How suncream blocks UV

How does sunscreen protect your skin?
10 July 2018

Interview with 

Andrew Farrer, Cambridge Science Centre


According to Cancer Research UK, there were almost 16,000 new cases of melanoma skin cancer in the UK in 2015, with 86% of them preventable. So just how important is sun cream? Chris Smith found out how it works from Andrew Farrer from the Cambridge Science Centre...

Andrew - So what we're going to do is I've got a standard fluorescent highlighter pen, the kind that we've all used when we've had to revise, or marking important documents. What I'd like you to do with it is just write something on your arm please.

Chris - Ok, so I'm taking, I’ll choose yellow because that's nice and bright

Andrew - Yellow is a good colour, yes.

Chris - I think I'm going to write ‘naked’. There we are. So I now have the giant words ‘naked’ in fluorescent ink down my forearm of my right arm.

Andrew - So what's really cool about fluorescent markers is fluorescence is actually a chemical thing where UV light is too high an energy wave from the sun for our eyes to detect. But if it hits a fluorescent chemical, what actually happens is that chemical bounces the UV light back, but it lowers the energy into something visible.

Chris - Right, so it takes the energy out of the UV and then converts it into a color of light that we can see .

Andrew - Effectively, yes. It's the electrons in the atoms and the molecules are absorbing the energy of the UV wave, and they bounce it back - they reflect it back; this is how we see all colour. But the electrons don't give all of the energy back again; they lengthen the wavelength of the light wave, so this becomes visible. So if we have these UV torches that we have here in the studio and shine them on your arm, what we should see is that the word ‘naked’ suddenly becomes very bright.

Chris - So I now have a very brightly glowing ‘naked’ on my forearm!

Andrew - Exactly, and this is the conversion of UV back to visible light and makes it glow. We use this in police vests, in the kind of vest you see on workmen using on the side of the road. So this makes that bright yellow-green that they’re wearing even brighter because the UV from the sun brightens it up again. Well what we have here is a tub of a factor 50 sunscreen, and what I'd like you to do now is just take a small dab of it on your fingers and just cover part of that word, the word ‘naked’.

Chris - So you want me to smear a dab of...

Andrew - Just to smear - dab the other half of the world so we can see the comparison.

Chris - Right. So I’m just - I’m just liberally putting this over half the word naked. I might have got a bit carried away, but I’m now covered in suncream as well - I won't touch any more knobs in the studio. Right. Okay.

Andrew - So the point of our suncream is to block the UV from reaching out skin, so it’s going to absorb all of that UV light before it hits your skin and also now before it hits the ink of the highlighter. So if we shine the UV on it now, the part of the word that is still exposed, that you didn’t put suncream on, still glows, when underneath the cream, there’s no glow at all. We can still see the yellow colour faintly, the actual ink, the normal colour, but we don’t get that fluorescence happening.

Chris - I mean the amount of UV light from this UV torch I’m shining on my arm that’s reaching the dye to make it glow is massively reduced where the suncream is. How exactly does the suncream do that?

Andrew - Exactly. Well, a factor 50 absorbs the UV light. Well, all suncream absorbs the UV light, about 2 percent of the UV rays will get through a factor of 50. But just like any light is absorbed by the molecular structure of whatever chemical, whatever substance it’s bouncing off, in some cases it’s bounced back at us. In this case the suncream is just retaining all of that energy, converting it in a different way.

Chris - And Jane when sun cream says factor 50 or factor 30, in practical terms, what does that mean?

Jane - that number number relates to the amount of time you should be able to spend in the sun, so that you can judge how much damage the sun is likely to do to you. But the problem about the way you were putting the cream on, if I might say, is that you had a big thick smear. And nobody wanders round the street, or lies on the beach, or goes for a swim with that sort of thickness of sunscreen on, and the tests that were done, when they're done, they assume quite a thick layer of sunscreen, and most people put a tiny smear on, and so they're not really getting the proper protection from their sunscreen.

Chris - So you’re saying you should be doing what I did on my arm, because when I go on the beach or when I go sailing and things, I do put a really thick smear on my nose, because I'm aware of the fact my nose has been burned in the past and also it's quite exposed. So that’s what you should do.

Jane - Absolutely, the ears and nose and top of your head are really, they get a lot of sun. So in an ideal world you would put a thick layer on, but of course nobody is going to do that, so you do need to make sure you repeat application during the day. It's estimated that if you are lying on a beach, let's say in your swimming trunks, and you’re just going to stay there all day, you should certainly apply cream at least three times a day and that you should use about 30 mls or 30 grams for each application. So most people when they go on holiday they probably take one bottle of sun protection cream with them, and that's way not enough. So the other thing is the two different types of sun protection in sunscreens, and the best ones to go for are ones that both reflect and also absorb. You were talking about the absorbing, absorbing properties of suncreams, but the ones that reflect which of course perhaps do look white when you put them on, they give you extra and better protection than just using the energy absorbing ones.


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