Sunscreen is a must everyday to protect your skin against skin cancers and premature aging but picking the right one isn’t as simple as you think. Dr Haran Sivapalan explains.

We all know that we ought to apply sunscreen when we’re out and about. And it’s not just for the beach, dermatologists recommend that you apply it everyday – even if it’s cloudy. It protects us from harmful UV radiation, prevents sunburn and reduces our risk of skin cancer. When it comes to what sunscreen to buy, the phrase “high factor” is often bandied about – a reference to the Sun Protection Factor (SPF) rating of sunscreens. But just what is SPF, what is meant by high factor and how well does it protect us from sunburn and skin cancer?

Let’s explain from the beginning.

UV Radiation

All life on planet Earth relies on a giant ball of gas burning about 150 million kilometres away – the Sun. The sun emits all sorts of different waves, which are all forms of what we call “electromagnetic radiation.” Thanks to the sun’s visible light rays, we can see during the day. Infrared radiation warms our continents and oceans. The sun also emits radio waves, microwaves, X-rays and, of relevance to the skin, ultraviolet (UV) radiation.

When it comes to ultraviolet radiation, there are three different types emitted by the sun:

• UV-A rays – these have a longer wavelength (315 – 400 nm) and make up about 95% of all UV radiation reaching the Earth’s surface. They can penetrate deep into the skin, reaching the dermis (middle layer of skin). While UV-A rays are responsible for tanning, they can also damage the skin.

• UV-B rays – have a shorter wavelength (280 – 315 nm), but a higher energy compared to UV-A rays. They penetrate less deeply into the skin, but can cause significant damage to cells due to their higher energy.

• UV-C – these have the shortest wavelength (100 – 280 nm) of all the UV rays. Although they are potentially the most damaging to the body, they are blocked by the Earth’s ozone layer. As such, they do not reach the Earth’s surface.

So, UV-A and UV-B rays from the sun can reach the Earth’s surface and penetrate our skin. But why should we be worried about this? The answer is because they can damage cells, causing signs of aging, sunburn and, more seriously, skin cancers. This is particularly true of UV-B radiation, which has a higher energy and is therefore more damaging to cells.

How does UV radiation cause skin cancers?

UV rays from the sun help our skin to produce Vitamin D and can also give us a tan. The trouble with UV rays, however, is that they can damage the cells in our skin too. Lots of pre-clinical laboratory studies attest to this.

Studies show that UV radiation can cause inflammation, suppress the immune system and also directly damage the DNA within skin cells. DNA is the genetic code that controls, among other things, the way cells grow and divide. By damaging DNA, UV radiation can cause uncontrolled cell growth – a hallmark of cancer. To guard against this uncontrolled growth, cells have various defence mechanisms to both repair DNA damage and to prevent the cells from dividing and growing. These protective mechanisms are encoded by genes known as “tumour suppression genes.” Unfortunately, UV radiation can also cause mutations in these tumour suppression genes, thereby inhibiting the cell’s protection mechanisms.

This combination of damaged DNA, uncontrolled cell growth and disabled cell defence mechanisms can give rise to various skin cancers. Indeed, exposure to UV radiation (particularly UV-B) is responsible for 90% of cases of malignant melanoma (a cancer of the melanocytes in the skin). The non-melanoma skin cancers, which include basal cell carcinoma and squamous cell carcinoma, are also very strongly associated with UV exposure.

There’s lots of epidemiological evidence to support this link between UV exposure and skin cancer too. People who work outdoors, fair-skinned people living closer to the Equator and people with a childhood history of sunburn – all groups with a higher lifetime to exposure UV radiation – have a greater risk of developing skin cancers.

Given this heightened risk of skin cancer then, how can we reduce our exposure to UV radiation? The obvious answer is to stay out of the sun – we can avoid going out during hours of peak sunlight. We can wear hats and headscarves and long-sleeved clothing. Or we can reduce the amount the UV rays penetrating our skin by applying sunscreen.

Sunscreen and Sun Protection Factors (SPFs)

Sunscreens protect us from UV radiation in two main ways:

• inorganic (not containing carbon) ingredients such as zinc oxide or titanium dioxide reflect UV rays away from the skin. They act like mirrors and are sometimes called mineral sunscreens.

• organic (carbon-containing) ingredients such as oxybenzone or avobenzone absorb UV radiation so that it is not absorbed by cells in the skin. They act like sponges and are sometimes called chemical sunscreens.

But just how well does sunscreen protect us from UV radiation? Is there a way of quantifying this protection? Yes, there is. On a bottle of sunscreen you may have seen a figure for Sun Protection Factor (SPF). SPF is a measure of how well a sun cream protects your skin from UV radiation, specifically UV-B rays. Recall that UV-B radiation is the higher energy, more damaging type of UV radiation that contributes more to sunburn and skin cancers. SPF does not measure protection against UV-A rays which is mainly responsible for deeper cell damage, Basal Cell Carconomas and photoaging (see below).  The SPF is calculated experimentally as follows.

First, scientists expose an area of bare skin to UV-B rays from a lamp. They find the dose of UV radiation that makes the skin turn red or sunburned. This is known as the Minimal Erythemal Dose. Afterwards, another area skin is covered with sunscreen and subjected to UV-B radiation. As the sunscreen reflects and safely absorbs UV-B rays, a higher dose of UV radiation is needed to make the covered skin turn red. In other words, the Minimal Erythemal Dose for sunscreen-covered skin is higher. The SPF is simply a measure of how many times higher the Minimal Erythemal Dose is for protected versus bare skin. For example, skin covered with SPF 15 sunscreen will require a 15 times as much UV-B radiation to cause redness and sunburn compared to bare skin. Skin covered in SPF 30 sunscreen requires a 30 times higher dose of UV-B radiation to cause redness and sunburn.

