Sunscreen, and the four things people get wrong

If we could only keep one intervention in the clinic, it would be sunscreen, and it would not be close. It prevents the pigment we spend courses removing, it protects the collagen we spend courses rebuilding, and it costs less than a single session of anything. It is also, consistently, the thing patients do least well.

Quantity. The protection number on the bottle is measured at a thickness almost nobody applies. Most people use around a quarter of the tested amount, which does not give you a quarter of the protection — the relationship is worse than linear. For a face and neck, think of roughly two finger-lengths of product, which will feel like far too much the first few times.

Timing. Chemical filters need fifteen to twenty minutes to organise themselves into a film. Applied as you leave the house, they are doing substantially less than the bottle claims for the first part of your journey. Mineral filters work on contact but rub off more easily.

Reapplication. This is where courses are lost. Protection degrades over roughly three to four hours of daylight exposure, sooner with sweat or touching your face. If you are outdoors during the day, a midday reapplication is not optional — and a powder or stick over makeup is a perfectly acceptable way to do it, since the alternative in practice is not doing it at all.

The indoor question. Ultraviolet A passes through window glass and is a principal driver of pigment and photoaging. If you sit by a window or drive regularly, you are being exposed. Visible light also drives melasma specifically, which is why patients with pigment often do better with a tinted mineral product than with a clear one.

None of this is complicated, and none of it is exciting. It is simply the part of the plan that happens on the three hundred and sixty days a year when you are not in the clinic — which is why it decides the outcome.