Margen
You have an SPF 50 bottle in your bag and you think it has bought you the whole morning. It has not bought you time. An SPF 50 sunscreen demands exactly the same reapplication as an SPF 15, and the number on the bottle is not a clock: it does not turn twenty safe minutes into a thousand minutes in the sun.
SPF stands for sun protection factor, and what it compares is two doses: the ultraviolet radiation needed to redden your skin with cream on against the dose that would redden it without, measured in a laboratory. It talks about accumulated radiation. The intensity changes with the time of day, the latitude, the clouds, the altitude and the glare off water, so multiplying "the minutes I can take" by 50 only produces a false sense of safety.
The numbers are not as far apart as they look, either. Applied at the tested amount, an SPF 15 filters around 93 % of UVB radiation, the kind that burns; an SPF 30, 96.7 %; and an SPF 50, 98 %. Going from 15 to 50 gains you five points of filtering. It lets 2 % through instead of 7 %.
Here comes the condition almost nobody meets. To certify an SPF, 2 milligrams of cream are spread over every square centimetre of skin: a thick, even layer that, for an adult body in a swimsuit, works out at some 30 to 35 millilitres, the contents of a shot glass. In daily life people apply between a quarter and half of that, and on top of it they skip the ears, the neck, the tops of the feet, the hairline and the edge of the swimsuit.
What happens then is not simple arithmetic. In 2007 the Danish dermatologists Faurschou and Wulf measured in the British Journal of Dermatology how the factor falls as the amount drops, and the fall turned out to be exponential: at half the dose, an SPF 50 behaves closer to a 7 than to a 25. That is the number you are missing when you wonder why you got burnt with sunscreen on.
Everything else follows from that. The amount is the decision that matters, and you take it before leaving the house, with time to spread it unhurriedly over all the skin that will see sun. Two drops per leg is nowhere near the tested dose; the shot glass is. If a thick layer feels uncomfortable, split it into two passes: the first shows you the patches you skip and the second completes the film. And do not mix it with moisturiser in your palm: you dilute a formula that was tested on its own.
Those 2 milligrams per square centimetre rule the convenient formats too. With a spray you have to see the product on the skin and then go over it with your hand, because a quick cloud drifts off and leaves gaps. With a stick you need several overlapping passes. And a foundation with SPF delivers the advertised factor only if you apply the amount used in its test, far more than a natural finish allows. The format makes the habit easier and leaves the arithmetic untouched.
Then there is the clock, which does exist, though not the one you think: reapply every two hours while you remain exposed, and sooner if you have been swimming, sweating heavily or drying off with a towel. "Water resistant" means it survives a swim measured in minutes, not that it is waterproof. And putting a 30 on top of another 30 does not give you a 60: it replaces part of what has gone, nothing more.
It is also worth looking beyond the number. SPF mainly describes UVB, while UVA penetrates deeper and accumulates as ageing and cell damage, so look for broad-spectrum protection on the label. And treat the cream as the last barrier in a queue headed by shade, clothing, a hat and the middle hours spent indoors.
What does not work is using a high factor as permission to stay out longer. A 50 forgives small mistakes better than a 15, but it does not delay the sun or stop the product coming off on the towel. The instruction that makes the number on the bottle true fits in one line: apply far more than feels elegant, and apply it again two hours later.