SPF 100 blocks only a little more UVB than SPF 30
SPF measures how much longer skin takes to redden from UVB—the rays chiefly responsible for sunburn—when sunscreen is applied correctly, compared with bare skin. It is not a percentage score, and the difference between SPF numbers shrinks at the high end.
In laboratory conditions, SPF 30 filters about 97% of UVB, while SPF 100 filters about 99%. That is roughly a two-percentage-point increase in the share blocked. But the small amount that gets through changes more dramatically: about 3% passes through SPF 30, compared with about 1% through SPF 100. So SPF 100 lets through roughly two-thirds less UVB than SPF 30 under the test conditions—not zero.
The advantage depends on how sunscreen is used
Those figures assume an even, generous application. People often apply less than the amount used in testing, which can reduce the protection they actually get. A higher SPF can provide a larger margin for error, but it cannot make a thin layer behave like a properly applied one.
Sunscreen also wears away with swimming, sweating, and wiping, so it needs to be reapplied. And SPF describes UVB protection, not protection from UVA rays, which also damage skin; a broad-spectrum label indicates protection against both types.
The practical tradeoff is therefore modest but real: SPF 100 blocks only a little more of all incoming UVB than SPF 30, yet that difference can substantially reduce the UVB that slips through—provided the sunscreen is applied well and kept in place.