So, the higher SPF the better? 

Well, yes and no. Owing to the way the maths works, there isn’t a simple linear relationship between SPF and protection from UV rays. A SPF 30 sunscreen does not afford you twice as much protection as SPF 15.

Instead, when it comes to blocking UV-B rays:

• SPF 15 blocks 93% of UV-B rays
• SPF 30 blocks 97% of UV-B rays
• SPF 50 blocks 98% of UV-B rays

So going up from SPF 30 to SPF 50 affords only 1% greater protection from UV-B rays. While higher SPFs block more UV-B radiation, SPFs above 30 give only negligible further benefit – like wearing a t-shirt over full body armour suit.

To simplify matters, the EU regulations suggest manufacturers classify sunscreens as follows:

• SPF 6 – 14 = Low Protection
• SPF 15 – 29 = Medium Protection
• SPF 30 – 50 = High Protection
• SPF > 50 = Very High protection

In order to sufficiently protect against sunburn and skin cancer, the British Association of Dermatologists recommends using a sunscreen with an SPF 30 or above.

And what about UV-A rays? 

Figures for SPF do not tell us anything about protection from UV-A radiation. Recall that UV-A rays have less energy but UV-A rays can penetrate deeper into the skin, where they can damage keratinocytes in the basal layer of the dermis (middle layer of skin). There is now mounting evidence suggesting UV-A rays do indeed play a role in the development of melanoma, basal cell carcinoma and squamous cell carcinoma. Choosing a sunscreen that also offers protection against UV-A rays is therefore also important. Sunscreens that protect against both UV-B and UV-A rays are known as “broad spectrum.”

When it comes to broad-spectrum sunscreens though, SPF only tells us about protection against UV-B radiation. How do we know how well it protects against UV-A rays? The answer is a star system. Sunscreens have a rating from 0 to 5 stars. For a particular sunscreen, the stars tell us how much UV-A radiation is blocked in comparison to the  amount UV-B radiation blocked by that sunscreen.

In terms of blocking UV-A rays, the star system rates sunscreens as follows:

• One star – blocks 20-40% of the amount UV-B radiation blocked
• Two stars – blocks 40-60%
• Three stars – blocks 60-80%
• Four stars – blocks 80-90%
• Five stars – blocks 90-100%

Of course, this can be confusing. A sunscreen with a low SPF (meaning it blocks a low amount of UV-B rays) can still have 5 stars – because the amount of UV-A rays blocked is still a high proportion of this low SPF figure.

Still confused? Well, to ensure the best protection against both UV-A and UV-B radiation, the British Association of Dermatologists recommend using a sunscreen that has both an SPF of 30 or above and a star rating of 4 or above.

Do sunscreens work? 

Just because sunscreens have been shown to block UV radiation, we cannot immediately assume that they must therefore reduce cancer risk. So what do the clinical studies say?

Large Randomised Controlled Trials do indeed show that the regular application of sunscreen reduces the incidence of squamous cell carcinoma, solar keratoses (rough, scaly patches due to chronic sun-damage) and melanoma. For example, a large study of 1,383 residents of Queensland, Australia found that daily application of sunscreen significantly reduced the risk (by about 40%) of developing squamous cell carcinoma in the next 4-5 years. The same patient groups of this trial (sometimes called the Nambour Trial) were studied again 10 years later, with those who applied sunscreen having significantly fewer cases of melanoma.

Interestingly, the evidence for sunscreen having an impact on basal cell carcinoma (BCC) is not thoroughly established. This is odd, given that laboratory studies do show UV radiation to damage cells and cause basal cell carcinomas. This lack of overwhelming clinical evidence might be due to issues with study design.

Taking an epidemiological approach, one study compared the incidence of skin cancer in the Australian population to those of the UK and Scandinavia. Unsurprisingly (especially to Brits), people in sunny Australia are exposed to much higher levels of UV radiation. The researchers estimated that virtually all cases of melanoma and 63% of cases of non-melanoma skin cancers (basal cell carcinoma and squamous cell carcinoma) in Australia are due to heightened exposure to UV radiation. By applying sunscreen, about 10-15% of these cases of skin cancer were prevented.

Finally, studies show that SPF makes a difference too. Analysing data from the Norwegian Women and Cancer Study, researchers estimated that women using a sunscreen with an SPF greater than 15 had a 33% lower risk of developing melanoma compared to those using sunscreen with an SPF lower than 15. For optimum protection though, dermatologists recommend using a sunscreen with an SPF of 30 or above.

And even if you don’t think it’s relevant to you in the UK, skin cancer rates here have risen by 300% since the 1970’s. Sun beds and summer holidays in the Med may be the culprits – but all this exposure isn’t just risking cancer – it’s also aging your skin. So, even when it’s cloudy in the UK it’s a good idea to slap on some sunscreen.

Take home points

•  There are two main types of UV radiation that damage our skin – UV-A and UV-B.
• Both UV-A and UV-B radiation have been associated with sunburn and skin cancers, but UV-B is thought to be more damaging.
• Applying sunscreen has been demonstrated to protect the skin from UV rays and reduce the incidence of sunburn and certain skin cancers
• SPF (Sun Protection Factor) refers to the amount of UV-B radiation blocked by sunscreen.
• Dermatologists recommend using sunscreen with an SPF above 30
• The star system refers to the amount of UV-A radiation blocked, as a proportion of the amount of UV-B radiation blocked.
• Dermatologists recommend using sunscreen with 4 or 5 stars.
• It is important to apply sunscreen regularly (every 2 or so hours) and in adequate amounts (2 mg per cm2 of skin – about two tablespoons per body part).