Sunscreen, and the four claims worth believing

If a shelf has to be reduced to one product, this is the one that survives, and by a distance. Photoageing accounts for the large majority of visible skin change over a life, which means a sunscreen used properly outperforms every serum ever sold. It is also the product most often bought correctly and used wrongly.

The number, and what it measures

SPF measures protection against UVB, the wavelength that burns, and it measures it under laboratory application of two milligrams per square centimetre. That is a thick, visible layer. Real people apply between a quarter and a half of it, and protection falls faster than proportionally, so an SPF 50 applied at a third of the test dose is behaving like something closer to SPF 10.

The step between numbers is also smaller than it reads. SPF 15 blocks about 93 per cent of UVB, SPF 30 about 96.7, SPF 50 about 98. Going from 30 to 50 buys roughly one and a third percentage points, which matters at the margins and matters far less than applying enough of either.

The number that is missing

UVA, the longer wavelength that drives ageing and passes through window glass, is not measured by SPF at all. In Europe the circled UVA mark means UVA protection is at least a third of the SPF figure, which is a floor rather than a triumph. In Asian labelling the PA system runs from PA plus to PA four plus, and four plus is the one worth having. If a product carries a high SPF and no UVA indication whatsoever, it is half a sunscreen.

How much, in units you can actually measure

• Face and neck: a quarter of a teaspoon, roughly two full finger lengths of product squeezed along the index and middle finger.
• Each arm: half a teaspoon. Each leg: a full teaspoon. Front of body: a teaspoon. Back: a teaspoon.
• A whole adult body, properly covered, is about 30 millilitres, which means a 200 ml bottle is six full applications.
• If a bottle bought in May is still half full in September, it has not been protecting anybody.

The best sunscreen is the one you will put enough of on. Elegance in the formula is not vanity, it is compliance.

Mineral and chemical, without the argument

Mineral filters, zinc oxide and titanium dioxide, sit on the surface and scatter light. They are broad spectrum, stable, well tolerated by reactive skin and tend to leave a cast on darker skin tones, which is a real and frequently dismissed problem. Chemical filters absorb light and convert it to heat. Modern European and Asian filters are excellent, cosmetically far better and offer stronger UVA coverage than what is approved in some other markets.

Neither category is dangerous. The systemic absorption studies that made headlines measured blood levels of filters without establishing any harm from them, and the risk being weighed on the other side of that scale is skin cancer, which is not hypothetical.

The reapplication question, answered honestly

Two hours is the rule for actual exposure: outdoors, in daylight, or swimming and towelling. A day at a desk two metres from a north facing window does not degrade a morning application in any meaningful way, and the advice to reapply over make up every two hours indoors sells a great deal of product to people who do not need it.

What does need attention is the boundary: the hairline, the ears, the back of the neck, the tops of the feet in summer, and the hands, which are the first place on most people where photoageing becomes visible and the last place anybody thinks to cover.

Spotted an error? Tell the desk