What causes the barking cough associated with croup?

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Medical illustration of the human chest showing the lungs, diaphragm, ribs, trachea, bronchus, and bronchioles, with one lung cut away to reveal internal structures. — respiratory disease The bronchioles of the lungs are the site where oxygen is exchanged for carbon dioxide during the process of respiration. Inflammation, infection, or obstruction of the bronchioles is often associated with acute or chronic respiratory disease, including bronchiectasis, pneumonia, and lung abscesses.

The characteristic barking cough associated with croup is caused by the inflammation of the larynx (voice box) and the area around the trachea (windpipe).1

This inflammation leads to swelling and edema of the laryngeal membranes, which, combined with potential spasms of the laryngeal muscles, results in airway obstruction and the distinct, low-pitched cough that resembles the bark of a seal. While most commonly triggered by a viral infection—frequently involving parainfluenza or influenza viruses—the condition can also result from allergy or physical irritation of the upper airway tissues.123