Sunscreen: What SPF Means, How Much to Use and What It Really Protects Against

Sunscreen lowers the dose of ultraviolet light reaching your skin — if you put on enough and keep reapplying. Mainly it buys fewer sunburns and a lower risk of skin cancer; slower skin ageing is an extra benefit confirmed in a randomised trial. It works alongside shade, clothing and a hat, not instead of them.

When to see a doctor

Right away, call an ambulance: signs of heatstroke after time in the heat or sun — confusion or loss of coordination, a fit, passing out, a very high temperature, hot skin, fast breathing or breathlessness, or no improvement after 30 minutes in a cool place with fluids (NHS, UK). The skin may be dry or still sweating: the NHS describes hot skin that's not sweating, while NIOSH/CDC (US) list hot, dry skin or profuse sweating. Call 999 in the UK, 911 in the US, 112 in the EU.

The same day: sunburn with blistered or swollen skin; a very high temperature or feeling hot, cold or shivery; feeling very tired, dizzy or sick; headache; muscle cramps; any sunburn in a baby or young child. In the UK, an urgent GP appointment or 111 (NHS).

Routinely: a mole that changes size, shape or colour, itches, bleeds or crusts, or a new unusual mark that does not heal within a few weeks (NHS, UK). How to look: check your skin regularly.

What is happening

What SPF means. SPF tells you how much more solar energy it takes to burn protected skin than unprotected skin; it does not tell you how long you can stay in the sun (FDA, US). It measures protection against UVB, the rays that burn (NHS). Simple arithmetic from the definition: applied as tested, SPF 15 lets through about 1/15 of the burning dose (7%), SPF 30 about 1/30 (3%), SPF 50 about 1/50 (2%). The step from 30 to 50 is smaller than the numbers suggest — and too thin a layer wipes it out.

Most people use too little. SPF is measured at 2 mg per square centimetre of skin; people actually apply roughly 0.4–1 mg, so they get much less protection than the label says (Petersen and Wulf, 2014).

UVA needs its own label. In the US, “Broad Spectrum” means the product passed a UVA test; only broad-spectrum products with SPF 15 or more may claim to reduce the risk of skin cancer and early skin ageing when used with other sun protection (FDA). In the UK, stars (up to 5) rate UVA protection, and a “UVA” logo in a circle means the product meets the EU standard (NHS). “Water resistant” in the US means 40 or 80 minutes of swimming or sweating (FDA); reapply after water anyway (NHS).

What is proven. In the Nambour trial (Australia, 903 adults under 55), people who applied broad-spectrum sunscreen daily for 4.5 years showed no detectable increase in skin ageing and aged 24% less than those who used it at their discretion (Hughes et al., 2013). The same trial found fewer squamous cell skin cancers, though no effect on basal cell carcinoma (Green et al., 1999). Ten years later there were fewer melanomas in the daily group — a reduction that was borderline for all melanomas and clearer for invasive ones (Green et al., 2011). That is one well-run trial in a sunny place: strong support, not an overwhelming body of evidence. The WHO adds that childhood sunburn raises skin cancer risk later, as do sunbeds. A tan does not protect you, and in the UK you can burn on a cloudy day (NHS).

Vitamin D. Older trials with SPF around 16 found no effect on vitamin D levels (Neale et al., 2019), but the Sun-D trial (Australia, 2025, 639 adults) found that daily SPF 50+ for about a year lowered vitamin D levels and made deficiency more common than using sunscreen at will. The authors' advice: keep using sunscreen and consider testing or a supplement — see supplements without a test.

Absorption. In an FDA trial, filters reached the blood at levels that call for more safety studies; the authors stressed this is not a reason to stop using sunscreen (Matta et al., 2020).

What to do

Use enough: in the US, at least an ounce (about a shot glass) for the whole body (FDA); in the UK, about 6–8 teaspoons for an adult's whole body (NHS).
Choose: in the US, broad spectrum, SPF 15 or more (FDA); in the UK, SPF 30 or more with at least 4 stars (NHS).
Apply before going out — for long spells, 30 minutes before and again just before (NHS) — and reapply every 2 hours and after swimming, towelling or sweating (FDA, NHS).
Cover all exposed skin — face, ears, neck, scalp if your hair is thin — or wear a wide-brimmed hat (NHS).
Check the UV index: protect from index 3 (WHO). Seek shade midday — in the UK 11 am to 3 pm from March to October (NHS), in the US 10 am to 2 pm (FDA). Sunscreen is not a reason to stay out longer (WHO).
Protect your eyes: sunglasses protect your eyes. Daylight itself is not the enemy: sunlight, sleep and vitamin D.

When this is not the case

Babies under 6 months: keep them out of direct sun (NHS); the FDA does not recommend sunscreen before 6 months. Older children need clothing, shade and sunscreen on exposed skin (NHS).
Medicines that raise sun sensitivity: some antibiotics (doxycycline, ciprofloxacin), anti-inflammatories such as ibuprofen and naproxen, some diuretics, retinoids (including isotretinoin) and others (FDA). Not everyone reacts; check your leaflet and ask a pharmacist.
Pregnancy makes overheating more likely: the body works harder to cool itself and dehydrates more easily (CDC, US). Rest, shade and fluids; any symptoms of heat illness need care at once.
Brown or black skin has some natural protection but can still burn and get skin cancer, and needs more sun to make vitamin D (NHS).
Noise damage adds up silently too: protect your hearing.

Sources

Checked: 24 September 2026