Aphrodite blogged below about SPF and in the comments is a discussion about the merits and whys and wherefores of various factors.
Well, interestingly, as I was reading the current fab issue of U Mag this morning (amazing, get it ladies, it's practically 100% devoted to gorgeous goodies this month) there was something very interesting in there about SPFs. A lady called Sally Penford, from the International Dermal Institute said that an SPF 15 blocks out 93.33% of UVB rays. But in order to get an SPF 30, double the amount of chemicals is needed to provide only 3% more protection!
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Because chemical sunscreens can irritate skin (as Aphrodite indicated below), this is often why a factor of 15 is used - as a sort of lowest common denominator. Interesting stuff, eh?
