[1] The Centers for Disease Control and Prevention estimate that 1 in 11 children and 1 in 12 adults have asthma in the United States of America.

[1] During an asthma episode, inflamed airways react to environmental triggers such as smoke, dust, or pollen.

)[4][5] In response to exposure to these triggers, the bronchi (large airways) contract into spasm (an "asthma attack").

[6] The normal caliber of the bronchus is maintained by a balanced functioning of the autonomic nervous system, which both operate reflexively.

The parasympathetic reflex loop consists of afferent nerve endings which originate under the inner lining of the bronchus.

The mechanisms behind allergic asthma—i.e., asthma resulting from an immune response to inhaled allergens—are the best understood of the causal factors.

[8][9] The suggestion is that for a child being exposed to microbes early in life, taking fewer antibiotics, living in a large family, and growing up in the country stimulate the TH1 response and reduce the odds of developing asthma.

[32] If gastro-esophageal reflux disease (GERD) is present, the patient may have repetitive episodes of acid aspiration.

Researchers found a link between the preterm birth and exposure to air pollution in asthmatic pregnant women.

[36] Researchers suggested that asthmatic episodes in pregnant women were associated with ongoing exposure to nitrogen dioxide and carbon monoxide - types of air pollutants.

"[36] In other studies, Scientists have found a link between asthma in children and prenatal exposure to air pollution.

The results suggested that a high exposure to prenatal air pollution was strongly correlated with increased susceptibility to asthma during childhood.