Nasal congestion, also known as a stuffy nose, occurs when something irritates the tissues lining the inside of your nose, which sets off a chain reaction of inflammation, swelling, and mucus production, making it hard to take in air through your nose. The following are some of the common causes of nasal congestion:

• Infections: Colds, flu, and sinusitis are some of the infections that can cause nasal congestion.

• Allergies: Allergic rhinitis (hay fever) and nonallergic rhinitis are two types of rhinitis that can cause nasal congestion. Allergens such as pollen, dust mites, animal dander, and many others can trigger allergic rhinitis, while nonallergic rhinitis can be caused by inflammation due to viral illness or exposure to certain triggers.

• Nasal polyps: These are sac-like growths of inflamed tissue lining the nose or sinuses that can cause nasal congestion.

• Deviated septum: A deviated septum is a condition where the nasal septum, the thin wall between the nostrils, is displaced to one side, making one nasal passage smaller than the other, which can cause nasal congestion.

• Vasomotor rhinitis: This is caused by dysregulation of the nerves that supply the nasal cavity, leading to clear, watery nasal discharge that usually occurs in response to certain triggers, like eating or changes in temperature when stepping inside or outside.

• Use of some nasal sprays or drops: Overuse of nasal decongestant sprays like oxymetazoline can cause "rebound congestion" and leave people worse than they started, leading to another form of rhinitis called "rhinitis medicamentosa".

Nasal congestion typically clears after a few days, but congestion that lasts for a week or more may be a sign of an infection. If left untreated, nasal congestion may cause sinusitis, nasal polyps, or middle ear infections.