Australians have absorbed the sun safety message of  ''slip, slop, slap'' as well as could be expected; slopping on sunscreen is now a habit when we’re going out on a sunny summer's day. But what else are we absorbing with the 'slop' part of the process?

As the use of nanoparticles in sunscreen has become more prevalent over the past few years, concern has risen over the potential risks of something that's meant to be keeping us and our families safe. We look at how sunscreens work, what nanoparticles actually are and the latest findings on whether they are a health risk.

How do sunscreens work?

Sunscreen contains one or both of the following types of active ingredients:

  • Chemical absorbers, which absorb UV radiation and stop it reaching your skin. They can irritate and even cause allergies, but of deeper concern is their role as endocrine disruptors and skin penetration enhancers (which have implications for people in contact with other chemicals, such as agricultural pesticides).
  • Physical blockers, such as zinc oxide and titanium dioxide, reflect and scatter UV radiation. They have generally been considered safer and more effective than chemical absorbers, are better for sensitive skin and renowned for their broad- spectrum UV radiation-blocking abilities. It was discovered that micronizing physical blockers resolved the unwanted ghostly look these sunscreens gave, but there have been concerns that these small particles fall into the nano range. Fortunately, the latest advice is that nanoparticles of zinc oxide and titanium dioxide in sunscreen do not pose a health risk.

What are nanoparticles?

Nanoparticles are particles with one or more dimension less than 100nm (where one nanometre is one-billionth of a metre). They exhibit different properties compared with larger particles of the same material, due mostly to the high surface area-to-volume ratio, which can make the particles very reactive. There are various health and environmental concerns around nanoparticles because they're able to penetrate cells in organisms, and there's still more to learn about their interactions with biological systems.

Are nanoparticles in sunscreens a health risk?

In 2013 the Therapeutic Goods Administration (TGA) published a review of relevant scientific papers on the subject, including both published papers and public domain reviews by international authorities. These included several studies, both in-vitro (i.e. laboratory studies) and in-vivo (i.e. studies using humans, other animals or plants). Studies using both animal and human skin have shown that nanoparticles do not penetrate the underlying layers of skin, with penetration limited to the outermost layer of the skin. This suggests that systemic absorption is unlikely.

The Cancer Council Australia reported further in early 2014 that a recent Australian study found that human immune cells (macrophages) exposed to zinc oxide nanoparticles effectively absorbed the nanoparticles and broke them down.

So, zinc oxide and titanium dioxide nanoparticles aren't absorbed through the skin into the bloodstream and, in the event that they do get into the bloodstream (for zinc oxide at least), the body's immune system can deal with them effectively.

The overall conclusion is that there's no known health risk from these nanoparticles and you can safely use sunscreens that contain them.

Then why the concern?

Several years ago, Colorbond painted roofing showed accelerated deterioration in fingerprint-shaped patches. Sunscreen used by builders was suspected, and research published by Colorbond manufacturer BlueScope Steel in 2008 confirmed that certain nanoparticles in titanium dioxide-based sunscreens were the culprit.

Other lab tests have indicated that nanoparticles of zinc oxide and titanium dioxide may create free radicals that cause damage to cellular DNA and mitochondria, particularly in the presence of UV light. Free-radical damage may also lead to cancer. So, if it's destroying painted surfaces and DNA, it's natural there would be concern about putting it on our skin.

The TGA and Cancer Council reports mentioned above show that BlueScope's findings, while valid in themselves, don't necessarily translate to actual health risks from using sunscreen. But while there's no need to avoid using sunscreen that contains nanoparticles, consumers should still be able to choose for themselves, and we believe nanoparticles should be labelled on all products. In 2010, we tested 12 sunscreens and found while several contained nanoparticles, only one contained a significant amount and four were nano-free.

And different manufacturers may use different rules for their claims; some sunscreens may contain nanoparticles, but are labelled as ''nano-free'' because the manufacturer claims the particles actually aggregate or clump into larger particles.  In the absence of regulated, compulsory labelling that forces manufacturers to reveal the presence of nanoparticles, consumers have either had to rely on the assurances of manufacturers that their sunscreens are nano-free – though as it turns out, this is no guarantee – or take their chances with one of the handful of sunscreens that claim to be "natural".

Nanoparticles explained

In order for nanoparticles in sunscreens to be considered dangerous, they have to first penetrate the skin, then go somewhere they can do significant damage. There is currently no solid proof this can happen. 

