How Sunscreen Works: What SPF and UVA Ratings Really Mean

It’s a sunny day and you’re in the chemist’s, looking at a wall of bottles. SPF 15, 30, 50, 50+. Some have stars, some have a small circle with “UVA” in it, others say PA+++ or “broad spectrum”. Prices range from cheap to eye-watering.

What do all these labels actually mean, and which ones matter?

What sunscreen protects you from

Sunlight includes ultraviolet (UV) radiation, which you can’t see or feel. Two kinds reach the ground:

• UVB mainly affects the outer layer of skin. It is the main cause of sunburn, and it is strongest in the middle of the day and in summer.
• UVA goes deeper into the skin. It contributes to wrinkles, sagging and age spots, and it is present at fairly steady levels all day and all year, even through clouds and window glass.

Both UVA and UVB damage the DNA in skin cells and raise the risk of skin cancer, including melanoma, the most dangerous kind.

SPF tells you about sunburn protection. It tells you nothing about UVA.

How sunscreen works

Sunscreens contain UV filters, ingredients that absorb or scatter UV rays before they can damage your skin.

• Mineral filters, such as zinc oxide and titanium dioxide, sit on the skin and mostly absorb UV, also reflecting a little. Modern versions are made of tiny particles, so they leave less of a white layer.
• Organic filters, sometimes called chemical filters, absorb UV and release the energy as a small amount of heat.

Both types work well. Many products combine several filters to cover the full range of UVA and UVB.

What SPF means

SPF stands for sun protection factor. It is a measure of protection against sunburn, which is mostly caused by UVB. In testing, the time it takes for skin to redden with sunscreen is compared with the time without it.

The protection doesn’t increase in a straight line:

So SPF 50 isn’t “twice as good” as SPF 30. But the gap matters more than it looks, because SPF 30 lets through about 3% of UVB and SPF 50 only about 2%. Many health bodies recommend at least SPF 30.

How UVA protection is labelled

Because SPF says nothing about UVA, other labels show it. They differ by country:

The simple rule: choose a product that shows both a good SPF and a UVA mark used where you live.

Why most people get less protection than the label says

The SPF on the bottle is measured using 2 mg of sunscreen for every square centimetre of skin. That’s a thick layer. Studies have found that people usually apply only a quarter to a half of that amount, which can reduce the real protection a lot.

The NHS suggests about two teaspoons to cover your head, arms and neck, and about two tablespoons if you’re covering your body while wearing a swimming costume.

Sunscreen also wears off. It is rubbed off by towels and clothes, washed off by water and sweat, and some filters break down in sunlight.

How to use sunscreen well

1. Apply generously, about 15 to 30 minutes before going out, to all exposed skin, including ears, neck, feet and the backs of hands.
2. Reapply at least every two hours, and straight after swimming, sweating or towelling, even if the product says “water resistant”.
3. Don’t use sunscreen to stay out longer. Use it alongside shade, a wide-brimmed hat, sunglasses and clothing that covers your skin.
4. Check the UV index. The World Health Organization recommends sun protection when the UV index is 3 or higher. Many weather apps show it.
5. Check the expiry date. Most sunscreens last two to three years unopened. Throw out any that has changed colour or texture.

Does it actually prevent skin cancer?

Yes, the evidence is good. In a large trial in Nambour, Australia, adults who were asked to apply sunscreen every day for four and a half years were found, over the following years, to have about half as many new melanomas as those who used it whenever they chose. Regular sunscreen use also reduced a common type of skin cancer called squamous cell carcinoma, and slowed visible skin ageing.