Wheezing is a whistling noise of varying pitch and loudness that occurs when the small airways of the lungs become narrower. This can happen because of inflammation or a buildup of mucus and dead cells in the airway. As these small air passages become narrower, wheezing may be heard when the person breathes out. As the passages continue to narrow, wheezing may be heard when the person breathes both in and out.

When a person has severe difficulty breathing, wheezing may no longer be heard because very little air is moving through the narrowed airways.

A doctor may order a chest X-ray or breathing test (spirometry) to determine the cause of a person's wheezing. Some causes of wheezing include bronchitis, allergies, or asthma. Treatment depends on the cause of the wheezing and may include medicines, an inhaler, or a machine to help a person breathe.