During normal breathing, the airways to the lungs are fully
open, allowing air to easily move in and out. In a person with
asthma, the airways are inflamed and overly sensitive to certain
things that wouldn't usually bother others. These can be
substances, events, or activities, and they're known as triggers
because they "trigger," or bring on, asthma symptoms such as
cough, wheezing, and shortness of breath. Triggers can vary from
person to person and from season to season and can change as a
child grows older. Some common triggers include: allergens
(substances that cause allergies), viral infections, cold air,
exercise, and smoke.