They also answered about cleaning:
There are several mechanisms. The trachea and large bronchi are covered with ciliated epithelium. It is a tissue made up of cells with tiny cilia on the surface. The vibrations of these cilia create a retrograde (i.e. back to the external environment) movement, and the smallest particles are removed from the bronchial tree. If the particles are too small and get into the alveoli, there they are captured and destroyed by special cells-lymphocytes.
But in smokers, for example, the ciliated epithelium is destroyed very quickly, it seems to "go bald", there is no cleansing mechanism ... That is why the smoker's lungs look so unpleasant - they are simply "dirty".