Sun Protection: What SPF Numbers Actually Mean

With a heatwave arriving in London this week, here is what the evidence actually shows about SPF, UVA and UVB protection, how much sunscreen you need to apply, and the mistakes that make most people's sun protection less effective than they think.

Skin cancer is now the UK's fifth most common cancer, with 17,500 new cases of melanoma annually. Yet 86% of melanoma cases are preventable through simple measures like using SPF 30 or above — making it one of the UK's most pressing and most avoidable public health challenges.

The gap between the knowledge that sun protection prevents cancer and the application of that knowledge in practice is significant. A May 2025 APPG UV Safety Inquiry found that despite decades of public health messaging, sun protection behaviours in the UK remain inconsistent — particularly among younger adults, outdoor workers, and people with darker skin tones who incorrectly assume their melanin provides sufficient protection.

With a heatwave forecast for London this week and temperatures expected to remain high through the weekend, now is the right time to understand what sun protection actually does — and what most people get wrong when applying it.

TL;DR

• Skin cancer is the UK's fifth most common cancer. 86% of melanoma cases are preventable with consistent sun protection. Skin cancer treatment costs the NHS approximately £720 million every year.
• The highest-quality evidence suggests sunscreens do prevent skin cancer — particularly when broad-spectrum (UVA and UVB) products are used consistently.
• Cancer Research UK recommends SPF 30 or above with a 4 or 5 star UVA rating, applied generously and reapplied regularly — including products labelled once-a-day or water-resistant.
• Most people apply approximately 25% of the amount of sunscreen used in SPF testing — meaning an SPF 30 applied at a typical volume provides closer to SPF 8 to 10 in practice.
• Over 90% of UV rays pass through cloud cover and can cause sunburn even on overcast days in the UK.
• UVA rays penetrate glass — working near windows or commuting in a car provides meaningful UV exposure without the UVB that causes visible sunburn.
• Sunscreen is one layer of protection — shade and clothing are equally important parts of the protection strategy.

What SPF Numbers Actually Mean

SPF — Sun Protection Factor — measures how much UVB radiation a sunscreen filters out relative to unprotected skin. An SPF 30 filters approximately 97% of UVB. An SPF 50 filters approximately 98%. An SPF 100 filters approximately 99%.

The numbers are not as dramatic as they appear. SPF 30 is not half as effective as SPF 60. The difference between SPF 30 and SPF 50 is approximately 1% more UVB filtered. For most everyday purposes, SPF 30 applied correctly and consistently provides very good protection.

The critical word is correctly. SPF ratings are established in laboratory tests where sunscreen is applied at 2mg per square centimetre — roughly 35ml of product for the entire body, or about a full shot glass. Research consistently shows that most people apply between 0.5 and 1mg per square centimetre in real-world use — approximately 25 to 50% of the test dose. At a quarter of the test dose, the effective SPF of an SPF 30 product drops to approximately 5 to 10.

This is the most important practical piece of information in sun protection: the number on the bottle is only accurate if you apply the right amount. Most people do not.

UVA vs UVB: Why Both Matter

SPF measures only UVB protection. UVB is the wavelength that causes sunburn and is the primary driver of squamous cell carcinoma and a significant contributor to melanoma. Understanding what it does not measure is equally important.

UVA — the longer wavelength — makes up over 90% of the UV radiation that reaches the Earth's surface. It penetrates deeper into the skin than UVB, reaching the dermis where it damages collagen, generates reactive oxygen species, and — critically — can cause DNA damage that drives melanoma. UVA does not cause the visible redness of sunburn, which is why its damage is often not felt until it accumulates over years.

UVA also penetrates glass. UVB is largely blocked by standard window glass. UVA is not. Working near windows indoors exposes skin to cumulative UVA equivalent to approximately 20 minutes outdoors. People who work near windows, commute by car, or spend time near glass assume they are protected because they are indoors. They are protected from UVB. They are not protected from UVA.

In the UK, look for a UVA star rating of 4 or 5 stars alongside the SPF, or the UVA circle symbol which indicates the product meets the EU minimum standard for UVA protection. Cancer Research UK recommends SPF 30 or above with a 4 or 5 star UVA rating.

The UV Index: When Sun Protection Is Necessary in the UK

In the UK, the sun's UV rays are often strong enough to cause damage between mid-March and mid-October.

