SFTPD and Airway obstruction: In addition, there was a correlation of serum (r = -0.35, p = 0.013) and BAL SP-D levels (r = 0.62, p < 0.0001) with the degree of airway obstruction (FEV1/FVC) within the group of smokers (S1, S2, C, n = 50, Figure 3A, B) and in line with this a significant positive correlation was observed for the BAL/serum ratio (r = 0.60, p < 0.001).