Fractional exhaled nitric oxide (FeNO) in bronchitis and COPD patients (COPD), asthmatics (A) and asymptomatic smokers (S) at the beginning of the study. The values of non smokers (NS) have been gathered from previously published materials where non-smokers were combined from two groups of subjects i.e. never smokers and ex smokers who had quitted smoking over 20 years ago. The levels of each marker in these two groups of non-smokers have been found to be very similar [8, 9, 15, 20]. P-value was calculated with Kruskall-Wallis test.