The Friday Question: How Does Suncream Work?

Hello again, Team Science! Apologies for the extended blog break, after I got back from eating All The Pasta and Gelato in Italy, I had a busy week at work, I was lucky enough to get to spend some time attending two regional Big Bang Near Me fairs, in Crawley and London, where young people exhibit STEM projects that they’ve carried out. They were brilliant fun, and I’m now completely in awe of the young scientists and engineers of the future!

So, it is now officially summer. It’s July, and in the UK the sun is Actually Shining, which has pretty much stunned the whole nation. I’m going to have to go shopping, because I haven’t yet bothered to buy summer work clothes, I didn’t think it was going to be necessary. Anyway, the point is, the sun is out, and that means so is the suncream. You can probably imagine that with my obsession with understanding how things work on a molecular level, I’ve always been pretty intrigued by suncream. In fact I spend a good 30 minutes reading the back of my suncream whilst lying by a pool in Italy. Never let it be said that I don’t know how to have fun.

A large amount of the energy released by the sun is in the form of UV waves. UV waves are invisible to the naked eye, but that doesn’t make them any less damaging, sadly. UV rays are so potent that if you hung around in outer space stark naked, you’d be burnt in mere seconds. And, of course, you wouldn’t be able to breathe, but that’s not entirely the point. Luckily for us, the Earth’s atmosphere absorbs a lot of UV rays, so we don’t have to spend our whole lives wrapped up in space suits to avoid frying. Some UV-A and UV-B rays do make it through, though, and they are what suncream protects against.

When UV-A enters your skin, it produces reactive oxygen species, chemically active molecules that in high numbers can swarm and attack DNA, I’m envisioning them as the minions in Despicable Me, although they are actually far more efficient than that at causing DNA damage. UV-B by contrast, acts differently. It doesn’t bother creating an army of minions to do its evil bidding and harm your DNA, it ploughs right on in there and does it alone. UV-B causes photo-damage to DNA by forming pyrimidine dimers; these are formed when bits of DNA that shouldn’t really be sticking together, stick together, which messes up the whole beautiful helical thing that DNA has got going on, and prevents the DNA from being replicated or used to produce proteins, or anything else useful like that.

Nefarious UV-V rays messing with your DNA.
Image credit.

Luckily, the human body does have defence against the nefarious UV rays. In the multi-storey scaffold that are the layers of your skin, there live cells called melanocytes, whose main job is to produce melanin, the pigment responsible for your skin colour.  When reactive oxygen species are produced by UV-A waves, this leads to a state known as oxidative stress. Under oxidative stress, melanocytes release any melanin that they have in storage, to try and absorb the radiation and stop it creating minions and causing havoc. This might create a temporary tan, as the melanin reacts, but no extra melanin has been produced, so there’s no additional protection against any more incoming UV rays.

When large amounts of UV-B radiation hits the skin, however, there’s no time to stop it causing photo-damage to your DNA. Melanocytes react to this photo-damage by producing more melanin, to absorb the UV-B. This increase in melanin leads to a change in skin tone, which we see as a suntan or as sunburn. The important thing here though is that to develop a tan, you already have to have suffered damage to your DNA. The production of melanin is stimulated by DNA damage, so if your skin has changed colour, then your DNA has taken a bit of a hammering. There are of course mechanisms in your body to try and fix or correct DNA damage, but you know, it’s usually better not to have to.

So, what can you do to stop DNA damage? Well that’s where suncream comes in. There are actually two types of protection in suncream, sunscreen and sunblock. Sunscreen contains organic and inorganic chemicals, which absorb UV rays. For example,  para-aminobenzoic acid and cinnamates absorbs UV-B waves, and benzophenones and ecamsules absorb UV-A waves. They’re like the front line of sun protection chemicals, a row of cavalry that will stop most, but not all of the UV filtering through. By comparison,  sunblocks reflect the UV rays, they’re like an army wielding shields, scattering the UV waves away so that they never even make it as far as your skin. When suncream seems to make your skin look pale and white, this is the reflective particles, usually zinc or titanium oxide.  Most suncreams these days, however, use tiny reflective nanoparticles, that are invisible to the naked eye, but still pretty bad-ass when it come to deflecting UV.

Your defensive army cannot stand forever though, as many films and books have taught us, so the SPF factor in suncream refers to how long you can stay in the sun without UV-B rays effectively making it through the defensive suncream barriers. Lower SPF factors provide less protection, so with them you can stay in the sun for shorter periods of time without opening yourself up to damage. They also might be more useful for people whose skin is naturally darker, this is because it contains more melanin and is better protected naturally. The paler your skin is, the more you want to supplement your natural protection.

So, that’s suncream. Now next time I get to lie on a sun lounger, I won’t need to wonder why I’ve just spent 20 minutes covering myself in the stuff. Instead I can enthusiastically tell everyone around me about it. I’m pretty sure they’re all looking forward to that.


One thought on “The Friday Question: How Does Suncream Work?

  1. I love sunscreen – it is my saviour – thanks for sharing this – I need ot stop thinking of UVA as having a cute impact though (I love the minions) and remember that it is hurting me (and minions do punch each other…)

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