"No SPF, not even 100+, offers 100 percent protection. What’s more, both UVA and UVB radiation can lead to skin cancer, which is why dermatologists now advise using sunscreens with an SPF of at least 15 and UVA-fighting ingredients like an avobenzone that doesn’t degrade in light or Mexoryl SX.
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).
A sunscreen’s SPF number is calculated by comparing the time needed for a person to burn unprotected with how long it takes for that person to burn wearing sunscreen. So a person who turns red after 20 minutes of unprotected sun exposure is theoretically protected 15 times longer if they adequately apply SPF 15. Because a lot of sunscreens rub off or don’t stay put, dermatologists advise reapplication every two hours or after swimming or sweating."
"It has long been assumed that applying half the recommended ounce meant half the SPF protection. But a small 2007 study...found that 'If you apply half the amount, you get the protection of only the square root of the SPF.'...So applying a half-ounce of SPF 70 will not give you the protection of SPF 35, but 8.4."
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