How it works

To get through the skin, the particles have to be small enough. Zinc oxide and titanium dioxide are often manufactured in nano-size particles, but they tend to clump together. These clumps are known as aggregates, which can in turn clump together to form agglomerates. While agglomerates break up fairly easily, aggregates require enormous amounts of energy to break them, and wouldn't do so in the normal use of sunscreens.

Sunscreen maker Hamilton Laboratories commissioned testing of their own sunscreens, which contained primary particles in the nano range, but in the finished products found no nanoparticles, suggesting the primary particles had formed larger aggregates. When we tested sunscreens in 2010, we found few nano-size particles present in sunscreens, suggesting either the primary particles were mostly aggregating or that the primary particle size used was larger than the nano-range.

What manufacturers say

Manufacturers argue that aggregates over 100nm don't qualify as nanoparticles because they're too big; however, critics argue the aggregates still have similar properties to nano-particles – one of which is high reactivity due to a large surface area: volume ratio – which could be an issue if they penetrate the skin.

Some manufacturers prefer to keep the particles from aggregating because it improves the transparency of the product, and nanoparticles can be coated so they don't aggregate. However, these coated nanoparticles are less reactive than uncoated particles, so even if they do penetrate the skin, they may not do much damage.

What scientists say

Can nanoparticles really penetrate the skin?

The key to the safety debate is whether the nanoparticles can penetrate the outer 'dead' layer of skin into underlying live tissue where they can penetrate living cells and cause damage. Under laboratory conditions with pieces of skin tissue, scientists have been able to get nanoparticles to pass through the tissue.

However, research to date has not found this to be true in live, healthy, intact skin, although there's some suggestion they may penetrate mechanically stressed skin (when exercising, for example) or broken skin (caused by acne, psoriasis or wounds). 
An Australian study found that when zinc nano-compounds were applied to skin, tiny amounts of zinc had penetrated. But it was unclear whether it was actually the nanoparticles themselves, or zinc ions from the nanoparticles which had dissolved and diffused through the skin. Ions are not the same as nanoparticles – and zinc itself is an essential nutrient. If they penetrate, the nanoparticles have to be sufficient in quantity to have a measurable or noticeable impact.

As noted above, the TGA and Cancer Council have published findings showing that there is no evidence that nanoparticles in sunscreen can penetrate the skin in measurable amounts.

How much gets through?

In studies where researchers successfully get metal oxide ions to penetrate skin, the amount getting through appears to be tiny.

Do metal pigment nanoparticles cause DNA damage in living cells?

If the nanoparticle makes its way to living tissue, it has to enter the cell and access cellular components such as mitochondria and DNA to have any effect. Studies using cultured human or mammalian cells, bacteria or naked DNA to test the effect of nanoparticles of all kinds have found damage to DNA. However, in almost all cases the nanoparticles tested were not directly comparable with nano metal oxides: they were different forms of particle or different substances (and the substances tested were often more hazardous than zinc or titanium). As such, results can't be generalised between different forms of nanoparticles and different substances.

Living cells have all sorts of protective mechanisms against oxidative damage that aren't necessarily present in the lab cells, and certainly aren't present in naked bacterial DNA. Some scientists have expressed confidence that these protective mechanisms will kick into action against nano-sunscreens and a recent study supports this.

A study of mice that had been given titanium dioxide nanoparticles in their drinking water found significant DNA and chromosomal damage occurred. So, there was no protective effect evident in that case. Note, though, that consuming water containing titanium dioxide will result in far greater absorption of the particles than applying a sunscreen to your skin, where there is effectively no absorption.

What the regulators say

In Australia, the TGA has jurisdiction over sunscreens – and, as mentioned above, based on a recent review of the evidence its current position is that neither zinc oxide nor titanium dioxide nanoparticles are likely to cause harm when used as ingredients in sunscreens.

Regulators in most other countries also tend to focus on 'new chemicals' and existing chemicals produced at the nanoscale are not considered to be 'new' for regulatory purposes, despite their differing properties. In Europe, however, cosmetics are required to indicate the presence of nanoparticles in their labelling. The rule applies only where the nanoparticles are biopersistent or insoluble, which according to some commenters are ambiguous definitions.

The TGA, in cooperation with international agencies, is continuing to monitor scientific research.

CHOICE verdict

The weight of evidence is clear. There's more risk from not using sunscreen than there is from using it, whether or not it contains nanoparticles. Not using sunscreen can increase your risk of skin cancer, while using it may result in nanoparticles sitting on the surface of your skin (helping to protect you from UV radiation), but they aren't absorbed into your body. Nevertheless, consumers should still be able to choose nano-free sunscreens if they want, and we want products containing nanoparticles to be labelled accordingly.