Allergies are the primary offenders.
The most common allergens, or allergy-causing substances, are pollen, grass, dust, mold, tobacco smoke, and animal dander. When inhaled, these substances can trigger the release of histamine and other body chemicals, causing an allergic reaction and asthma episodes. Lung infections can also induce asthma. Bronchiolitis, a viral respiratory infection that usually affects children aged 2 and younger, is a common cause of childhood asthma. Adults may develop asthma as a result of an upper respiratory infection, such as bronchitis.