COPD mostly develops from a chronic bronchitis. According to the criteria of WHO, the latter one occurs if the patient suffers for at least three months from a permanent cough, increased slime formation and sputum in more than two successive years. In most cases this remains as bronchitis and no COPD is developed.
If besides the mentioned bronchitis symptoms, a contraction of the airways (obstructive component) occurs, then COPD has set in. All lower airways, from the bronchial tubes over the smaller bronchioles up to the air sacs (alveoli) contract increasingly. The most important risk factor is smoking. But also people who have never smoked can suffer from this disease. This indicates an involvement of genes.