How Does Sunscreen Protect our Skin

Even though the sun is almost 150 million kilometres away from us, its UV rays are still able to cause significant damage to humans here on Earth. Whilst UV radiation is able to mutate your DNA and promote the formation and growth of tumours, it is also beneficial to us as it facilitates the natural production of vitamin D as well as endorphins in the skin.¹ UV radiation can be split into three categories depending on wavelength, UVA (320 – 400 nm), UVB (280 – 320 nm) and UVC (200 – 280 nm), with only UVA and UVB reaching the earth’s surface since UVC rays are mostly absorbed by ozone in the atmosphere.

Approximately 95% of the UV rays that reach the Earth’s surface are UVA rays which penetrate deeper into the lower dermis layer of the skin and damage the elastin, causing significant sun damage in the form of wrinkles and premature aging.² The other 5% are UVB rays which affect the top layer of the skin, damaging the skin cells in the epidermis- the layer of skin in which most skin cancers develop.

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In the epidermis are melanocytes, a group of cells responsible for the production of the skin pigment melanin. Melanin is naturally efficient at blocking UV photons and absorbs incoming UV rays, preventing them from permeating into as much of the epidermal layer. Melanin is much less abundant in fair-skinned people and on average, the fairer your skin is the more harmful exposure to UV radiation will be.¹ Melanin alone is not potent enough to protect humans from all UV rays and so using sunscreen is very important.

There are two main types of sunscreens: physical and chemical. Physical sunscreens work by creating a barrier on the surface of the skin that can reflect, scatter or absorb the UV rays, preventing them from penetrating deeper into the skin. Two of the main chemicals used in these creams are zinc oxide (ZnO) and titanium dioxide (TiO₂), both photostable inorganic compounds usually used as microsized or nanosized particulates. ZnO primarily absorbs UVA radiation whilst TiO₂ mostly absorbs UVB and so together they offer protection against a broad spectrum of incoming radiation. Unfortunately, physical sunscreens only work whilst on the skin and can be rubbed or washed off relatively easily, exposing the skin to UV radiation.

Using a chemical sunscreen can be much more advantageous as it tends to be a thinner consistency, making spreading across the skin easier. Two chemicals commonly incorporated into sunscreens are oxybenzone and avobenzone, aromatic ketones capable of absorbing UV radiation. Oxybenzone absorbs predominantly UVB rays, whilst avobenzone absorbs UVA. Upon absorption of UV radiation, excitation occurs within the molecules and subsequent relaxation occurs by an alternative pathway, thought to be through keto-enol isomerisation.⁴

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When it comes to physical vs chemical sunscreens, each have their own advantages and disadvantages. Whilst physical sunscreens are usually less irritating on the skin, chemical sunscreens are much more efficient for someone who may sweat a lot in the heat or want to go swimming.

References

1. J. D’Orazio et al, Int. Mol. Sci., 2013, 14, 12222-12247
2. S. G. Coelho et al, Pigment Cell Melanoma Res., 2010, 23(1), 57-63
3. T. G. Smijs et al, Nanotechnology, Science and Applications, 2011, 4, 95-112
4. L. A. Baker et al, Sci Prog., 2016, 1;99(3), 282-311