There’s some sort of arms race between sunscreen lotion companies to offer the absolute highest protection possible, but the New York Times points out that it’s mostly a marketing gimmick.
The difference in UVB protection between an SPF 100 and SPF 50 is marginal. Far from offering double the blockage, SPF 100 blocks 99 percent of UVB rays, while SPF 50 blocks 98 percent. (SPF 30, that old-timer, holds its own, deflecting 96.7 percent).
What’s more, SPF only applies to UVB rays, not UVA, which are also cancer causing. To protect yourself from UVA, look for added ingredients “like an avobenzone that doesn’t degrade in light or Mexoryl SX.”
One consequence of the rising SPF numbers is it can trick consumers into thinking that their crappy SPF 45 sunscreen is inadequate. It isn’t. Considering you’re advised to reapply sunscreen every 2 hours, any cream with an SPF of 15 or higher will work—just be sure to use enough:
Skimp and you lose. To get the SPF advertised, you must use a full shot glass on your body. That’s an ounce, which means a three-ounce tube should last, at most, a few outings.