How Is The SPF Number Determined?

The SPF--sun protection factor--of any sunscreen is determined in an FDA-approved, independent lab and must be determined by using a panel of at least 20 human subjects. It is determined by measuring the time it takes to develop skin redness (erythema) to a known amount of radiation. The most important factor you need to consider when choosing sun protection is the ingredients in the sunscreen, not the SPF number.