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.
How is My Sunscreen’s SPF Number Determined?
