The bottle says „water resistant” and we read it as „no need to do it again”. That is the mistake dermatologists repeat every summer, and every summer somebody comes back from the beach burned and confused because they „put it on in the morning”.

Dr Miguel Sánchez Viera, director of the Institute of Integral Dermatology, puts it very directly: „Water resistant doesn't mean waterproof. These products are formulated to retain part of their effectiveness while you swim, but you always lose some of the product in the water. When you dry yourself with a towel, you remove even more, even if you're careful.”

So that's two losses, not one. First the water, then the towel. And the second is bigger than we think, because friction does precisely what a cream cannot defend itself against.

The mechanism is simpler than it sounds. Sunscreen forms a very thin film on the skin that intercepts ultraviolet radiation. Over hours, that film stops being even - sweat, friction and water strip it unevenly, leaving patches with no protection. On top of that, the filters themselves gradually lose their capacity to absorb or reflect radiation. The cream doesn't stop working all at once; it works worse and worse, and you can't see it happening.

There is another myth about the number on the bottle. SPF shows how much UVB radiation it filters and by how much it multiplies your time without burning - around 300 minutes with SPF 30, around 500 with SPF 50, against ten minutes with nothing. But the key part comes next: both SPF 30 and SPF 50 must be reapplied equally often. The higher number doesn't buy you more time between applications. It buys you higher protection within the same interval.

In practice, three things carry the whole difference. Apply the cream 20 to 30 minutes before going into the sun, so the filters bind and form an even layer. Reapply after every swim, without exception. And where possible, dry the skin before applying, because sunscreen holds better on dry skin.

None of this requires a new product. Which is also why we rarely hear it said out loud - advice that isn't for sale rarely gets an advertising budget.