Relationship between CYP2A6 activity, CPD, expired CO, and plasma COT. Self-reported CPD and expired CO did not differ by CYP2A6 genotype (A, B) or 3HC/COT quartiles (D, E), while CYP2A6 slow metabolizers had significantly higher plasma COT levels compared to normal metabolizers (* p < 0.01, C) and those in the slowest 3HC/COT quartile had significantly higher plasma COT levels compared to those in the fastest quartile (#, p < 0.001, F). NM = normal metabolizers, IM = intermediate metabolizers, SM = slow metabolizers. Analyses were performed on log-transformed variables (CPD, expired CO, plasma COT) although raw data is plotted.