The UV Index is the most useful practical tool for deciding when to apply sun protection. It runs from 0 to 11 and above. A UV index of 3 or above means the sun can be strong enough to burn some skin types — sun protection steps should be taken. During a UK heatwave, the UV index in London regularly reaches 7 to 8 — classified as high — and can reach 9 in peak conditions.

The UV index is available on most weather apps. Checking it before going outside is a more reliable guide than temperature or cloud cover — it is possible to burn on a cool, cloudy day if the UV index is high, and the assumption that overcast conditions mean no UV risk is one of the most consistent causes of unexpected sunburn in the UK.

Over 90% of UV rays pass through cloud cover and can cause sunburn even on overcast days.

During a heatwave, UV index values peak between 11am and 3pm. This is the window when shade, clothing, and sunscreen together provide the most important protection.

How to Apply Sunscreen Effectively

How much to apply. For the face — including ears, neck, and the back of the neck — approximately half a teaspoon (2.5ml) is the appropriate amount. For the full body, approximately 35ml — six to eight teaspoons — is required to achieve the labelled SPF protection. Most bottles of sunscreen contain 200ml, meaning a bottle should last approximately five to six full-body applications when used correctly. If a 200ml bottle is lasting a two-week holiday, not enough is being applied.

When to apply. Apply sunscreen 15 to 30 minutes before sun exposure to allow it to bind to the skin. This is particularly relevant for chemical sunscreens — mineral sunscreens (containing zinc oxide or titanium dioxide) provide protection immediately on application.

When to reapply. Reapply sunscreen regularly throughout the day, including products labelled once-a-day or water-resistant. The labelling of products as once-daily or water-resistant does not mean they do not need reapplying — it means they maintain their SPF rating for longer under controlled conditions than standard products. In practice, reapplication every two hours during outdoor sun exposure, and after swimming or towel-drying, is the appropriate approach for all sunscreen products.

Coverage. The most commonly missed areas are the tops of the ears, the back of the neck, the back of the hands, the tops of the feet, and the scalp along the parting. Spray sunscreens often deliver less than 50% of the labelled SPF unless applied for at least six seconds per limb and rubbed in. Lotions applied by hand provide more consistent coverage than sprays for most people.

The Clothing and Shade Argument

No sunscreen, no matter how high the SPF, can provide 100% protection from the sun. Sunscreen should be used together with shade and clothing to give skin the best protection.

This is the framing that most sun protection guidance leads with — and it is the correct one. Sunscreen is one component of a layered protection strategy, not the whole of it.

Clothing is actually the most reliable form of UV protection for the areas it covers. A standard white cotton T-shirt has a UPF (Ultraviolet Protection Factor) of approximately 5 to 7 — considerably lower than most people assume. Darker, more tightly woven fabrics provide significantly better protection. Purpose-made UPF clothing with ratings of 50 or above blocks 98% of UV radiation and is worth considering for prolonged outdoor exposure.

Wide-brimmed hats — with a brim of at least 7.5cm — protect the face, ears, and neck more reliably than sunscreen alone. Wrap-around or close-fitting sunglasses that carry a CE mark and UV400 marking protect the eyes and the delicate skin around them from both UV damage and cumulative cataract risk.

Shade between 11am and 3pm — when UV intensity is at its peak — reduces UV exposure significantly. This does not mean avoiding the outdoors entirely: sitting under a tree or umbrella, moving into shade for lunch, and planning outdoor activity for the early morning or evening during a heatwave achieves protection without restricting enjoyment of the weather.

The Vitamin D Balance

The most common counter-argument to rigorous sun protection is the vitamin D concern — the idea that blocking UV exposure will cause vitamin D deficiency.

This concern is real but often overstated. A UK Biobank study of 502,492 participants found that by prioritising cutaneous cancer risk and treating vitamin D as the sole justification for intentional sun exposure, current guidance adopts a narrow view of sunlight that sits uneasily alongside emerging evidence for broader cardiometabolic and immunological effects of UV radiation.

The balance is nuanced. Some UV exposure is necessary for vitamin D synthesis and may have broader health benefits beyond vitamin D. The public health argument for sun protection is not that all UV exposure is harmful — it is that unprotected overexposure is the primary preventable driver of skin cancer.

In practical terms: incidental sun exposure during normal outdoor activity — a 20 to 30 minute walk, gardening, outdoor lunch — produces meaningful vitamin D synthesis without requiring deliberate tanning or sunburn. The concern about vitamin D deficiency from sun protection is most relevant to people who wear full-coverage clothing outdoors for cultural or religious reasons, and supplementation with vitamin D is the appropriate response for those individuals — not reducing sun protection.

