It’s drummed into us – and rightly so – from an early age that using SPF to protect our skin from sun damage is crucial to avoid cancer, at one end and premature ageing at the other.
Even in the depths of winter, or during a global pandemic when you’re indoors more than you’re out, it’s important to make an SPF cream part of your daily routine. But what is SPF, how does it work and why is it so important?
SPF stands for Sun Protection Factor and refers to how well a cream, oil or lotion will protect you from the sun. In particular, it’s a relative measure of how long a sunscreen will protect you from ultraviolet rays, specifically UVB.
Light from the sun includes a broad spectrum of wavelengths, from visible light, the light we see, to infrared, the light that warms the planet, and ultraviolet (UV). If you think about a rainbow being the spectrum of visible light, infrared has a longer wavelength than red, and ultraviolet has a shorter wavelength than violet (hence their names).
UV light comes in two variations, UVA and UVB – well, technically there is a third type called UVC, but its short wavelength can’t penetrate the atmosphere so doesn’t reach us on Earth’s surface.
UVB causes sunburn and increases the risk of skin cancers by penetrating the upper layers of the skin. UVA also causes skin cancer, but in the deeper layers of the skin. Up to 80 per cent of these UV rays penetrate through clouds, even in the winter, so wearing SPF is vital for any skin exposed to sunlight at any time of the year. Even if you’re only popping out for a few minutes.
Sun protection creams tend to have two parts to them, an active ingredient – something doing the protecting, and an emulsion – an oil or cream that makes it easy to apply. Active ingredients then fit into two categories, UV absorbers and UV reflectors.
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“UV absorbers are chemicals that absorb UV radiation and convert it to a very low level of heat,” explained Terry Slevin from Cancer Council Western Australia (CCWA). “So low most don’t notice it, but a small proportion of people do report sunscreens make them feel uncomfortably warm.” UV absorbers usually contain carbon.
If you’re using a UV absorber, look for one labelled “broad spectrum”, which will protect from both UVA and UVB. UV reflectors are usually oxides, such as zinc oxide or titanium dioxide, which scatter UV radiation and prevent it from reaching the skin.
An SPF number i.e. SPF 15; SPF 30, SPF 50, gives an indication to how long the sun’s UV radiation would take to redden your skin, which is measured by how much UV light is allowed to pass through, versus how long it would take if you weren’t wearing any SPF cream.
SPF 20 allows one-twentieth of UV to reach the skin, or 5 per cent, blocking the other 95 per cent. On average, cream with an SPF 30 rating lets in around 3% of UVB rays, while an SPF 50 cream allows 2%.
This may seem like an inconsequential number but it means that SPF 30 creams allow 50% more radiation to reach your skin.
Theoretically, wearing lotion or creams rated SPF 30 would protect you from skin damage for up to 30 times longer than without protection.
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As an example, if you burn within 10 minutes of being in the sun, SPF 30 would protect you for around five hours, but it may protect someone else, who is less genetically likely to burn as quickly, for longer.
It’s not a hard and fast rule, however. The intensity and wavelength distribution of UVB rays varies throughout the day and depends on where you are in the world. This rating also doesn’t apply to UVA rays.
To rate UVA protection, SPF creams additionally feature a star rating. It should be noted, however, that this star rating refers to the ratio of UVA protection compared to that same cream’s UVB protection. It’s confusing, but in summary, it means you should opt for creams with high SPF and a high star rating.
We can’t see UV, but it can be incredibly damaging to our skin. This is because exposing skin to UV light causes the production of melanin, the pigment that makes skin darker and gives us a tan, while also promoting the creation of molecules called cyclobutane pyrimidine dimers (CPDs).
These CPDs bond with the molecules linking skin’s DNA together, particularly in the skin’s upper layer.
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These bonds cause the skin to become thicker, and the connective tissues to break, which makes the skin less elastic and loose. This is why sun exposure causes wrinkles and saggy skin. It can also cause dark patches, know as age spots or liver spots.
In extreme cases, like sunburn, skin becomes inflamed a few hours after exposure to too much UV light. How susceptible someone is to sunburn is genetic. Melanin is produced to defend skin against the sun, but some people produce more than others – which is why some people tan quicker. People with lighter skin are more prone to sunburn. However, a tan and sunburn are both signs of UV damage to your skin. It can also lead to age spots, also known as dark spots or liver spots. Read more in our how to get rid of age spots guide.
DNA damage can also cause cells to grow abnormally, causing cancer.
Exposure to UV light is the biggest risk factor for the three most common types of skin cancer, with or without sunburn. These skin cancers are called basal cell carcinoma, squamous cell carcinoma and malignant melanoma. Malignant melanoma, often referred to as simply melanoma, is the deadliest form of skin cancer.
Basal cell carcinoma (BCC)
Basal cell carcinoma (BCC) is the most common type of skin cancer. It grows slowly and is easily cured if caught early enough. However, it is still a serious condition and if untreated, can become life-threatening. BCC occurs when basal cells in the outer layer of the skin are damaged and start to grow out of control. They can look like red patches, shiny bumps, scars or open sores.
Squamous cell carcinoma
Squamous cell carcinomas are the second most common form of skin cancer and are caused by the abnormal growth of squamous cells, another type of cell in the skin’s top layer. SCCs look like red patches, sometimes itchy sores, rough patches, warts or growths. These can appear all over the body, and again, if allowed to grow they can become dangerous.
Malignant melanoma is the most dangerous type of skin cancer and it begins in melanocytes, the cells that produce melanin. This type of cancer can spread much more quickly than BCC and SCC, to other organs, making it the most life-threatening of the three.
Melanoma is usually spotted by something new, changing or unusual on your body. For example, an existing mole that has started to change shape or a new one growing rapidly. The signs of melanoma can appear anywhere on your skin but usually somewhere that has had a lot of exposure to the sun. The Skin Cancer Foundation has a comprehensive list of early warning signs for melanoma.
The signs of all skin cancer types often look different from one person to another, so make sure to see a doctor if you are concerned about any changes to your skin’s appearance.
Abigail is a leading science journalist writing about space, sustainability, technology and culture. She is author of The Art of Urban Astronomy, a must-have guide to the night sky that guides you through the seasons and learn about the brightest stars and constellations, the myths and legends of astronomy and how to identify star clusters and galaxies.