Ecological Evidence for Lower Risk of Lymphoma with Greater Exposure to Sunlight and Higher Altitude.

Introduction: Sunlight exposure increases vitamin D-related immune modulation and motility of T lymphocytes. Blue light exposure from the sun can stimulate immune function and help promote healthy circadian rhythm. Hence, greater sunlight exposure may lower the risk of Hodgkin lymphoma and non-Hodgkin lymphoma (NHL). Altitude may also lower the risk of these cancers through an oxygen-related mechanism, and because cosmic radiation has less shield from the atmosphere at higher levels, it allows for radiation hormesis. Methods: An ecological study design was used, with county-level lymphoma, sunlight, altitude, urban residency, poverty, smoking, obesity, and leisure-time physical inactivity data for 16 cancer registries (607 counties) in the contiguous United States, 2012-2016. Relative rate estimates were derived from two-level mixed effects Poisson regression models. Results: Higher rates of NHL are associated with being older, male, and white. Higher rates of Hodgkin lymphoma are associated with ages 20 years and older, male, and white or black. The risk of NHL or Hodgkin lymphoma is lower among those living in poverty. Urban residency, smoking, obesity, and physical inactivity are not associated with these cancers. Both increased sunlight exposure and higher altitude are simultaneously associated with lower rates of Hodgkin lymphoma and NHL in adjusted models. The inverse association between sunlight and NHL is more pronounced with higher altitude. The inverse association between sunlight and Hodgkin lymphoma is only in altitudes below 500 m. Conclusions: Greater sunlight exposure and higher altitude are simultaneously associated with lower rates of Hodgkin lymphoma and NHL. The inverse associations are dependent on altitude, with the relationship only in lower altitudes for Hodgkin lymphoma and more pronounced in higher altitude for NHL.