There are many asthma triggers. Two large trigger categories are allergens and irritants. Allergens are substances that cause little problem for a majority of people, but will trigger an allergic reaction in sensitive people. During an allergy attack, the body releases chemicals called mediators. These mediators often trigger asthma episodes. Irritants such as cold air, cigarette smoke, industrial chemicals, perfume, and paint and gasoline fumes can trigger asthma. These irritants probably trigger asthma symptoms by stimulating irritant receptors in the respiratory tract. The receptors, in turn, cause the muscles surrounding the airway to constrict, resulting in an asthma attack.

The worst offenders of indoor environmental asthma triggers are predominately of the allergen category and include (but are not limited to): house dust, dust mites, cockroaches, molds, pets (any animals with fur or feathers), and pollen (tree, grass and weed). Irritants include (but again not limited to): second hand smoke, scented candles, air fresheners, ozone, and combustion by-products.

The EPA acknowledges house dust, dust mites, mold, mildew and other common indoor allergens trigger asthma episodes in sensitive individuals. Because Americans spend up to 90% of their time indoors, exposure to such indoor allergens (and irritants) plays a significant role in triggering asthma episodes.