There are several reasons:
- Change in temperature and humidity. Walking into a cold, dry room can trigger a runny nose. This is similar to how walking outside on a cold winter day can also cause runny nose (vasomotor rhinitis). The cold air may trigger nervous system reflexes in the nose that cause glands in the nasal membranes to produce mucus. This may be more common in people with allergies. However, changes in temperature and humidity should trigger only brief congestion.
- Allergens and pollutants/irritants get trapped in AC filter. People with allergies might develop congestion because small particles like pollen, mold spores, pollutants and dust mites can get trapped by air-conditioning filters. Pollutants and bacteria wouldn’t cause an allergic reaction but could irritate the nose via VOCs (nonallergic rhinitis).
The key is to change AC filters regularly.
Can Air-Conditioners Cause Sinus Congestion? http://buff.ly/2rjbjsb