How Does Sunscreen Work?

Image designed by Matthew Bird using Canva
"The Sun". Licensed under CC BY-SA 2.5 via Wikimedia Commons.

Sometimes understanding the science behind something will not only benefit you intellectually, but physically as well. Sunscreen is one of the those cases. By knowing how sunscreen works you'll understand why you need to use it and in what way. So let's be curious and find out how sunscreen works.

UV and tans

Sunscreen reduces the amount of UV light that reaches your skin. However, not all UV light interacts in the same way. There are three categories used to split UV light: UV-A, UV-B, and UV-C.

UV Spectrum

UV-C, having the shortest wavelength, is absorbed by the atmosphere, so is nothing to worry about when out in the sunshine. It can be a problem when using tanning beds, so be careful with that. UV-A, with the longest wavelength, causes skin to tan, but also is a carcinogen (causes cancer). UV-B causes the skin to burn and is the main target of sunscreen.

Sunbathers are primarily after a good tan, so I think it is appropriate to be curious about how this process actually works. UV-A light interacts with parts of the skin called melanocytes causing the release of melanin. This then reacts with oxygen (oxidises) and darkens, hence a tan forms. UV-A may cause the skin to tan, but the amount of tanning depends on the amount of melanin. Where UV-A releases melanin, UV-B increases its production. It should be noted that an increased melanin production is actually a side-effect of UV-B damaging the DNA so should in general be avoided (why we use sunscreen to some extent).

Reducing UV Exposure

All sunscreens reduce the amount UV-B that reaches (and penetrates) your skin. This is what the SPF number dictates. SPF 30 means 1/30 of the UV-B light reaches the skin.Another way of looking at this is it will take 30x longer for your skin to burn assuming a constant layer of the sunscreen is applied. SPF only applies to UV-B.

"Octyl methoxycinnamate" by Ed (Edgar181) - Own work.
Licensed under Public domain via Wikimedia Commons.

In the same way that the atmosphere absorbs UV-C, some of the active ingredients in sunscreen absorb UV-B and some UV-A. Once they do this energy is given out in the form of heat. If you look at the ingredients on a sunscreen bottle you might find an ingredient with the suffix 'cinnamate', for example 'ethylhexyl methoxycinnamate' (or octyl methoxycinnamate). It sounds and looks complicated, but it is made up of only three elements: carbon, oxygen, and hydrogen. But the role of that molecule is to absorb UV-B light.

Many sunscreens also reduce UV-A exposure. You can tell this by the circle with UV-A in the middle. One of the most common active ingredients for reducing UV-A exposure is benzophenone. It should be noted at this point that some people find the word 'chemical' scary, and especially find chemical names with 'benz-' scary. The source of this fear is that benzene is carcinogenic. But in the same way that chlorine gas would kill you, but table salt (sodium chloride) is safe to put on your chips, the benzophenone in your sunscreen is not going to give you cancer. In fact studies have shown it is pharmacologically active against tumours!

You may have noticed I used the phrase 'reduces exposure' rather than 'blocking' with regards to UV light. This is because the ingredients listed so far won't completely block UV radiation. Some still gets through. However, there are some inorganic molecules that do block UV. These are what make sunscreen white (especially older types). For example titanium dioxide is used in some sunscreens and is also the main molecule used to make paper white. Zinc oxide is also used, along with many other inorganic (i.e., contains a metal) compunds.

If those molecules make things white, how come some sunscreens are clear liquids? The answer is nanotechnology. Scientists have been able to create tiny nano-sized particles of these molecules such that they are invisible to the naked eye (let visible light through), whilst still being opaque to UV light. This can happen because UV light has a shorter wavelength than visible light.

How much?

Sunscreens advertise SPF numbers as a measure of their effectiveness. But those numbers only apply when you apply the same amount of sunscreen that those figures were calculated for. SPF numbers are quoted assuming you apply an even layer of 2 milligrams of sunscreen per square centimetre. To cover your body you would need to 'fill a shot glass'. (gizmodo.com/)

It is also important to remember that although SPF 15 means you can stay out 15x longer in the sun than without sunscreen, it still needs to reapplied to be effective and that there are other health hazards associated with staying in the heat for than length of time.

Curious Fact

SPF 15 reduces UV-B exposure by 93 %, SPF 30 by 97 %, and SPF 50 by 98 %. That's a range of just 5 % between SPF 15 and 30!

Do you have anything to add to the discussion on sunscreen? Any bad experiences to share? Let us know in the comments below. As always you can share this post using the social media buttons on the left, and directly beneath this post, and you can find It Is All Science on the social networks using the buttons on the right. Please like, +1, tweet, etc... this post if you enjoyed it and found it useful.

And remember, it is all science. So let's be curious!