What causes nasal congestion?

Inflammation : Anything that is irritating or sets off an allergic reaction by breathing in airborne particles of pollen, pollutants, dust, viral oarticles, etc., trigger an inflammatory process that causes mucosal swelling, irritated nasal passages, and congestion from white cell release of histamines. It can be relieved by removing the irritant, antihistamines, or if chronic, antibiotics to reduce infection