On sun screen creams you will find a SPF, or sun protection factor figure, which attempts to give you some idea how long you can stay out in the sun without getting sunburn after applying it.

This figure is experimentally determined by taking a volunteer, placing them in the sun and timing how long it takes their skin to redden, with and without the sunscreen in question. The compound is applied in a standard dose of 2 milligrams per square centimeter of skin, and the SPF is given by the following equation :-

SPF = time taken for skin to redden with sunscreen / time taken for skin to redden without sunscreen

One major problem is skin reddening is caused by UVB radiation (280-320nm), no information is given about UVA protection. Most sun creams have this information given seperately now however. Also none of the creams as far as I now provide protection against UVC, as the ozone layer does that for us, at the moment.....

Source :- The Right Chemistry, Jeffery Hancock, Hodder & Stoughton ISBN 0-340-70194-3

I, like just about everyone I have talked to about this, was under a false assumption about the SPF. I think that most people assume, like I did, that the higher the SPF number, the more protection you get; SPF 40 protects 4 times more than SPF 10. Well if you paid close attention to the above writeup you will notice that SPF is all about time. SPF 10 and 40 protect you the same amount, but the 40 does it longer. This is crucial information for someone like me who burns from little more than watching an episode of Baywatch.

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