Why do people tan differently?

The Fitzpatrick skin type scale^[1] is a method for classifying people’s expected reaction to UVB exposure based on how they respond to sun exposure of about three minimum erythema doses (MEDs), or about 45–60 minutes of noon exposure in northern latitudes (north of Mexico through Oregon) during early summer. Not surprisingly, skin pigmentation plays an important role in determining the Fitzpatrick skin type. There are six categories that a person may fall into, with the first four associated with white skin, the fifth with brown skin, and the sixth with black skin.