Smoking is a major risk factor for chronic obstructive pulmonary disease or COPD. This includes emphysema as well as chronic bronchitis. In emphysema, there is a destruction of lung tissue around the alveoli or air sacs. These sacs are unable to maintain their correct shape during exhalation. Tobacco smoking a major culprit causing this disease. COPD causes the airways to become narrower. There is shortness of breath and this becomes worse over time. Patients with COPD are required to use oxygen therapy or may have to have a lung transplant. Smoking can lead to oral and pharynx(throat) cancer. It can also increase the chances of pancreatic cancer as well as lung cancer. Exposure to carbon monoxide and cyanide in cigarettes is believed to do the damage seen in emphysema to the alveoli or air sacs. People exposed to second-hand smoke with asthma often have more asthma attacks and respiratory infections. Second-hand smoke can cause cancer in the people who are exposed to it.
I agree with the above answer. I would like to add a few thoughts. Smoking is a constant irritant to the mucous membrane of the respiratory tissue. This is one reason smokers suffer from illnesses like chronic bronchitis more so than nonsmokers. Additionally, in the disease of COPD, chronic obstructive pulmonary disease, the cilia in the trachea become damaged over time from the smoke. The purpose of the cilia that line the trachea is to help the person expectorate mucus particles that line the mucous membrane. When the cilia are damaged, they can't effectively help push the mucus up towards the mouth. This series of events leads to a trapping of mucoid material in the lower airways and causes obstruction of the airways. Hence the word obstructive in COPD. Note the two different spellings......mucous is the membrane and mucus is the material expectorated.