Why are nanoparticles used in sunscreen

Louise Sales

You may have come across recent media reports that "nanoparticles in sunscreen are harmless" on the basis of a recently published study.[1] Sounds good huh? However, unfortunately these reports don't reflect either the study's own conclusions or the current state of the science.

Whilst the results of this lab study are interesting, the study is restricted to zinc oxide and importantly draws no conclusions about the safety of nano-ingredients in sunscreen. It also doesn't look at other nano sunscreen ingredients such as titanium dioxide and cerium oxide. More studies are needed reflecting real life conditions before any conclusions even about the safety of nano zinc oxide in sunscreen can be drawn.

The study found that when white blood cells are exposed to zinc oxide nanoparticles in the lab, they absorb some of them and some of the particles dissolve. Only one white blood cell was looked at for this analysis. It is completely inappropriate to make inferences about safety based on the results of one in vitro study.

What do we know about the safety of nano-ingredients in sunscreen?

From the research that has been carried out, we know that surface area plays a key role in the toxicity of nanomaterials. As we reduce the size of particles, the larger relative surface area increases the potential for free radical production to damage proteins and DNA. Accordingly, the leader of CSIRO's Nanosafety group warned in 2008 that in a worst-case scenario, nano-ingredients in sunscreens could cause skin cancer.[2]

Our sunscreen regulator, the Therapeutic Goods Administration (TGA), argues that the majority of studies suggest that nanoparticles don't penetrate the skin so there is no reason for concern. However the majority of studies that have been conducted are short term; use excised skin in a lab; fail to consider the role of skin condition (e.g. eczema, acne, sunburn, children with thinner skin); and do not assess the role of penetration enhancers − despite the prevalence of these substances in sunscreens, cosmetics and workplaces.

A 2010 study by Gulson et al. found small amounts of zinc from sunscreen in the blood and urine of human trial participants.[3] Some scientists have argued that, since the amounts of zinc found in the blood and urine were small there is no cause for concern.

However, one interesting finding reported in a later paper by Gulson et al. was that the highest levels of zinc isotope were actually found nine days after the five day application period had ended. The scientists aren't really sure why this was the case. They suggested that the nanoparticles could be accumulating in the skin and acting as a long-term chemical reservoir. This is obviously of concern if they react with sunlight and produce free radicals while they are there. Or they could be accumulating elsewhere in the body − such as the liver or muscle.[4]

The study was not able to show whether the zinc oxide was absorbed in nanoparticle form or whether it dissolved, so this requires further research. Zinc oxide is fairly soluble so it is possible that it dissolves in the body. This may mean that the body's defences will be able to deal with it – since our body has mechanisms to regulate zinc levels. This is why the James et al. study [1] is interesting – as it shows that in the lab white blood cells may be able to take up and dissolve zinc oxide nanoparticles. However, further studies are needed before conclusions can be drawn about what really happens in the body.

Furthermore, these findings can't be extrapolated to other nanomaterials used in sunscreen such as titanium dioxide and cerium oxide. Titanium dioxide for example, is less soluble that zinc oxide and not a chemical that our body is naturally exposed to. In fact, the European Commission's Scientific Committee on Consumer Safety recently recommended that certain nano titanium dioxide (TiO2) ingredients not be used in sunscreen because they strongly react with sunlight to produce free radicals. It also recommended that nano TiO2 and nano zinc oxide (ZnO) not be used in powder or sprayable products because of the toxicity risk associated with inhalation.

A recent Italian study using pig ear skin found that nano TiO2 damaged the outer layer of skin. The researchers warned that this could allow nanoparticles and other unwanted chemicals to penetrate the skin − posing a potential human health risk.

The European Chemicals Agency is also currently reviewing the safety of titanium dioxide (including the nano form) because of concerns it may be harmful to the environment and human health. Meanwhile our regulators here have taken no action to remove these ingredients from sunscreen.

Some industry spokespeople have argued there are nanoparticles in everything and that our body does appear to have defences to deal with these. It's true that any finely ground powder will have a small tail end of nanoparticles. However, it can't be assumed that because our body has the defences to deal with occasional nanoparticles that it can deal with products that are entirely comprised of nanoparticles.

It's also important to realise that not all nanomaterials are the same. Their properties, toxicity and the extent to which they penetrate the skin will vary depending on a range of factors including shape, size, surface coating and charge. This illustrates why it is important that all nanomaterials undergo thorough safety testing before they are used in consumer products.

So what can I slip, slop, slap on my skin?

Friends of the Earth recently selected several Australia sunscreens which we hoped were free from untested and unsafe nano-ingredients. We submitted these sunscreens for testing by the Government's National Measurement Institute, with the hope of being able to offer some non-nano sunscreen options to stay sun-safe, while avoiding participating in the nano-experiment. Sadly, all the tested sunscreen products were found to contain a high proportion of nanoparticles. Therefore, we are in the difficult situation of not being able to recommend any non-nano sunscreen products at the moment.

This is not to say we believe all zinc oxide or titanium dioxide based sunscreens are using nano-ingredients. We will continue to research potential options and are hopeful that we will have some brands that we can recommend in the near future.

Unfortunately, many sunscreen products that don't use zinc oxide or titanium dioxide instead rely on endocrine-disrupting chemicals such as 4-methylbenzylidene camphor and octyl methoxycinnamate which we would not recommend using either.

Given the uncertainty over what to slop on your skin, we advise people to closely follow the SunSmart guidelines:

• Slip on sun-protective clothing that covers as much skin as possible;
• Slop on SPF30+ sunscreen – make sure it is broad spectrum and water resistant;
• Slap on a hat that protects your face, head, neck and ears;
• Seek shade;
• Slide on sunglasses – make sure they meet Australian Standards.

References:

[1] James, S.A. et al. (2013) Quantification of ZnO Nanoparticle Uptake, Distribution, and Dissolution within Individual Human Macrophages, ACS Nano, 7(12):10621-10635.

[2] 7.30 Report: Safety concerns over high-tech sunscreens, 17 Dec 2008 www.abc.net.au/7.30/content/2008/s2449409.htm

[3] Gulson B. et al. (2010) Small amounts of zinc from zinc oxide particles in sunscreens applied outdoors are absorbed through human skin. Toxicol Sci 118 (1): 140-149.

[4] Gulson, B. et al. (2012) Comparison of dermal absorption of zinc from different sunscreen formulations and differing UV exposure based on stable isotope tracing, Sci Total Environ. 420:313-318.

Louise Sales is the Nanotechnology Project Coordinator at Friends of the Earth Australia. www.nano.foe.org.au, [email protected]

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