The etiology of chronic bronchitis is not very clear at present, and it is now believed to be caused by the long-term interaction of various factors. The most closely related factors are harmful gases or particles that act on the respiratory tract for a long time, causing damage to some structures of the respiratory tract, such as cigarette smoke, fumes, dust, irritating gases, sulfur dioxide, nitrogen dioxide, etc. These physicochemical factors cause damage to the airways. Additionally, some infectious agents, such as bacteria, mycoplasma, or viral infections, along with some immune-related factors, are believed to play a certain role in the development of chronic bronchitis.