The invention relates to a method for determining an integral sun protection factor (∫SPF or iSPF) which encompasses both UVA and UVB radiation and can be used for classifying cosmetic and dermatological sunscreens. The method comprises the steps of applying a defined amount of the sunscreen onto a defined area of a skin substrate impregnated with a spin trap, exposing the skin substrate to a defined amount of UVA- and UVB rays by a sun simulator, subjecting the skin substrate exposed to radiation to an electron spin resonance (ESR) measurement, recording the number of free radicals trapped as a real number×1012 free radicals/mg of skin sample, and determining the integral sun protection factor ∫SPF according to the equations (1) and (2) and (8) ∫ S   P   F = E 0 E ( 1 ) log 10  ( E ) = a · log 10  ( X · 1000 ) + b ( 2 ) E = 10 log   10  ( E ) . ( 8 )