Causes of Asthma

The cause of asthma is not known, but there is evidence that many factors play a part.

• Genetic factors: asthma tends to run in families, and many people with asthma also have other allergic conditions such as rhinitis (inflammation of the nose lining). "Allergy" is a hypersensitivity to some proteins foreign to the body; a small dose of the "allergen" will produce a violent reaction in the person concerned.
• Environmental factors: in wealthy, hygienic Western countries, most babies are not exposed to bacterial infections that "kick start" the immune system in early life and may be important in directing the immune system away from allergic responses. They also grow up in warm, well-furnished, carpeted homes that don't allow much airflow. This encourages the rapid breeding of large numbers of house dust mites in bedding, carpets and furnishings. Many children, instead of playing outside in fresh air, spend most of their time indoors. This further increases dust mite sensitisation. Exposure to tobacco smoke, whether during the mother's pregnancy or in early childhood, predisposes children to developing asthma. It also makes their symptoms more severe. Children can also become sensitised to animals, pollens moulds and dust in the environment if they are genetically predisposed.
• Dietary changes: changes in diet in Western countries, such as a high proportion of processed foods, a higher salt intake, a lower antioxidant intake and a lack of fresh oily fish (lower intake of omega-3 fatty acids) may contribute to the development of asthma.
• Lack of exercise: spending more time inside in front of the television means that children get far less exercise. Reduced exercise may mean less stretching of the airways, and a greater tendency for the muscle in the airway walls to contract abnormally when exposed to minor irritants.
• Occupational exposure: in adults, asthma can develop in response to irritants in the workplace - chemicals, dusts, gases, moulds and pollens. These can be found in industries such as baking, spray painting of cars, woodworking, chemical production, and farming.

Reviewed February 2011