Epidemiology of multiple sclerosis in U.S. veterans: 2. Latitude, climate and the risk of multiple sclerosis.

An analysis of ten climatic factors and elevation for the counties of birth of 4371 U.S. white male veterans with multiple sclerosis and matched controls has been made in relation to birthplace latitude. The climatic factors include an air pollution index, concentrations of minerals in ground water, measures of annual solar radiation, both in energy per unit area and in hours of sunshine, mean annual periods of high and low temperatures, and measures of annual rainfall and average humidity. These variables all significantly influence the risk of multiple sclerosis when analyzed alone, but when they are adjusted for latitude, their effect is found to be due to their correlation with this variable.