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