Allergens: According to Webster's II New Riverside Dictionary, an allergy is an abnormal or pathological reaction to environmental substances. Allergens are substances which function as catalysts and cause the body to release chemicals called mediators. This bio-chemical reaction may set off an asthma attack in certain individuals. The principal allergens that trigger asthma or asthma symptoms are indoor air allergens. The most common indoor air allergens are dust mites, cockroaches, mold, and animal dander.

Irritants: Irritants that generally trigger asthma attacks are cigarette smoke, strong smells, pollutants, and hot or cold air. These irritants probably trigger asthma symptoms by stimulating irritant receptors in the airways. These receptors, in turn, cause the muscles surrounding the airway to constrict resulting in an asthma attack.