Asthma is a long-term (chronic) lung disease that causes your child’s airways to become sensitive to certain things (triggers). Several things happen to the airways when a child is exposed to triggers:
The lining of the airways swells.
The muscles around the airways tighten.
The airways make more thick mucus than normal.
Link to article: https://www.hopkinsmedicine.org/health/conditions-and-diseases/asthma/asthma-in-children