COPD is one of the leading causes of morbidity and mortality in the industrialized and the developing countries. According to the prediction of the World Health Organization, COPD will become the third leading cause of mortality and the fifth cause of disability in 2020 worldwide. In epidemiology, distinct phenotypic entities converge on the term COPD, so that prevalence and mortality data may be inclusive of chronic bronchitis, emphysema and asthma; moreover, the assessment of prevalence rates may change considerably according to the diagnostic tools used. Thus, a considerable problem is to estimate the real prevalence of COPD in the general population. COPD is determined by the action of a number of various risk factors, among which, the most important is cigarette smoking. However, during the last few decades, evidence from epidemiological studies finding consistent associations between air pollution and various outcomes (respiratory symptoms, reduced lung function, chronic bronchitis and mortality), has suggested that outdoor air pollution is a contributing cause of morbidity and mortality. In conclusion, epidemiological studies suggest that air pollution plays a remarkable role in the exacerbation and in the pathogenesis of chronic respiratory diseases. Thus, respiratory physicians, as well as public health professionals, should advocate for a cleaner environment.