If air pollution were the culprit, asthma rates hardly would rise as air becomes cleaner. It is true that asthma is becoming almost epidemic. Scientists have several theories about what is causing the increase. That explains why asthma rates are rising most rapidly among inner-city dwellers and children. Doctors also theorize that asthma is increasing because our immune systems aren't getting enough exercise.
Modern life is sanitary.
Lacking sufficient bacteria to prey upon, the immune system fights allergens. The increase in asthma could be an evolutionary phenomenon as well. Modern homes and offices are super-insulated, allowing very little fresh air to circulate. Therefore modern Americans are exposed to interior threats such as dander, dust mites, and cockroaches, just as people were regularly exposed to horses 100 years ago when the animal was a chief mode of transportation and allergies to horses were common. After filling out the checklist, the children are given lung-function tests and generally perform just fine.