This is because when smoke enters the mouth, it passes through the throat and airway into the lungs as the air passes through the throat and airway in turn. It is the same reason that you can breathe through your mouth even if you pinch your nose. In medicine, the respiratory tract is usually divided into upper and lower parts by the lower edge of the cricoid cartilage of the larynx. The upper respiratory tract includes the nose, throat and larynx. The lower respiratory tract includes the trachea, bronchial tubes and lung tissue. The main harmful substances in tobacco are mainly nicotine, tar and carbon monoxide. Smoking can lead to the accumulation of harmful substances in the smoke on the walls of the airways and alveolar walls, affecting the immune function of the respiratory system, which in turn triggers a variety of respiratory diseases. Smoking increases the risk of respiratory infections such as acute and chronic bronchitis, emphysema, chronic obstructive pulmonary disease, and even causes cancer. It is recommended that smokers should actively quit smoking and maintain good living habits, at least three times a week for aerobic exercise, each exercise duration to maintain more than thirty minutes; reasonable diet, daily consumption of fresh vegetables, moderate amount of fruits; to ensure that sufficient sleep.