Stridor is usually loud enough to be heard at some distance. The sound is caused by turbulent airflow through a narrowed upper airway.
In children, the cause may be
An inhaled foreign object
In adults, the cause may be
A tumor
Swelling (edema) in the upper airway
An inhaled foreign object (occasionally)
Stridor that causes shortness of breath Shortness of Breath Shortness of breath—what doctors call dyspnea—is the unpleasant sensation of having difficulty breathing. People experience and describe shortness of breath differently depending on the cause... read more 
when the person is at rest is a medical emergency.
Tumors of the head and neck such as laryngeal cancer Laryngeal Cancer Laryngeal cancer is cancer originating in the larynx, also known as the voice box. People may be hoarse or have a lump in the neck or difficulty breathing or swallowing. A biopsy is needed for... read more can cause stridor if they partially obstruct the upper airway.
When both vocal cords are paralyzed, the space between the paralyzed cords is very small, and the airway is inadequate so that even moderate exertion causes difficulty in breathing and a harsh, high-pitched sound (stridor) with each breath.
Brief episodes of stridor can result when a person inhales a food particle or bit of water into the lungs, causing contraction of the voice box (larynx). If an inhaled foreign object is a likely cause and the person is outside of a hospital, another person, if trained, can sometimes help expel the foreign object. For example, in an adult, the other person may use the Heimlich maneuver Choking Maneuvers to relieve choking are frequently life saving. Adults most often choke on a piece of food, such as a large piece of meat. Infants do not have well-developed swallowing reflexes and... read more .
If a person has stridor and is in a hospital or emergency department, a tube may be inserted through the person's mouth or nose (tracheal intubation) or through a small surgical incision directly into the trachea (tracheostomy) to allow air to get past the blockage and prevent suffocation.
The cause usually becomes clear during tracheal intubation, when the doctor can see the upper airway directly. If tracheal intubation is not done, the diagnosis is usually determined by inserting a flexible viewing tube through the nose and upper airway (a procedure called nasopharyngeal laryngoscopy).
