Asthma's primary cause is not known, but researchers believe that sufferers have a genetic predisposition towards allergies and allergic reactions. It is also believed that early exposure to allergens and infections in infancy, when the immune system is developing, is another primary asthma cause.
Genetics
According to the Encarta dictionary, allergic atopy is a hereditary tendency to react to certain allergens such as pet dander, pollen, dust and a list of other triggers. The ADAM33 gene is hypothesized as being related to the development of asthma.
Environmental Pollutants
Smog, consisting of ammonia, sulfur dioxide, nitrous oxide and a long list of industrial pollutants, is a primary cause of chronic asthma and asthma exacerbation. The relationship between air quality and asthma is well documented.
Respiratory Infections
Another primary cause of asthma is viral and bacterial upper and lower respiratory infections. Respiratory syncytial virus (RSV) is the most common cause of asthma in children from infancy through age five. The common cold in adults is the most common cause of asthma exacerbation. These infections irritate an already compromised airway.
Exercise
Exercise-induced asthma, particularly in cold weather, is the result of introducing a barrage of allergens carried in cold air into a warm and moist airway through an increased respiratory rate. A heightened reaction occurs because of breathing air through the mouth, instead of the nose, where the allergens can be filtered before reaching the airway.
Smoking
Cigarette smoking, both first- and second-hand smoke, is the most controllable common cause of asthma.
