Asthma Prevalence Associated with Geographical Latitude and Regional Insolation in the United States of America and Australia

Background: It has been proposed that vitamin D deficiency may be responsible for an increase in the prevalence of allergic diseases and asthma worldwide. Human ability to generate physiologically required quantities of vitamin D through sun exposure is decreasing with increasing geographical latitude. Objectives: Considering that vitamin D deficiency is usually due to lack of outdoor sun exposure, this study is designed to test the hypothesis that a higher prevalence of asthma should be expected at high relative to low geographical latitudes. Methods: Linear regression analyses are performed on asthma prevalence in the U. S. adult population vs. geographical latitude, insolation, air temperature, and air pollution (PM2.5) for 97 major metropolitan/micropolitan statistical areas of the continental United States of America and on general population asthma prevalence vs. geographical latitude in eight metropolitan areas of Australia. Results: A 10 degrees change in geographical latitude from southern to northern regions of the Eastern Seaboard is associated with a 2% increase in adult asthma prevalence (p < 0.001). Total insolation in winter months is almost as strong as latitude in its ability to explain the observed spatial variation in the prevalence of asthma (r(2) = 0.43; p < 0.001). Similar results are obtained using the Australian data (r(2) = 0.73; p < 0.01), suggesting a consistent association between the latitude/insolation and asthma prevalence worldwide. Conclusions: The results of this study suggest that, as a known modulator of the immune response closely linked with the geographical latitude and erythemal UV irradiation, vitamin D may play an important role in the development/exacerbation of asthma.