What SPF measures, and what it leaves out

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Researched. We have not used this one long enough to give you a verdict, and we would rather say so than invent one. What follows is how it works, who it suits, and what it will not do. When we have used it properly, this page carries the verdict and the date.

SPF is a ratio. Once you can see the ratio, the rest of the label stops being mysterious.

The number comes out of a lab. A patch of skin gets a measured dose of ultraviolet until it just turns pink; the same is done with sunscreen on. The SPF is how much more ultraviolet that took. SPF 30 means the burning radiation reaching skin is roughly one thirtieth of what would otherwise have arrived.

That is the whole definition. Not a percentage, not a number of minutes, not a promise about your morning.

Why the jump from 30 to 50 is smaller than it looks

Write the fractions down. Bare skin: all of it arrives. SPF 30: a thirtieth. SPF 50: a fiftieth.

The distance from nothing to 30 is enormous. The distance from 30 to 50 shaves a sliver off a remainder that was already small. The packaging reads as though it offers two-thirds more protection, because 50 is two-thirds more than 30, and that is not what the number does.

This is not an argument for buying 30. Higher is higher and the sliver is free. It is an argument against treating a high number as a licence: longer outside, thinner layer, no reapplication.

The number only describes half the problem

The SPF test measures redness, and redness is mostly UVB — the shorter wavelength, the one that burns, strongest in the middle of the day and the middle of summer.

UVA is the longer wavelength. It burns very little, and the SPF test barely registers it — which is how a sunscreen can post a large number while doing almost nothing about UVA. It reaches deeper, into the layer that holds the skin's structure, and it is the wavelength associated with photoageing and with pigment. It is also less obliging: more constant across the day and the year, and it comes through window glass.

So a high SPF with weak UVA protection is a product that stops you noticing. Sunburn is a warning signal, and removing the signal without the cause is not a neutral outcome.

If pigment is why you are here, UVA is the line to look for, and what the brightening ingredients can and cannot do about a mark is the other half of it.

How to find the UVA answer on a label

It depends on where the bottle was labelled.

PA — Japanese in origin, and on most Korean bottles too. PA+ through PA++++, from a measured UVA test on skin. The scale began with the Japanese cosmetics industry rather than with a regulator, and Korean rules use the same marks, which is why so much of what people import carries one. Each plus is a band, and the bands roughly double as you climb, so PA++++ sits nearer four times PA++ than twice it. Two extra plus signs is a much larger step than two extra plus signs look.

The circled UVA mark, in the EU. The letters UVA inside a circle, from a European Commission recommendation the industry follows. It means UVA protection of at least a third of the labelled SPF — a ratio, not a fixed value, so it scales: a circled-UVA SPF 50 must carry more than a circled-UVA SPF 15. A floor, not a grade.

"Broad spectrum", in the United States. The FDA's own, and pass or fail. A product clears one threshold test or it may not use the words, and there is no gradation above the line. An American label gives you the burn number to two figures and the UVA answer as a single bit. The silence is a labelling rule, not a fact about the product.

Mineral and chemical, without the moralising

Mineral filters are zinc oxide and titanium dioxide, sitting at the surface. Marketing calls them physical blockers that reflect light; mostly they absorb it, like the others. Zinc has the useful reach across UVA. The honest complaint is the white cast, and its consequence is that people apply less than they should.

Chemical filters — organic filters, properly — absorb ultraviolet and shed the energy as heat. They are cosmetically far better, and they are why a sunscreen can feel like nothing. Some are less stable in light and need a partner ingredient to hold up; avobenzone, which does most of the UVA work in American formulas of that kind, is the well-known case.

Neither category is the moral one, whatever the packaging implies, and we would not choose between them on that basis. The more useful complaint is the list. In the United States sunscreen is regulated as an over-the-counter drug; in the EU, Japan and Korea it sits under cosmetic rules. A new filter therefore has to clear a drug pathway, which is slow, expensive, and rarely worth it for an ingredient rather than a product. So the American list is short and old by international standards, and filters that are unremarkable in European, Japanese and Korean sunscreens are not on it. That is most of the reason those sunscreens feel better. Not better chemists. A longer list.

The quantity problem, which is the real one

Here is the part that undoes everything above.

The SPF on the bottle is measured at a fixed application density: two milligrams of product per square centimetre of skin. That is a thick layer, far more than almost anybody applies. The rules of thumb that circulate — two fingers' length for face and neck, about a shot glass for a whole body — are attempts to turn that figure into something you can do at a sink. They are not standards, but they exist because the honest amount surprises people.

The shortfall is probably not proportional either. One of the accounts in circulation, and the one quoted most, has protection falling away faster than the quantity does: halve the amount and you land nearer the square root of the number than half of it. On that account, half a dose of SPF 50 is not behaving like an SPF 25. It is behaving like a single-figure number. Other measurements come out closer to a straight line, which would be less alarming and still not good.

You do not have to settle that argument to act on it. The direction is not in dispute: the amount you apply matters more than the number you bought.

Used properly, a bottle empties fast — which is its own argument for the cheaper one you will use generously, the same arithmetic we ran on pads against cotton rounds.

Reapplication, the step nobody takes

Filters degrade in light, and sunscreen comes off on towels, sleeves and your own hands. The standard guidance is roughly every two hours outdoors, and after swimming, sweating or towelling.

Water resistance is a labelled claim rather than a property. American labels may state 40 minutes or 80 minutes, tested; the absolute version of that claim — the one implying a film that never comes off in water or in sweat — is not permitted at all, because it is not true of anything.

Almost nobody reapplies, and it is worth being honest about why rather than repeating the instruction louder. Over makeup it is awkward. Powders and sticks beat skipping it, but you apply less of them than you think.

So match the effort to the day. An afternoon outdoors is a different problem from a commute, and only one of them is settled by the number on the bottle.

What it will not do

• It will not make time in strong sun free. It reduces a dose; it does not delete one. Shade, a hat and the hour you go out do work no bottle does.
• It will not undo pigment already there. It stops you adding to it, which is why a brightening routine is built on top of sunscreen rather than beside it.
• It will not survive being buried. It is the last step of a morning, and which step goes where matters here more than almost anywhere.

If your routine contains an exfoliating acid, none of this is optional. The acid that gets down into the pore lining leaves skin more sensitive to sunlight, and an acid with no sunscreen behind it is money spent on ground you give straight back.

Texture is the specification

The best sunscreen on paper — highest number, most modern filters, the one everybody online agrees on — is worth nothing if it pills under makeup, stings your eyes or leaves a cast you hate. You will skip it on the mornings you are in a hurry, and those are most mornings.

So make feel the specification rather than the tiebreak. An unremarkable sunscreen applied properly every day beats a superb one applied twice a week. That is not the consolation prize. It is the finding. Choose on texture and smell and whether you will reach for it, check that the label answers the UVA question, and buy the largest size you will use generously.

And unlike everything else on the shelf, you will never see it working. There is nothing to check in the mirror and none of the waiting and watching the rest of a routine asks for. The evidence that a sunscreen did its job is a photograph nobody took, of a face you do not have.

The product

We are not naming one here at the moment. The sunscreen this section recommended is no longer listed at the only place we would send you, and an unavailable product is not a recommendation — it is a dead end with our name on it. The space stays empty until there is one worth naming.

Written by JamieG in Garner Town, Tennessee.