It is common knowledge that many skin disorders, such as atopic dermatitis, get worse in the winter, but there is little agreement about the reasons for this phenomenon. Recently, a group of investigators in Australia observed that the severity of atopic dermatitis is related to latitude, and proposed that this could be the result of decreased vitamin D production from the weaker light at higher latitudes. We have suggested an alternate explanation for their findings: the skin dries out (develops xerosis) in the winter, because we spend more time indoors, exposed to forced-air heating, which lowers the environmental humidity. A tighter barrier is needed when the difference between the water content of the body and that of the surrounding atmosphere is large.
Bottom Line: Low humidity puts additional stress on the skin barrier. Normal skin is able to compensate by generating a tighter permeability barrier, but this is not the case for people at risk for atopic dermatitis. Many of them already have a vulnerable barrier due to inherited defects in a protein (filaggrin) that is needed for normal barrier function.