Figure 2.

Relationship between lung function, vitamin D deficiency, and smoking (pack-years). Shown here is the cross-sectional relationship between FEV₁% predicted and pack-years of smoking in subjects who were vitamin D deficient (vitamin D ≤ 20 ng/ml) compared with those who were not (vitamin D > 20 ng/ml). In subjects who were vitamin D deficient, the effect of pack-years of smoking on FEV₁ was twice as severe as in subjects without vitamin D deficiency.