Derby British Science Association

Science and Engineering in Derby and Derbyshire

Let the sun shine

Picture by Nick Ares (Aresauburn on Flickr)

It’s sunny! Yes unbelievably that rarest of things has made it to the Midlands, and it is summer. All around people are sporting sunglasses, stripping to skimpies and sensibly slapping on sunscreen.
Years of public health messages, and more than a few adverts, have eventually convinced most of us that before we step out to enjoy the weather we need to protect ourselves. What though, is sunscreen protecting against? And what does it do? Or not do?
It’s a shame about rays…
If you’ve ever spent much time examining those sunscreen bottles you will have noticed they are festooned with a range of initials. The most prominent of which is generally UVB, and increasingly, UVA is mentioned too. Now you’ve probably guessed that the UV part stands for Ultra Violet. Even if you don’t remember much of school physics, you probably remember that light is on a radiation spectrum with Gamma at one end, through to Radio at the other. Gamma is short very energetic waves and very dangerous. Whereas Radio is longer, low energy waves, and only really dangerous if you fling your wireless at someone. Whilst Visible light is pretty much in the middle of the spectrum, sandwiched between Infrared and Ultraviolet.
Ultraviolet rays are divided into three types, depending on the length of their waves. The furthest from Visible light is UVC, which has the shortest type of wavelengths. It is dangerous, but fortunately as it has such short wavelengths it has not been found to penetrate the atmosphere. UVB is the wavelength sunscreen is most concerned with, and has medium sized waves. Though most of the UVB that comes from the sun is filtered by the atmosphere, enough gets through to help people develop burns, wrinkles and skin cancer. All sunscreens will mention something about their UVB protection levels. Finally there is UVA, which is the closest to Visible light, and a relatively long length wave. This is what forms most of the UV on earth (about 95%), and is responsible for tanning and freckling. It is only recently that it has been found that UVA may be playing a role in skin cancer development too, and for this reason many sunscreens have begun to incorporate UVA protection.
Not much, too young
The next initialisation, and the one that gets beauty counter ladies very excitable, is SPF. Sun Protection Factor is a measure how much exposure, or more commonly how long you can be exposed for, without being burned. The SPF number is reached by taking the amount of time it would take protected skin to be burned, and dividing that by the amount of time it would take unprotected skin to be burned. When I was at school my mum had to get factor 50 on prescription, nowadays everywhere has at least SPF60, and some ranges go up to 100. Which sounds amazing, doesn’t it? Indeed SPF 100 sort of sounds as though you have some kind of X-Men level of sun protection.
Sadly though, you don’t. For several reasons. The first reason is what IT support might refer to as a user error, however, unlike using your CD drive as a cupholder, this isn’t really user fault. In amongst all that tiny writing on the back of the sunscreen it should tell you how much to use. The amount of sunscreen used to test SPF levels in the laboratory, and the amount that you are supposed to use is 2mg per cm2. Which, unless you happen to have amazing visual acuity for mass, or are a chemist, could frankly mean any amount. At such a small number you couldn’t even weigh yourself before and after. So it is no surprise really that researchers (Kim et al 2009) found that most of the people they surveyed were only using 0.5mg/cm2. SPF16 when applied at quarter levels means effectively only SPF2, which is considerably less protection.
The second reason that you still get a burned nose is frequency of application. High summer isn’t the only time you should be slathering the sunscreen on, as regular snow bunnies will know. You should be applying it almost half an hour before you go en piste, or en plage, and then at least every two hours. In a study of 4837 adult skiers (Buller et al 2010) only 4.4% were found to be following the instructions fully. Those that were least likely to reapply included men and those who believed their skin to not be sun sensitive.
Thirdly, and partly the reason you should reapply, is the effect of sand, water, and even just sweating. In an earlier study (Diffey 2001) it was found that people overestimate the abilities of sunscreen to protect them against melanoma. Which some critics suggest has lead to an over-reliance on sunscreen, and increased the risk of skin cancer because people believe they can stand longer exposure.
It’s different for sunscreens
Not all sunscreens are created equally. Even when you’ve got past the are they UVB, or UVB and UVA, thing, you’ve got the mix of ingredients to consider. That tiny writing reappears once more, and this time with more syllables per word, than meals in the day. The likes of butyl methoxydibenzoylmethane and benzophenone-3 might trip off the tongue of the average chemist, but they’re likely to be things most people don’t pay much attention to. However, if you can stand that miniscule font, you maybe should. Many sunscreens make use of minerals as protective agents, and one research group (Serpone, Dondi and Albini 2005) found that generally these begin to lose their efficacy after a relatively short exposure to UVB and UVA. Whilst two more sets of research teams (Pescia et al 2011, and Gaspar and Campos 2005) investigated the specific combinations of ingredients used in the creams and found that even those labelled with the same SPF differed in the level of protection they provided. Those creams with photo-unstable ingredients, that is they lose their level of protection relatively fast, were found to provide less long lasting protection against UVA. The first group compared four preparations of sunscreen, and the more recent study compared two. The best performing sunscreens in both studies contained benzophenone-3, and octocrylene.
Sunscreen is important, and can help protect skin against damage, but only if used appropriately. What the research can tell us, to get the most out of it is: put more on, put it on more often, and read the back of the bottle.
Research Mentioned
David B. Buller et al “Compliance with sunscreen advice in a survey of adults engaged in outdoor winter recreation at high-elevation ski areas” in Journal of the American Academy of Dermatology January 2012

Brian Diffey “Sunscreen isn’t enough” in Journal of Photochemistry and Photobiology B: Biology 15 November 2001

L.R. Gaspar and P.M.B.G. Maia Campos “Evaluation of the photostability of different UV filter combinations in a sunscreen” in International Journal of Pharmaceutics 13 January 2006

Sang Min Kim et al “The relation between the amount of sunscreen applied and the sun protection factor in Asian skin” in Journal of the American Academy of Dermatology February 2010

Nick Serpone, Daniele Dondi, and Angelo Albini, “Inorganic and organic UV filters: Their role and efficacy in sunscreens and suncare products” in Inorganica Chimica Acta 15 February 2007

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About Lotte

Likes making a mess and arguing.

One comment on “Let the sun shine

  1. Administrator
    May 28, 2012

    It’s easier (and safer) just to stay indoors and read blogs

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This entry was posted on May 26, 2012 by in Biology, Chemistry, General Science, Medicine and tagged , , , .

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