Fig. 1From: Vitamin D deficiency is associated with respiratory symptoms and airway wall thickening in smokers with and without COPD: a prospective cohort studyVitamin D concentration by GOLD stage: Boxplots demonstrating the median (bold black line), middle quartiles (box), and 1st and 4th quartiles (whiskers) for vitamin D concentration, stratified by GOLD stage. There was no significant difference in median vitamin D concentration by GOLD stageBack to article page