Minimal Erythema Dose. The skin's reaction to sun exposure varies according to the individual. Each skin type is able to accept a specific amount of UV rays before burning. The amount of time an individual can stay in the sun before burning reflects the minimal erythema dose (MED). The MED and expected sun exposure, in turn, determines the proper SPF for a skin type. For example, if you burn in 20 minutes without sunscreen, an SPF of 15 allows you to stay in the sun 15 times longer (20 mins. X 15 = 300 minutes or 5 hours).