The pharynx will block most of the intrusions, but a small amount will enter the trachea. The mucus secreted by the trachea will adhere to the impurities that have been stolen in and then expel them from the body together, which is actually what we call phlegm. If you have sputum, you must cough up it. It's better to cough it out when the sputum is not too much and the symptoms are not too serious. If sputum is left in, harmful bacteria may stimulate the bronchus and lungs, cause infection and inflammation, make mucus secrete more, and increase the burden of ciliary columnar epithelium, which is not conducive to mucus discharge, but also easier to breed bacteria and aggravate pathological changes.