What Is Asthma?
In many people, asthma appears to be an allergic reaction to
substances commonly breathed in 
through
the air, such as animal dander, pollen, or dust mite and cockroach
waste products. The catch-all name for these substances, allergens,
refers to anything that provokes an allergic reaction. Some people
have a genetic predisposition to react to certain allergens.
When these people breathe in the allergen, the immune system
goes into high gear as if fighting off a harmful parasite. The
system produces a molecule called immunoglobulin E (IgE), one
of a class of defensive molecules termed antibodies. The IgE antibody
is central to the allergic reaction. For example, it causes mast
cells, a type of specialized defensive cell, to release chemical
"weapons" into the airways. The airways then become
inflamed and constricted, leading to coughing, wheezing, and difficulty
breathing -- an asthma attack.
Without treatment, such as inhaled corticosteriods to reduce
the inflammation, asthma attacks can be deadly. The overall death
rate for asthma, however, is low.
Why is Asthma on the Rise?
Although several theories exist about why asthma rates have risen
during the last two decades, there probably is no simple answer,
says Calman Prussin, M.D., head of the clinical allergy and immunology
unit at NIAID.
One theory is that people today, especially in developed countries,
are spending more time indoors, Dr. Prussin says. We are therefore
exposed to more indoor allergens, such as dust mite allergen,
that cause asthma. "Our houses are now hermetically sealed
to save heating and cooling energy," he notes, "and
unfortunately this causes more indoor allergen exposure."
Another reason may be that people today live in cleaner, more
sanitary conditions than they did before the industrial revolution,
relatively free of disease-causing viruses and bacteria, he says.
This clean living affects our immune system. The immune system's
defensive white blood cells, called T cells, have two basic "settings,"
he explains. Th1 cells fight infectious viruses and bacteria.
Th2 cells fight parasites but are also involved in allergic reactions.
"We are exposed to fewer viruses and bacteria than people
were 100 years ago, so perhaps our immune systems have not learned
to make Th1 cells as well," Dr. Prussin says. "That
means we have a greater proportion of Th2 cells in our bodies,
which might lead to more allergies and asthma."
Other theories point to increased levels of air pollutants, a
decline in the amount of exercise people get, or rising obesity
as factors in the increase of asthma.
Asthma Statistics
In 1998, an estimated 17 million Americans, or 6.4 percent of
the population, had asthma. Children account for 4.8 million of
Americans with asthma.
Asthma affects slightly more African Americans (5.8 percent) than
whites (5.1 percent). In 1993 however, African Americans were
3 to 4 times more likely than whites to be hospitalized for asthma.
In 1996, African Americans were 4 to 6 times more likely than
whites to die from asthma.
More than 5,000 people die from asthma each year in the United
States. Although asthma deaths are infrequent, they have increased
significantly during the last two decades. From 1975-1979, the
death rate was 8.2 per 100,000 people. That rate jumped in 1993-1995
to 17.9 per 100,000.
In 1994, asthma caused 451,000 hospitalizations. Children under
15 accounted for 169,000 of these.
In 1995, asthma caused more than 1.8 million emergency room visits.
Asthma cost the U.S. economy an estimated $10.7 billion in 1994,
including a direct health care cost of $6.1 billion and indirect
costs, such as lost work days, of $4.6 billion.
