UK heatwave: How does sunscreen work?

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As the UK sweats its way through a prolonged stretch of unusually warm weather, Reality Check wanted to make sure we know our SPF from our UVB.

Most of us are by now familiar with sun protection factor (SPF), the big number on the front of your sunscreen bottle, and look for a high one to give us more protection from the sun.

Many brands carry a five star rating too, and that might be just as important to understand.

SPF tells you how much protection your sunscreen provides from UVB radiation while the star system tells you the percentage of UVA radiation that is absorbed by the sunscreen in comparison to how much UVB is absorbed.

What are UVA and UVB?

Ultraviolet A and B refer to different wavelengths of radiation from the sun that enter the earth's atmosphere. A third wavelength of radiation transmitted by the sun, UVC, doesn't penetrate our atmosphere, so we don't need to worry about that one.

UVA is associated with ageing of the skin and pigmentation as well as skin cancer (particularly squamous cell carcinoma - the second most common type). It can affect human skin even through glass.

UVB causes sunburn and is linked to particular types of skin cancer - basal cell carcinoma (the most common type of skin cancer) and malignant melanoma.

Sunscreen doesn't stop all types of skin damage so it's also important to cover up and seek shade when the sun is strongest.

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What do the numbers mean?

The number of stars refers to the percentage of UVA absorbed as a ratio of how much UVB is absorbed.

So a low SPF sunscreen could have a high star rating despite not providing the maximum protection, because the ratio of UVA to UVB protection is the same - for that you need to look for both high SPF and a high star rating.

The SPF number on a bottle of sunscreen refers to how much UVB it allows in.

A sunscreen with SPF 15 allows one fifteenth of the sun's rays to reach your skin, or about 7%.

So it filters out about 93% percent of UVB rays while SPF 30 filters about 97%. The number refers to how much gets through rather than how much is screened out - the lower factor allows in about double the amount of radiation than the higher, but doesn't block double the amount of radiation.

Another way of looking at it is, if you could stay in the sun for 10 minutes unprotected without burning, SPF 15 would in theory give you 15 times that protection, or two-and-a-half hours before you'd burn.

However that's in a perfect world - in reality, most people don't apply sunscreen perfectly and it can rub off or come off with sweat. And experts think most people apply half the quantity of sunscreen as is recommended.

The British Association of Dermatologists says sunscreen with SPF 30 is a "satisfactory form of sun protection in addition to protective shade and clothing" and that it should be reapplied at least every two hours, no matter what SPF it is.

EU guidance states that sunscreen should only be marketed as having sun protection of "50+" and not the ratings of 80 or 100 which can be found in other countries, which could be misleading as to how much extra protection they provide (SPF 50 provides about 98% protection while 100 would provide less than 100%).

No product provides 100% protection from the sun's rays.

Once a day?

There are lots of "extended wear" sunscreens on the market that advertise themselves as being for use "once a day", or claiming to last for eight hours.

But dermatologists recommend that these products should still be applied at least every two hours, like any other sunscreen, since the risk that you may have missed a spot - or that it will rub or wash off in that time - are too high.

A Which? report in 2016 criticised these products for not living up to their claims.