Three different groups were studied. The first group consisted of hospitalized patients with an acute exacerbation of COPD (AE-COPD), the second group were subjects with stable COPD and the third group was composed of smokers without COPD. The groups were defined as previously published [20] . Briefly AE-COPD patients suffered from COPD as defined by GOLD [21] . Acute exacerbation was characterized by worsening in dyspnea, cough, and expectoration. A routine posterior-anterior chest radiograph was evaluated on admission by expert radiologists to exclude other other reasons for increased symptoms as pneumonia, tuberculosis, pulmonary fibrosis, bronchiectasis, bronchial carcinoma or congestive heart failure.Stable COPD patients did not have an exacerbation within the last 30 days prior to hospital admission and had no changes in therapy within the last 14 days (including inhaled and oral medication) and had been admitted for other medical reasons into departments of internal medicine other than pulmonary care. COPD subjects were recruited in a 2:1 ratio each month in order to prevent seasonal selection bias. Smokers have been smoking more than 10 pack-years, could have chronic symptoms like cough and phlegm but did not report dyspnea and did not have bronchial obstruction (FEV 1 /FVC>70%, FEV 1 >80% predicted). None of the smokers had a history of COPD or asthma, nor was using systemic or topic pulmonary medication. The smokers were recruited either from our smoking cessation initiative or by newspaper advertisement.