Wheezing is a whistling noise that occurs when the bronchial tubes, which carry air to the lungs, narrow because of inflammation or mucus buildup. Wheezing is often present in asthma.

During an asthma attack, the bronchial tubes become smaller. At first, the person may wheeze when breathing out. As the attack becomes worse, the person may also wheeze when breathing in. During a severe asthma episode, wheezing may go away because little air is moving through the narrowed bronchial tubes.

Wheezing can be a sign of asthma in children, but it does not always mean that a child has asthma. Children younger than 5 often develop wheezing during a respiratory infection. Children with a family history of allergies seem to be more likely than other children to have one or more episodes of wheezing with colds. Children with certain viral infections, such as respiratory syncytial virus (RSV), rhinovirus (which causes the common cold), and influenza virus, also are likely to develop wheezing.