinc oxide particles.

As for the human study, it is not yet known if the traceable zinc was absorbed as dissolved zinc ions or zinc oxide particles, or a combination of the two. However, the maintenance of total zinc concentrations suggests that the traceable zinc is present as zinc ions.

Effects on health from long-term use of sunscreens

Effects on health from long-term use of sunscreens are being investigated in animal studies.

Three commercially-available sunscreens are being tested, with and without exposure to simulated solar radiation.

Of the sunscreens being tested, one contains zinc oxide nanoparticles as the active ingredient, a second contains titanium dioxide nanoparticles, and the third contains no nanoparticles, just molecular UV-absorbers.

To aid our understanding of what happens when zinc oxide particles interact with cells, we are also carrying out experiments with different types of cultured cells to determine the effects of varying particle size, shape, coatings, and internal composition.

Information from these experiments may contribute to the design of safer particles (if necessary) for use in sunscreens and other applications.

Environmental effects

Environmental effects of nanoparticulate zinc oxide are being assessed in both soils and in aquatic systems.A study to determine the mechanism of toxicity of zinc oxide particles to an alga has been published in a peer-reviewed journal. Franklin NM, Rogers NT, Apte SC, Batley GE and Casey PE. 2007. Comparative toxicity of nanoparticulate ZnO, bulk ZnO and ZnCl2 to a freshwater microalga (Pseudokirchnerilla subcapitata): the importance of particle solubility. (abstract). Environmental Science and Technology. 41: 8484-8490. Both large and nano-sized particles of zinc oxide were found to be slightly soluble in water.

A dialysis technique was used to separate the particulate zinc oxide from the truly dissolved zinc ions in solution.

Toxicity to a sensitive fresh-water alga was caused by zinc ions (from dissolution of both the large and nano-sized zinc oxide particles) rather than by the particles directly.

The solubility of zinc oxide particles exceeded water quality guidelines for zinc, and so risks to aquatic systems would be well managed by current water quality guidelines.

For more information, refer to our work on Understanding nanosafety.