What SPF actually means
Most of what people believe about sunscreen is half a fact that stopped halfway. This is the long version, written by the people who make it, with nothing in it we would not say to a dermatologist.
The number is a ratio, not a strength
SPF measures one thing: how much longer skin takes to redden with a sunscreen on than without it, under laboratory conditions. It is a UVB number, and it is a ratio — which is why the arithmetic surprises people.
SPF 30 filters about 97% of UVB. SPF 50 filters about 98%. The whole distance between them is one percentage point, and the distance between nothing and SPF 30 is ninety-seven. Doubling the number on the bottle does not double anything. It is worth understanding this before paying for it.
What SPF 50 does buy is margin — for the part you rubbed off, the part you missed, and the hour you went past. That margin matters most when the conditions are hardest, which is why it is on the sport and the mineral formulas and not on the face fluid.
Almost nobody applies enough
This is the most useful thing on this page. SPF is tested at 2 milligrams per square centimetre of skin — a genuinely thick, visible layer. Study after study finds real people apply somewhere between a quarter and a half of that.
Protection does not fall off in proportion; it falls off faster. An SPF 50 applied at a third of the tested thickness does not behave like an SPF 17, it behaves worse. The practical consequence is simple and unglamorous: the amount is more important than the number. Roughly a shot glass for a body. Two fingers' length for a face, neck and ears.
UVA, UVB, and what "broad spectrum" claims
UVB is the burn. It is what SPF measures, it peaks in the middle of the day and in summer, and it is largely stopped by window glass. UVA is the longer wavelength — it penetrates deeper, it is roughly constant through the day and the year, it goes through glass, and it is the one associated with the slow changes people call ageing.
"Broad spectrum" is a test, not an adjective. A product earns it by holding protection across a defined span of wavelengths — the critical-wavelength test, at 370 nanometres. It is the phrase worth checking for, and it is the one most shoppers skip past on the way to the big number.
Mineral, chemical, and the mirror that isn't
The standard explanation is that mineral sunscreen sits on top of the skin and reflects UV like a mirror, while chemical sunscreen absorbs it. The first half is mostly wrong. Zinc oxide absorbs the large majority of the UV that hits it and converts it to a trace of heat, exactly as the organic filters do. It reflects and scatters only a small fraction, mostly at the visible-light end — which is precisely the part you see as a white cast.
Both categories work. They differ in finish, in how they behave on sensitive skin, in photostability and in cost. Choose on those, not on a diagram from a magazine. We use organic filters where invisibility and water resistance decide whether a product gets worn, and zinc where a single, simple filter is the better answer — on children, on reactive skin, and on a stick you want to see going on.
Water resistance, and the word that disappeared
"Waterproof" and "sweatproof" were banned from sunscreen labels because no sunscreen is either. What is allowed is a tested claim of 40 or 80 minutes of water resistance, and 80 is the maximum anyone can say.
The towel does more damage than the water. Swimming degrades the film gradually; drying off removes it mechanically, in one pass, from exactly the places you just wet. Reapply after the towel, not after the swim.
Altitude and snow
There is less atmosphere above you the higher you go, so less UV is absorbed before it arrives — roughly ten per cent more for every thousand metres gained. Fresh snow is the most reflective natural surface most people ever stand on, throwing back up to eighty per cent of what lands on it. Between the two, a clear day on a glacier delivers a dose that a beach in July does not approach.
Cold is what makes this dangerous rather than merely notable. The sensation that normally tells you to get out of the sun — heat on the skin — is absent at minus five with a wind. People come off a mountain burned across the nose, the ears and the underside of the chin, which are the surfaces facing the reflected light, and are genuinely surprised.
Reapplication, concretely
- Every two hours of continuous exposure. Set something to remind you; willpower is not a schedule.
- Immediately after towelling, whatever the bottle's water-resistance claim says.
- Fifteen minutes before you go out, not once you are already in the sun.
- Ears, the tops of feet, the back of the neck, the parting in your hair, and the underside of the nose on snow. These are the misses, every time.
What we leave out, and why
No oxybenzone and no octinoxate, in any formula. Both are restricted in Hawaii, Key West and a growing list of marine parks over reef-toxicity findings, and oxybenzone in particular shows systemic absorption at levels that regulators have asked more questions about than we would like to be waiting on. The performance is replaceable. The argument is not worth having.
No fragrance, anywhere. Fragrance is the most common avoidable irritant in skincare, and a sunscreen's job is to be worn daily by people who may already be reacting to something.
We do not say "chemical-free". Everything is a chemical, zinc oxide included, and a brand that says otherwise is telling you it thinks you will not check.