The Only Product With Proof

There is a great deal of debate in skincare and remarkably little of it is settled. Sunscreen is the exception. Daily broad-spectrum protection is the most evidence-backed intervention available over a counter, and the studies are not subtle: in a four-year randomised trial in Queensland, the group applying sunscreen daily showed no detectable increase in skin ageing while the group using it discretionarily did. Nothing else on the shelf has a result like that.

What the numbers mean

SPF measures protection against UVB, the wavelength responsible for burning. SPF 30 filters roughly 97 per cent of UVB and SPF 50 about 98 per cent, which sounds like a trivial difference until you notice that it halves the amount getting through. The step from 15 to 30 is the important one; the step from 30 to 50 is worth having and the step beyond 50 is marketing.

UVA, the wavelength that drives ageing and pigmentation, is measured differently and is where products diverge. In Europe, look for the UVA symbol in a circle, which certifies UVA protection at least a third of the SPF figure. In Asian products, the PA rating with three or four plus signs is the equivalent signal. A high SPF with poor UVA protection is a product that stops you burning while you age.

The best sunscreen is not the highest number on the shelf. It is the one you will put on again in August at four in the afternoon.

The quantity problem

Laboratory SPF is measured at two milligrams per square centimetre of skin. In real life, people apply between a quarter and a half of that, which means an SPF 50 applied normally is behaving like an SPF 15 to 25. This is the single largest gap between what sunscreen can do and what it does.

For a face and neck, the reliable measure is roughly a third of a teaspoon, or the two-finger rule: a line of product along the length of your index and middle fingers. It will feel like too much for the first week. It is not.

Mineral or chemical

Mineral filters, zinc oxide and titanium dioxide, sit on the skin and are excellent for reactive or rosacea-prone faces. They can leave a white cast, which modern formulations have improved but not eliminated on deeper skin tones. Organic filters, the ones often called chemical, absorb into the top layer and convert UV to heat; the newer European filters such as Tinosorb and Uvinul are photostable, elegant and give better UVA coverage than most mineral formulas.

Neither category is dangerous. The safety concerns that circulate about oxybenzone concern a filter that European formulators have largely moved away from, and the doses in the studies were far above real use. Choose on texture and cast, not on the mineral-versus-chemical argument.

What to do in practice

• Apply as the last step of the morning routine, before make-up, every day of the year including cloudy ones. UVA passes through cloud and window glass; UVB largely does not.
• Reapply if you are outdoors: every two hours, or after swimming and towel drying. Indoors at a desk away from a window, a single morning application is enough.
• Do not forget the ears, the hairline, the back of the neck and the backs of the hands, which are where photoageing shows first and where nobody applies anything.
• If reapplication over make-up is the obstacle, a powder or stick formulation makes it possible. Less protection applied at four o’clock beats perfect protection you skip.

The one caveat

Sunscreen is not a substitute for shade, a hat or sensible timing, and no product makes an hour of midday July sun harmless. It also does not mean avoiding daylight entirely; vitamin D, mood and sleep all depend on getting outside. The aim is protection, not avoidance.