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Sunscreen

“Do we have any sunscreen?” – the rallying cry of the Picture It team before venturing out into the sunshine, echoed all over the world in the summertime. But what are the key ingredients of sun cream, and how do they work to protect us? Here we investigate the chemical and physical mechanisms by which sunscreens work, and why we should be glad that they do.

Creams, sprays and lotions to protect against sunburn have, since the mid-20th century, become a staple of summer holidays and day trips, with a bewildering array of products available in pharmacies and supermarkets. Their effect is, in short, to prevent ultraviolet (UV) radiation in sunlight from reaching the skin. UV light makes up the part of the electromagnetic spectrum at shorter wavelength than the visible region, as shown on the diagram below, so UV rays carry more energy than visible light but less than X-rays. The UV spectrum is further divided up by wavelength into three segments, labelled UVA, UVB and UVC, each of which have different associated health effects. “Broad-spectrum UV radiation”, containing all three components, is a known carcinogen, and it is fairly well established that each type of UV radiation contributes to causing skin cancers.

EM_spectrum

The electromagnetic spectrum, detailing the visible and ultraviolet regions

In addition to this, UVB causes the short-term skin damage we all know as sunburn and also allows vitamin D to be synthesized in the skin – the latter process happens at much lower radiation doses so it is perfectly possible to get enough vitamin D from sunlight without burning. UVC, the highest energy UV radiation, is not a practical concern because almost all of it is absorbed by the atmosphere. In general, higher energy radiation is less able to penetrate through matter; this is why the lower-energy UVA is believed to cause DNA damage to cells deeper in the skin.

The protection offered by a sunscreen is usually measured by the Sun Protection Factor (SPF), clearly visible on every bottle. SPF in fact only describes protection against UVB, so a high-SPF sun cream will prevent sunburn but not necessarily mitigate the risk of long-term skin damage; for this UVA protection is also required.

SPF_graph

Graph showing the percentage of incident UVA radiation blocked by sunscreen of a given SPF

The numerical value of SPF simply denotes the fraction of burning radiation which reaches the skin through the sunscreen; as shown on the graph to the left, SPF 10 lets one tenth through, SPF 30 blocks all but a thirtieth and so on. This is in some ways equivalent to the definition “SPF x means that you can stay out x times longer without getting burned” – this is one way to measure SPF, by seeing how long a human volunteer takes to burn through the sunscreen, though reliable spectrometry experiments also exist. However, SPF does not have any bearing on the rate at which the active ingredients degrade so sunscreen of any factor must be reapplied to provide proper protection. As indicated by the graph, at large SPF values the differences between sunscreens is minimal; this has led to European and Australian authorities limiting the maximum SPF on a bottle of sunscreen to 50+ to prevent confusing and spurious claims by manufacturers.

UVA protection should also be considered important for long term healthy skin, but measuring and reporting it is a much less well-developed process. As UVA does not cause sunburn it is harder to directly measure UVA protection with a human subject, but laboratory experiments have been developed to measure transmittance through sunscreen. One such test is the basis of the star rating used in the UK, developed by the pharmacy chain Boots, in which the stars refer to the ratio of UVA:UVB protection. This means that a five-star SPF-50 sun cream offers more UVA protection than a five-star SPF-25 cream.

Let’s now turn to the chemical details of exactly how sunscreen works. The active ingredients can be divided into two classes – “physical” and “chemical”. These physical agents are nanoparticles, which reflect light, while the chemical agents are molecules capable of absorbing UV rays and dissipating the energy safely.

Physical protective substances are usually tiny particles of inorganic compounds, the most common examples being the simple minerals TiO2 and ZnO, which provide protection by reflecting incident radiation. This is an effective way of blocking harmful UV rays but, as it is not selective for particular wavelengths of light, it leads to the white appearance of sun cream as visible light is also reflected. Over time, manufacturers have been able to make the particles smaller and smaller, reducing the chalky appearance of sunscreen. Both of these substances are used as pigments in paints, called titanium white and zinc white, and TiO2 particles are included in toothpaste for its white appearance and as an abrasive agent. They are not without their drawbacks, however; TiO2 is harmful to marine life, and a recent report from the European Centre for Research and Teaching in Environmental Geoscience (CEREGE) found that beachgoers’ sunscreens release significant amounts of it into the sea during the summer. The extent of the resulting environmental damage is not clear as the nanoparticles will be dispersed in the sea.

carbonyls

Some common carbonyl groups found in organic molecules

Chemical agents work at the molecular level, so by looking at the structures of a few commonly used compounds we can identify their key features and mode of action. They tend to contain carbonyl groups – carbon double-bonded to oxygen – which form functional groups like esters and ketones by bonding in particular arrangements, some of which are depicted side by side for comparison.

Another common property of these molecules is that they have large networks of double bonds. This “conjugation” is necessary for absorbing light, as it is the electrons in the double bonding framework which will be excited by the UV rays, and as such is also a feature of natural and synthetic dyes. The similarities between these types of molecules can be seen from their skeletal diagrams. The wavelength absorbed by the molecule is determined by the chemical detail of the conjugated system,

sunscreen_molecules

Four molecules containing conjugated double bonds – three absorb UV light and are used in sunscreens, while one is a naturally ocurring dye which absorbs visible light

so by careful design a synthetic chemist can make a molecule intended to absorb at a particular wavelength. This enables us to protect against damaging UV radiation in sunlight, or in another context to make dyes of all different colours.

The basic mechanism by which all these chemical agents protect our skin from UV rays is as follows. When a conjugated molecule absorbs a UV photon, an electron in the double bonding system is excited to a higher energy level. The electron then relaxes back down again and the energy is converted to molecular vibrations, which slowly dissipate the energy as heat by interactions with other molecules. One notable exception to this is the widely used agent avobenzone, which undergoes photodegradation on exposure to UV light; that is, after absorbing a UV photon the molecule breaks apart rather than transferring the energy to vibrations. This is one reason for which sunscreens must be reapplied at regular intervals.

UV radiation carries a great deal of energy, and can therefore do severe damage to our skin if we are exposed to too much of it. The remarkable effectiveness with which suncream minimizes this risk demonstrates the power of synthetic chemistry in solving the problems which we face every day.

Contributors: Harry Morgan (writing, images), Alistair Sterling (editing, images), Natalie Fey (editing)

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