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Q. What factors determine the SPF performance of a given sunscreen?A. The SPF is the sun protection factor afforded by a given sunscreen. It is determined by dividing the lower back of untanned individuals is into small test sites which are exposed to UVB light until a minimum amount of erythema develops, known as the MED. Lightproof barriers are placed around the test sites to prevent light contamination from one test site to another. Once the MED for the test subject has been determined, the subject is invited to return to the test site the next day for application of sunscreen. The sunscreen is placed on the test sites and allowed to dry. The skin is then exposed to UVB light at the expected SPF of the sunscreen product. The expected SPF is roughly determined by spectrophotometric absorption. The amount of UVB light required to obtain the same degree of erythema as the preceding day is determined and the SPF calculated.