In both types the tracheobronchial tree becomes hypersensitive, and the diameter of the airways shrinks. Acute episodes are typically followed by symptom-free periods. Perhaps one in 10 adults with asthma contract it through exposure to occupational agents such as reactive dyes. In addition to household contaminants, asthma can be precipitated by exercise, cold air, emotional stress, viral infections, everyday chemical agents such as aspirin, and industrial air pollutants, including ozone and nitrogen dioxide. There is no evidence, however, that outdoor air pollution is an initiating cause of asthma.
Smoking exacerbates asthma, and maternal smoking during pregnancy increases the risk for the child.
Obesity is also associated with asthma. With such a variety of factors, it is no wonder that scientists don't fully understand the natural history of the disease. Even so, they have made remarkable progress, notably with drugs such as inhaled steroids. These and other new treatments, if used regularly by all asthmatics, could for the most part prevent deaths from the disease.