Asthma episodes can be triggered by a variety of factors, most notably: allergens, Infections, environmental pollutants or non-specific stimuli such as exercise and emotional states.
Between 50 and 70 per cent of adults with asthma suffer from allergies. In children under three years of age, viral infections are likely to be the most common trigger. After three years, allergies also begin to play an increasing role as a trigger.
After 20 years of age, occupational exposure to toxic substances and allergens also can be important triggers for asthma.