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Asthma

Asthma Basics

What is Asthma?
Why is Asthma on the Rise?
Asthma Statistics

What is Asthma?

Diagram of airwaysIn 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
  1. 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.

  2. 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.

  3. 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.

  4. In 1994, asthma caused 451,000 hospitalizations. Children under 15 accounted for 169,000 of these.

  5. In 1995, asthma caused more than 1.8 million emergency room visits.

  6. 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.
References

  1. Centers for Disease Control and Prevention (CDC), Morbidity and Mortality Weekly Report 47(47):1022-25 (December 4, 1998).

  2. C. Cannon et al. American Journal of Respiratory and Critical Care Medicine 158 (1): 320-34 (1998).

  3. CDC Surveillance Summaries, April 24, 1998, from Morbidity and Mortality Weekly Report 47(SS-1): 1-28 (1998).

  4. CDC, Morbidity and Mortality Weekly Report 45(17): 350-3 (1996).

  5. K.B. Weiss et al. Journal of the American Medical Association 264(13):1683-7 (1990).

  6. CDC, National Center for Health Statistics, National Vital Statistics Report 47(4) (1997).

  7. CDC Surveillance Summaries, April 24, 1998, from Morbidity and Mortality Weekly Report 47(SS-1): 1-28 (1998).

  8. National Center for Health Statistics, National Health Interview Survey, 1994.

  9. CDC Surveillance Summaries, April 24, 1998, from Morbidity and Mortality Weekly Report 47(SS-1): 1-28 (1998).

  10. K.B. Weiss et al. Journal of Allergy and Clinical Immunology 106(3): 493-99 (2000).
Last updated August 30, 2001 (ldr)