Asthma results from a combination of inherited and trigger factors which cause inflammation of the bronchi. Most, but not all, children with asthma have a family history of allergic conditions (asthma, eczema, or allergic rhinitis). Children with asthma often have other allergic conditions.

Inflammation of the bronchi results in:

1. Mucosal oedema (swelling of the linings of the bronchi)
2. Bronchospasm (contraction of the smooth muscle in the bronchi)
3. Increased secretion of sticky mucus

These factors cause narrowing of the bronchi, especially in small children who normally have narrower bronchi than do older children.