Darker Skin Tones: A Specific Clarification

One of the most consistent misconceptions in UK sun protection messaging is that people with darker skin tones do not need sun protection.

Melanin does provide some protection against UV damage — higher melanin concentrations mean higher natural SPF. But this natural protection is estimated at approximately SPF 2 to 4 for dark skin tones — considerably lower than a standard sunscreen application.

Anyone can get sunburnt. For people with darker skin tones, sunburn might feel tender or itchy rather than look red or pink — making it harder to identify. Skin cancer rates in people with darker skin tones are lower than in lighter skin, but survival rates are significantly worse — partly because skin cancers in darker skin tones are often detected at a later stage due to the assumption that UV damage is not occurring.

Cancer Research UK's advice applies to all skin tones: SPF 30 or above with 4 or 5 star UVA protection, applied generously and reapplied regularly.

Frequently Asked Questions

What SPF should I use in the UK?

Cancer Research UK recommends SPF 30 or above with a 4 or 5 star UVA rating for protection against both UVA and UVB radiation. During a heatwave when the UV index reaches 7 or above, SPF 50 with 5 star UVA protection is appropriate. The star rating or UVA circle symbol on the packaging indicates UVA protection — an SPF number alone only confirms UVB protection.

How much sunscreen should I apply?

For the full body, approximately 35ml — around six to eight teaspoons — is needed to achieve the labelled SPF. For the face alone, approximately half a teaspoon. Most people apply a quarter of the necessary amount, which means their effective SPF is significantly lower than the product label suggests. Apply generously, do not miss commonly overlooked areas including ears, neck, back of hands and tops of feet, and reapply every two hours during outdoor exposure.

Does sunscreen need to be reapplied?

Yes — all sunscreen products need reapplying during prolonged outdoor exposure, including those labelled once-daily or water-resistant. Reapply every two hours outdoors and immediately after swimming or towel-drying. Water-resistant products maintain their rating for longer in water than standard products but still need reapplying after leaving the water.

Does SPF 50 mean double the protection of SPF 25?

No — the scale is not linear. SPF 25 filters approximately 96% of UVB. SPF 50 filters approximately 98%. The difference between SPF 30 and SPF 50 is approximately 1% more UVB filtered. Both provide very good protection when applied correctly. The amount applied matters more than the SPF number above SPF 30.

Can you get sunburnt through a window?

Yes — partially. Standard window glass blocks most UVB but allows UVA through. UVA is responsible for skin ageing and contributes to melanoma. Working near windows exposes skin to a cumulative UVA equivalent to approximately 20 minutes outdoors. Daily SPF on the face is relevant for anyone who spends significant time near windows, whether at home or at work.

Do people with darker skin need sunscreen?

Yes. While darker skin tones have higher natural melanin protection — equivalent to approximately SPF 2 to 4 — this does not provide sufficient protection against cumulative UV damage, skin ageing, or skin cancer risk. Anyone can get sunburnt. For people with darker skin tones, sunburn might feel tender or itchy rather than look red or pink. Cancer Research UK recommends SPF 30 or above for all skin tones.

The Bottom Line

86% of melanoma cases are preventable. The tools are available, inexpensive, and well-understood. The gap between knowing sun protection matters and applying it correctly is primarily a practical one — applying enough product, covering all exposed areas, reapplying consistently, and combining sunscreen with shade and clothing rather than relying on sunscreen alone.

During a heatwave, when UV index values in London reach 7 to 8, the priority is straightforward: SPF 30 or above with 4 or 5 star UVA protection, applied generously 15 to 30 minutes before going out, reapplied every two hours outdoors, and combined with shade between 11am and 3pm and appropriate clothing for prolonged outdoor time.

The skin you are protecting this week will thank you in twenty years.

For the evidence on what UV damage actually does to skin at a cellular level and what the cumulative effects look like over time, read our detailed piece on sun damage and oxidative ageing: Is It the Sun or Your Age That's Damaging Your Skin?.

For a calmer, more consistent skin and wellness routine through the summer, pair this with Companion — Vitae's AI wellness companion for personalised, evidence-led prompts on sleep, hydration, and recovery.

Related reading: 80% of Skin Ageing Is Sun Damage. Not Time. · Are Face Serums Worth the Hype? · Cortisol Explained — and How to Reduce It Without Making Things Worse

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