Mathematically, almost nothing: SPF 30 blocks 97% of UVB, SPF 50 blocks 98% - doubling the number shaves one percent.
The one practical argument for 50 is under-application: most people apply only 20 to 50% of the tested amount, so a higher label buys margin. The traps: high SPF lasts no longer than low - reapply every two hours - and the false confidence that licenses longer exposure. The AAD floor is broad-spectrum SPF 30. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper .
What to do
Common questions
SPF 30 vs 50: does the difference matter?
Mathematically, almost nothing: the American Academy of Dermatology notes SPF 30 blocks 97% of UVB rays, and a dermatologist at UT Dell Medical School lays out the full ladder - SPF 15, 30, 50, and 100 block 93%, 97%, 98%, and 99% respectively. Doubling the SPF number does not double the protection; it shaves one percent. But there is one practical argument for 50, and it is you: most people apply only a fraction of the tested amount - the AAD cites 20 to 50% - so real-world SPF 30 can behave like SPF 20, and starting higher buys margin against under-application. The traps: high SPF lasts exactly as long as low SPF - reapply every two hours regardless - and the false-confidence effect, where a bigger number licenses longer sun exposure, erasing the gain. The AAD floor is SPF 30, broad spectrum. Below that is under-protecting; above 50 is marketing; the shot-glass amount and the two-hour reapply are the variables that actually decide your burn.