The function of cilia in the trachea and bronchi is to protect the airways from being damaged or infected by particles of dust or foreign matter.

Air that is inhaled may not necessarily be free of dust particles. For this reason, there must be something to remove the potentially harmful matter from the body. The cilia in the trachea and bronchi act as a defence system for the body by keeping the airways clear of mucus, dust, dirt, and other foreign matter. This allows us to breathe easily without disruption. For example, in Beijing, the air is so full of dust that many people must wear masks to protect themselves from breathing in the harmful particles.