org/wiki/Nasal_congestion for various causes of
nasal congestion. Apart from allergy and sinusitis, deviated
nasal septum (the dividing wall between your two nasal openings is not properly placed) are the major causes of stuffy nose. For persistent congestion, it's usually best to see your ENT doctor.
Prolonged use of air conditioners does affect health. As you mentioned this could be the reason behind your stuffy nose. “Air conditioning aggravates the effects of arthritis and neuritis.