Studies of risk of non-Hodgkin's lymphoma have found both direct and inverse correlations with respect to solar ultraviolet (UV) doses. Reduced risk is generally found in midlatitudes, while increased risk is generally found at higher latitudes. It is suggested that reduced risk arises from vitamin D production from UVB irradiance, while increased risk arises from immunosuppression associated with UVA irradiance. The ratio of UVA to UVB increases with increasing latitude.