In essence, asthma is the result of an abnormal immune response in the bronchial airways. The airways of asthmatics are "hypersensitive" to certain triggers, also known as stimuli (see below). In response to exposure to these triggers, the bronchi (large airways) contract into spasm (an "asthma attack"). Inflammation soon follows, leading to a further narrowing of the airways and excessive mucus production, which leads to coughing and other breathing difficulties.
There are seven categories of stimuli:
- allergens, typically inhaled,
which include waste from common
household insects, such as the
house dust mite and cockroach,
grass pollen, mould spores and pet
skin and fur cells.
- medications, including aspirin
and beta blockers
- air pollution, such as ozone,
nitrogen dioxide, and sulfur
dioxide, which is thought to be one
of the major reasons for the high
prevalence of asthma in urban
areas.
- various industrial compounds
and other chemicals, notably
sulfites; chlorinated swimming
pools are known to induce
asthma.
- early childhood infections,
especially viral respiratory
infections. However, persons of any
age can have asthma triggered by
colds and other respiratory
infections even though their normal
stimuli might be from another
category (e.g. pollen) and absent
at the time of infection.
- exercise, the effects of which
differ somewhat from those of the
other triggers
- emotional stress, which is poorly understood as a trigger.
