Wheezing occurs when lower airways are narrow or constricted – breathing feels difficult and there is a whistling sound in the chest on breathing out. Wheezing is a symptom of asthma and other lower respiratory conditions (such as the viral condition bronchiolitis).

Narrowing in the lower airways can be caused by different things.

• In asthma, redness and swelling (inflammation) in the small airways deep in the lungs causes over-production of mucus and tightening of airway muscles. The combination of swelling, mucus, and muscle tightening all cause narrowing of the airways.
• With a viral infection in the airways, excess mucus production can build up and clog the airways. This is particularly likely in a child born with narrow or abnormally shaped airways.

See the ‘Asthma and Lung Function Tests’ brochure for more information on lung inflammation in asthma.

What is the link between asthma and wheezing?

Most people with asthma wheeze. But not everyone who wheezes has asthma.

Studies of babies and children have shown us that there are different types of wheezing in young children.

• Transient wheezing – In more than half of children who wheeze, the wheezing occurs when the child has an infection and stops when the child gets better. Transient wheezing usually stops altogether by about 3 years of age as the airways grow and widen.
• Bronchiolitis is a common virus causing transient wheezing in babies, particularly those under 6 months of age. If your baby has a lung condition such as bronchiolitis it does not necessarily mean that he or she will develop asthma as a child.
• Persistent wheezing – Children with wheezing that continues beyond the preschool years are more likely to have allergies than children whose wheezing stops. Signs of allergy include having eczema, hay fever, or a runny nose without a cold.

The combination of continuing wheezing and allergies, or parents having allergies or asthma, further increases the chance that wheezing will continue and asthma will develop.

Can we tell if wheezing is transient or persistent?

Even though we know about different types of wheezing, it is difficult to tell whether a young child has transient or persistent wheezing.

The doctor can’t be completely sure whether it is asthma until the child has a lung function test (see brochure on lung function tests).

To identify which children will get asthma, we need to understand more about how asthma develops.

Content Updated March 2005