Why doesn the skin tan equally on all sides?
The insides of the arms and legs do not tan so well because they have less pigment forming cells than the rest of the skin. The face does not tan easily because it has a thicker horny layer by nature, especially as a protection against UV-B rays. Freckles and pigmental spots have a high number of tanning pigments, and for this reason will turn darker much faster than other parts of the skin. Parts of the skin that we lay on have a poor circulation. A lack of oxygen may lead to a reduced coloration of melanin pigments. Tanning without pressure spots is thus provided best by vertical tanning units.
