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.