No SPF, not even 100+, offers 100 percent protection. SPF only rates UVB protection, but both UVA and UVB radiation can lead to skin cancer, which is why dermatologists now advise using sunscreens with an SPF of at least 30 – but look carefully to make sure the sunscreen gives UVA protection too, there is currently no rating system in the US for UVA protection.
The difference in UVB protection between an SPF 100 and SPF 50 is marginal, SPF 100 does NOT offer double the blockage,
SPF 100 blocks 99 % of UVB rays.
SPF 50 blocks 98 % of UVB rays.
SPF 30, blocks 96.7 % of UVB rays,
A sunscreen 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.
Dermatologists advise reapplication every two hours or after swimming or sweating, this is because sunscreens can rub off and degrade over time.
Consumers should worry more about wearing enough sunscreen.
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.
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