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