Chemistry of Sunscreen

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With summer more or less here (stifle those sniggers, English readers), it seemed as good a time as any to examine the chemicals in sunscreen. It’s a product that many of us may take for granted, but you’ve got chemistry to thank for it preventing your skin turning lobster red in the summer sun. There are a number of chemical molecules used in currently available sunscreens, with the exact formulation actually depending on where in the world you live. Additionally, the chemistry of these molecules can help explain why sunscreen has to be reapplied periodically.

To understand the protection that sunscreen affords, we first have to understand what we’re trying to protect ourselves from. Sunscreen is designed to protect us from UV radiation from the sun; this has a shorter wavelength than visible light, and a lot of the energy emitted by the sun is in the form of UV, which can be divided into three categories. One of these, UVC (with a wavelength of ~290-100nm) isn’t a problem, as it’s absorbed by ozone in the atmosphere before it can reach the Earth’s surface. There are two other categories, however, UVA & UVB, which can cause damage to skin.

UVB (wavelength ~290-320nm) is responsible for around 5% of the UV radiation reaching Earth, with the majority of it also being absorbed by the atmosphere. It causes your skin to produce more melanin, which is what causes the tanning effect of sitting in the sun. However, it can also cause sunburn, and direct DNA damage, which can increase the risk of developing skin cancer. Sunscreen has been available as a product since around 1928, and most early sunscreens were formulated to screen the skin against UVB rays.

UVA (wavelength ~320-400nm) is responsible for the largest proportion of the UV radiation from the sun that reaches the Earth’s surface – approximately 95%. UVA can penetrate much deeper into the skin than UVB, down into the connective tissue. This causes wrinkling and premature ageing of the skin. UVA can also generate reactive species in the skin, and thus indirectly causing DNA damage, and contribute to an increased skin cancer risk. It was for a time considered relatively harmless in comparison to UVB, but now the damage it can cause is beginning to be understood, sunscreens have included different chemicals to also shield against this portion of the UV spectrum.

So, how does sunscreen work chemically? Both inorganic chemicals and organic (carbon based) chemicals can be used to afford protection. The two inorganic compounds used are titanium dioxide and zinc oxide. These compounds are often described as merely providing a physical, reflective barrier to UV light, but in fact they can absorb the UV radiation, and dissipate it harmlessly. The first sunscreens containing only these chemicals would have left a visible white layer on the skin.

In present day sunscreens, a combination of inorganic chemicals with organic chemicals are used. Due to their chemical structures, chemical bonds in certain organic chemicals are able to absorb photons of UV light – this energy is then dissipated harmlessly in the form of heat. Variations in structure can lead to absorption at different wavelengths, meaning a mix of these organic chemicals is often used to ensure protection against the full range of UVA and UVB wavelengths.

These organic chemicals also explain, to an extent, why sunscreen has to be reapplied. Some of the organic chemicals used will be photostable; that is, they won’t break down when exposed to UV light. Some of them, however, will slowly break down as they absorb UV light over time – avobenzene, shown in the graphic, is one of the prime examples of this. Other chemicals can be added to help slow this breakdown, but it can be one of the reasons why it’s necessary to reapply sunscreen regularly. The more obvious reasons include the fact that even sunscreens that claim to be ‘water-resistant’ will still eventually wash off, which is why regulation now states sunscreens should also specify the length of time to which they remain water resistant.

If the the idea of this range of chemicals in your sunscreen has you concerned, it shouldn’t. All of them undergo a rigourous testing process before they are permitted to be used in sunscreens, and they all have limits on the amounts that can be used, to ensure that they’re nowhere near the levels that could be harmful. Some are absorbed more by the skin than others, but this is accounted for in safety testing, so it’s no cause for concern.

On a final note, interestingly, there are many more chemicals permitted for use in sunscreen in the EU (28) and Australia (34) when compared to the USA (17). In particular, a number of chemicals that block both UVA and UVB radiation have yet to be approved by the FDA despite being used for several years in other countries. This has something to do with the fact that, in the US, sunscreen is classed as an over-the-counter drug, whereas in other countries it’s classed as a cosmetic. As a consequence, the FDA’s approval process seems to be a little on the slow side – a new compound hasn’t been approved since 1999, and there are at least 8 new compounds waiting for approval, some of which have been waiting for over a decade.

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