PhD Research: How do sunscreens work?
(Full academic thesis available at: http://wrap.warwick.ac.uk/114335/1/WRAP_Theses_Rodrigues_2018.pdf; Winner of the Warwick Faculty of Science, Engineering & Medicine Thesis Prize 2019)
There is no denying that often all it takes is a bit of sunshine to lift your mood, especially if it is the first warm Spring day after a long, harsh Winter. As you enjoy those delightful rays of warm light, however, perhaps you remember the warnings regarding the risks of being out in the sun too long (if you have ever experienced a painful sunburn, maybe you won't need further warning!). The widespread advice regarding staying safe in the sun is clear: stay in the shade when the sun is strongest (which in the UK, according to the NHS, 'is between 11 am and 3 pm from March to October'), cover your skin with appropriate clothing and your eyes with sunglasses when out in the sun, and make sure you never burn. While these are the safest and most reliable methods to avoid sun damage to your skin, sometimes it is simply too tempting, and indeed irresistible, to stay in the sun for just a little bit longer; so where did I leave my sunscreen bottle from last year? Particularly if you are planning a holiday to a sunny part of the world, you may find yourself packing plenty of sunscreen and planning to use it as your main source of protection against the sun. But what do these protective lotions, on which we rely so much, actually do?
The answers to those questions have only recently started to emerge, despite modern sunscreen having been on the market since the 1930's, when Eugene Schueller, the founder of L'Oreal, released the first version of Ambre Solaire. We have known for a while that some sunscreen molecules are photoallergenic, that is, they don't cause any harmful or unpleasant reactions on the skin when first applied, but when the skin (and, therefore, the sunscreen molecule) is exposed to the sun, a rash may develop. This was the case for the famous sunscreen PABA (the chemists call it para-aminobenzoic acid), which a significant number of sunscreen users found to be photoallergenic and PABA was, therefore, removed from commercial sunscreen formulations. Nevertheless, the case of PABA demonstrates the issue with sunscreens rather well: there may be sunscreen molecules that absorb the radiation as they are supposed to, but then the extra energy has some effect that ultimately results in the sunscreen molecule being harmful to human skin. What the scientific community is now starting to come to understand is that if the extra energy that molecules receive from absorbing ultraviolet (UV) radiation hangs around for too long, it increases the chances of harmful chemistry taking place (the kind that results in photoallergy). In essence, if the sunscreen molecule does not find a quick sink for the excess energy, it may not have another option but to break apart. Molecules could potentially start to glow to release the extra energy, but glowing is a process that happens over several minutes, which is not ideal if we are trying to get rid of energy quickly. It is a shame, really, because if sunscreens did glow when exposed to the sun, going to the beach would be like an immense colourful glowing party!
Sunscreens are designed to protect us from the UV radiation we receive from the sun, which is usually classified as UVA and UVB. While UVB radiation is more aggressive to the skin and causes direct DNA damage, much higher levels of UVA actually reach our skin; UVA also generates free radicals which cause skin ageing and indirect damage to DNA in skin cells. The bottom-line is, however, that both UVA and UVB have the ability to prompt a cascade of processes that may ultimately lead to skin cancer. In fact, UV radiation (both UVA and UVB) is the primary origin of skin cancer, the incidence of which has risen in recent years. It is also important to remember that there isn't such a thing as a 'healthy tan': the darkening of our skin as a result of sun exposure, even if it was just a nice day out in the mild Spring sun, occurs as a response to damage caused by UV radiation. Therefore, if you are even mildly tanned, your skin has already suffered some level of damage. So you can imagine what tanning beds do to you!
The way sunscreens protect us from the destructive effects of UV radiation is, quite simply, by absorbing the harmful UV radiation from the sun before our skin has the chance to. Which means: yes, wearing sunscreen will hinder tanning, at least if you are using it right, which means re-applying every few hours and especially after you've been in the sea or the pool. When the minuscule components of any material, which we call molecules, absorb radiation, such as sunscreen molecules which absorb UV radiation from the sun to protect the skin against it, they get excited. I know what you are thinking: 'to be fair, I also get excited when I'm out in the sun!'. But that's not quite what it means here. Excited molecules have extra energy, which they get from absorbing the radiation from the sun. This extra energy, in turn, needs to go somewhere, it can not simply disappear (if you are ever asked about it in a pub quiz, this is called the law of conservation of energy). So where does the extra energy go? Different molecules deal with extra energy in different ways: some release it by emitting light (ever play with glow sticks?), others quite simply break apart and generate secondary chemicals (that can either be harmless or deadly), both of which are not ideal for sunscreens. So what do the sunscreen molecules do?
The remaining option is for sunscreen molecules to shake the energy off, essentially stretching back and forth until the energy is gone. And, as it turns out, good sunscreens can shake excess energy away quite quickly, on a timescale of femto to picoseconds. That is approximately 0.000000000000001 to 0.000000000001 seconds - very fast! The way in which sunscreen molecules shake, however, can be quite particular. Sometimes they shake so much their overall structure changes. Importantly, they sometimes shake differently depending on what liquid they are dissolved in. This can be quite significant, considering sunscreen formulations can be either water- or oil-based, for example. Do sunscreen work better in water? Oil? What kind of oil? Are there other liquids we may use? And how do other components of the sunscreen (fragrances, for example) affect how the sunscreens release the harmful extra energy? These are all questions that remain unanswered, and that the field of ultrafast sunscreen photodynamics - the study of how sunscreen molecules get rid of extra energy - is currently trying to answer.
Once we understand how the current sunscreen molecules work their magic, we may be able to alter them at a molecular level to enhance their performance. Importantly, we may use the new knowledge provided by the field of ultrafast sunscreen photodynamics to address some of the challenges currently facing the sunscreen industry. One of these challenges is to provide appropriate protection which covers both the UVA and UVB radiation ranges: there are few UVA sunscreen molecules, for example, and some of them tend to be unstable within commercial formulations, i.e. they degrade and lower the sunscreen's protective performance. In addition, some studies now reveal a negative impact of sunscreen molecules on the environment. When we apply sunscreens and go into the sea, the sunscreen molecules eventually end up having an effect on plant and animal sea life. Oxybenzone, for example, which is used in many commercial sunscreen formulations, has been linked to coral bleaching. In fact, this is the reason Hawaii has recently banned sunscreen which contain oxybenzone from their beaches!
The sunscreen molecules currently used in commercial sunscreen formulations have been consistently shown to provide important and safe protection against UV radiation from the sun by absorbing it before the skin has the chance to. However, currently we don't fully understand the mechanisms by which sunscreen molecules get rid of the extra energy they receive from solar radiation and, importantly, we don't know how the components of a commercial sunscreen formulation may affect these mechanisms. The field of ultrafast sunscreen photodynamics is unveiling these mysteries in the hope of guiding the development of a new generation of sunscreens designed for optimum UV protection. In the future, these innovative sunscreens may be incorporated in high performance commercial sunscreen formulations which absorb more UV radiation and dissipate the extra energy quicker, making them more effective against sunburn. The optimised protection provided by these innovative high performance sunscreens may also help curb the rising numbers of skin cancer incidence. Finally, the development of new sunscreens is also an environmental concern, since current sunscreen molecules have been shown to have a negative impact on the environment, and in particular on sea life.
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