Since asthmatics have hyperreactive airways, a number of allergens or irritants may cause the lungs' breathing tubes to close, restricting the breathing. Technically an airways irritant rather than an allergen, smoke has a severe effect on asthmatics. Dust mites . The excrement of this microscopic mite is a potent allergen. Mites infest bedding, upholstery, draperies and rugs and thrive on moisture and human skin cells.
Cockroaches are a primary allergen.
Pet danders . Cat, dog and horse danders, and bird feathers can cause severe reactions. Mildews and molds . Unvented gas appliances, heaters and stoves . Poorly vented wood-burning fireplaces can also be a hazard.