The science of sunscreen

Did you know that every two out of three Aussies will have some form of skin cancer by age 70?  We’re the skin cancer capital of the world.

The decision to use sunscreen, and what type to use, is often muddied by conflicting recommendations. As usual, a little understanding of the basic science could go a long way in helping you wade through these decisions.

The danger

When it comes to sunlight, it’s what you can’t see that hurts you. Beyond the blue end of the rainbow lies the ultra-violet (UV) spectrum. This invisible light (i.e. radiation) carries more energy than visible light, and packs enough punch to damage skin cells by splitting open sensitive biomolecules and creating free-radicals – the molecular equivalent of lobbing a hand-grenade at your DNA.

There are different forms of UV light, each with their own dangers.

UV-C is the highest in energy, and most dangerous, but luckily it’s blocked by the ozone layer. Unless you’re a spacewalking astronaut, you don’t need to worry about it. UVB is the next highest in energy. It damages the outermost layer of the skin and so is the main culprit behind sunburn. UVA light, the least energetic, doesn’t cause sunburn, but it can penetrate deeper into the skin, causing wrinkling and raising the risk of skin cancer in the long-term.

Tanning beds are designed to use UVA (rather than UVB) so that you tan, rather than burn, under their blue glow. It turns out that UVA does takes its toll. A 2014 study in the US found that indoor tanning beds are responsible for twice as many cancer diagnoses than smoking. Even one tanning bed session can increase your risk of getting melanoma – the deadliest form of skin cancer – by 75 per cent.

Choosing a Sunscreen

The most obvious factor that jumps out when choosing a sunscreen is its SPF (Sun Protection Factor) rating. But this rating is not as definitive as it sounds.

The main problem is SPF only refers to UVB protection. It says nothing about UV-A. You’ve probably noticed labels like ‘UVA/UVB’, ‘multi-spectrum’ or ‘broad-spectrum’ on sunscreens. These labels mean that UVA and UVB are covered.

What’s more, slapping on SPF-50, rather than SPF-25, does not necessarily double your protection. What SPF actually describes is how much of the UV light reaches your skin. SPF-50 stops 98 out of every 100 photons, while SPF-25 stops 96 out of 100. In this sense the 50-factor only provides an extra 2 per cent protection, not double!

The ‘stuff’ in sunscreen

Take a look at the active ingredients on a bottle of sunscreen and you’ll likely see a list of intimidating names like avobenzone, zinc oxide, titanium oxide, octocrylene, oxybenzone and so on. So what are these ingredients and what do they do?

The idea of sunscreen is to set up a shield around our skin against dangerous UV radiation. There are two main strategies to achieve this—one is to use tiny particles to reflect the light, the other is to use chemicals to absorb it. Many brands use a combination of both.

Chemical ‘sunscreen’

In some circles ‘chemical’ is a dirty word, but of course everything in life, including your own body, is built of chemicals.  Chemical (a.k.a. organic) sunscreens contain molecules which catch UV light like a net, turning it into heat. No one molecule can absorb the whole UV spectrum, so full protection only comes with a mix of different molecules—that’s one reason the list of a sunscreen’s active ingredients can run so long. Because these chemicals need to be absorbed into the skin to work, they should be applied at least 20 minutes (or as recommended on the bottle) before stepping out into the sun.

Physical ‘sunblock’

Physical sunscreens contain tiny particles of minerals like zinc oxide or titanium oxide. With this stuff, you are basically covering yourself with a fine powder that reflects or scatters the light before it hits your skin.  In fact, the use of zinc oxide paste as sun protection goes back thousands of years. These particles protect over a broader range than any organic molecules. The problem is these white particles leave the skin coated with a pasty-looking residue.

In the past couple of decades, scientists found that by shrinking the size of particles down to the nanoscale, they could make an effective sunblock that remained clear. Because materials can take on new properties when shrunk to the nanoscale, people began to wonder whether applying nanoparticles onto the skin could be dangerous. As these particles are so small, could they enter into cells and cause toxic effects?

This uncertainty sparked a huge amount of research into their possible dangers, and by now the results seem to be in. In 2012 the regulatory bodies in the US and the EU published reports saying that nanoscale particles were no more harmful than their microparticle equivalents. Australia’s CSIRO has come to a similar conclusion in several studies, most recently (in 2013) saying “neither TiO2 nor ZnO nanoparticles are likely to cause harm when used as ingredients in sunscreens.”

The general conclusion seems to be that nanoparticles applied to the skin stay there—they don’t migrate into the body and so don’t pose a threat, though nobody recommends ingesting the stuff! After two decades of nanoparticles in sunscreens, they seem to be safe—there have been no reports of adverse health effects in humans.

References:

[Fransen M, Karahalios A, Sharma N, English DR, Giles GG, Sinclair RD. Non-melanoma skin cancer in Australia. Med J Aust. 2012;197(10):565-8.]

[Staples MP, Elwood M, Burton RC, Williams JL, Marks R, Giles GG. Non-melanoma skin cancer in Australia: the 2002 national survey and trends since 1985. Med J Aust. 2006;184(1):6-10.]