The nasal mucous membranes consist of ciliated cells and mucous cells. When you breathe in dirt from the air, it gets stuck on the mucus. Small hair like filaments called cilia push the mucus with the dirt towards the throat, where it is swallowed.
Cocaine dries out the nasal membranes. That is because cocaine causes the capillaries to constrict, thus reducing the blood supply to the tissues. This desiccation of the mucous membranes makes them more vulnerable to inflammation and infection.
Often, the mucous membranes are destroyed altogether. Because these membranes also serve as natural protective barriers for the other cells in the nose, serious damage can result. There are even cases where the nasal septum becomes perforated.