> The actual difference between (say) SPF 30 and 50 is not a lot, 96.7% UV filtering vs 98% but I’m not 100% sure how that translates to actual rates of cancer.

Counterintuitively, higher SPF matters a _lot_. The difference is in the _durataion_ of the protection and in the amount of sloppiness you can afford while applying the cream.

Suppose that for you the half-life for the sunscreen is 1 hour. SPF 30 cream would thus decay to SPF 7 in 2 hours, providing little protection. But an SPF 90 cream would still offer quite reasonable SPF 25 protection.

The same applies to sloppiness. SPFs are measured in perfect conditions, with a prescribed amount of the cream spread evenly. So the higher the SPF, the more mistakes you can make while applying it.

Would different SPF sunscreens have same half-life? I have not dig into it, but I would think there is a few mechanism or chemicals and those would have different halflifes.