FeNO / controls (N = 34): 10, 5 ± 4, 2 ppb: FeNO / healthy smokers (N = 38): 6, 9 ± 2, 8 ppb: FeNO / ex-smokers (N = 32): 9, 2 ± 4, 4 ppb: FeNO / smokers with COPD (N = 26): 15, 6 ± 5, 5 ppb: FeNO / ex-smokers with COPD (N = 28): 21± 6, 3 ppb.