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Generated by an infinite conga-space of monkeys coerced by: Ben Carr

**The author writes:**

Ever wondered what the SPF number of a sunscreen means?

SPF *X* means that if it takes you *T* minutes to burn normally, then you will burn equally in *XT* minutes after proper application (about 30 grams across your body) of that sunscreen.

SPF 10 means that you should burn 10 times more slowly, by blocking out 90% of the sun's ultraviolet rays. SPF 100 means that you should burn 100 times more slowly, by blocking out 99% of the sun's rays.

In general: SPF *X* means that you should burn *X* times more slowly, by blocking out (100 - 100/*X*)% of the sun's rays.

You can infer from this that applying half the recommended amount blocks out only a quarter as much sun (taking the square root of the SPF rather than halving it). And in fact most people do under-apply sunscreen, using roughly half of the recommended amount. A half application of SPF 50 is the same as a full application of sunscreen with SPF of just 7 and a bit!