The nose and the sinuses are lined with a membrane that produces mucus.
When air enters the nose, it is warmed, moistened, and filtered before passing into the lungs.
Dust, smoke, bacteria, and other irritants often become trapped in the mucus that lines the nasal passages.
This mucus is pushed to the back of the throat by tiny hairs (cilia) that line the inside of the sinuses and nose, and the mucus is eventually swallowed.
If the mucous membrane becomes inflamed, it may swell, blocking the openings that drain the sinuses. Mucus inside the sinuses then becomes trapped. Both the inflammation and the pressure of trapped mucus cause pain. Also, bacteria trapped in the mucus can grow, causing a bacterial sinus infection.