Using high resolution CT in addition to spirometry and other types of lung function assessment will help to determine COPD prevalence among non-smokers, smokers of conventional cigarettes and those who quit smoking. COPD prevalence will be defined based on abnormal respiratory function assessed by standard spirometric tests and by using COPD assessment test (CAT) questionnaire, 6-minute walk test and a high-resolution computed tomographic (HRCT) scan of the chest of those patients with preserved pulmonary functions but with clinical symptoms.