The invention relates to a method for determining an integral sun protection factor (.intg. 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.times.10.sup.12 free radicals/mg of skin sample, and determining the integral sun protection factor .intg. SPF according to the equations (1) and (2) and (8).intg..times..times..times..times..function..function..times..times..t- imes. ##EQU00001##