[josu]

SPF is a Sun Protection Factor, meaning it multiplies the time it takes for your skin to burn. For example, if very light skin normally burns in about 10 minutes, SPF 20 stretches that to ~200 minutes, which is already over 3 hours. Since dermatologists recommend reapplying every 2 hours regardless, going beyond SPF 30–50 (which blocks ~97–98% of UVB) doesn’t add much practical benefit. Even for very fair skin, correct application and reapplication are far more important than chasing SPF 100.

[noosphr]

Where I live in summer I regularly get days with UV index above 15.

If you burn in 15 minutes under UV index 6 on the worst days that I've seen you'd burn in 5 minutes. So a SPF of 60 is as useful here like an SPF of 20 is wherever you live.

[anonym29]

Jesus H Christ, UV index of 15? I thought the 12 we see in the middle of Texas summers was bad. I've burnt in 10 minutes through a windshield with that.

[loeg]

This kind of SPF fatalism doesn't really make sense to me. There's absolutely no reason to quantize sun damage into "below burn time" and "above burn time." Damage is dose-dependent. Even burns come in different classes at different exposure durations; and maybe you'd prefer to get, you know, 30 seconds unprotected equivalent of sun damage instead of 3 minutes equivalent, at the same re-application interval.

If someone can make a true SPF 200 economically, it's valid for consumers to prefer that to a true SPF 100 or true SPF 50.