Asthma is a recurrent, inflammatory disease of the airways that causes them to periodically spasm, swell, and narrow, making it difficult to breathe.
Asthma causes changes in the airways (the bronchi, or “breathing tubes,” and smaller bronchioles of the lungs) that make them highly sensitive or reactive to stimuli that may not affect healthy lungs. Allergic asthma is the most common form of the disorder and can be triggered by foreign substances such as dust, air pollution, pollen, animal dander, or mold; however, some people without allergies are affected by asthma. Exercise can trigger attacks in some people with asthma, as well as in some people who are otherwise not affected by asthma.
During an asthma attack, muscle tissue in the walls of the bronchi begins to spasm, and immune-system cells called mast cells that line the airways swell and secrete mucus. These actions cause bronchoconstriction, a narrowing of the bronchi. Bronchoconstriction makes it more difficult to breathe in air in and expel it out. The mast cells also release certain immune-system chemicals, including histamine and leukotrienes. These chemical releases cause white blood cells to rush to the area, resulting in inflammation.
When a child's airways become sensitized to a particular allergen, subsequent exposure to that allergen can rapidly cause an asthma attack. In addition to common allergens, children may experience asthma attacks as a result of respiratory infections or colds; excitement or stress; exposure to cigarette smoke, cold air, or sudden changes in temperature; or exercise. Childhood asthma, however, is often characterized by intermittent attacks followed by asthma-free periods, and many children outgrow the disorder.
Adult-onset asthma differs from childhood asthma in various ways, although it may also involve allergies. Occupational asthma can be caused by inhaling workplace plastics, solvents, wood dust, or other foreign substances. Some adults develop sensitivities to various drugs, including aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs). Compared to childhood-onset asthma, adult-onset asthma tends to be ongoing, and attacks can occur even during sleep. Adult-onset asthma is often severe and associated with loss of lung function.
Exercise-induced asthma (EIA) or exercise-induced bronchoconstriction (EIB) occurs only during or immediately following physical activity. Although exercise can trigger attacks in people with asthma, EIA also affects people who do not otherwise have asthma. Exercising in cold air or engaging in aerobic activity for more than ten minutes or high-intensity exercise for shorter periods can trigger EIA in susceptible people. Certain pollutants or chemicals, such as chlorine in swimming pools or herbicides on playing fields, can increase the risk of EIA or worsen attacks.
In addition to allergies, a family history of allergies or asthma is a major risk factor for asthma. Obesity also is a risk factor. Certain ethnic and racial groups are at increased risk. Living or exercising in air-polluted areas and exposure to cigarette smoke increase the risk of asthma.
Worldwide, more than 300 million people are affected by asthma, and it is the most common chronic childhood disease. Asthma prevalence among both children and adults has been increasing in recent decades, especially among young children and adolescents in urban areas, paralleling increases in other allergic disorders. The severity of asthma also appears to be increasing. Although asthma can develop at any age, it most often first appears in childhood, with the majority of children developing symptoms before age five.
As of 2017, approximately 1 in 13 people in the United States had asthma. In 2014, 8.6% of U.S. children (6.3 million) and 7.4% of adults (17.7 million) had asthma. Almost 60% of these children and almost 65% of the adults had persistent asthma, with the remaining cases being intermittent. Asthma rates are significantly higher among African Americans, Native Americans, and Hawaiians/Pacific Islanders than among non-Hispanic white Americans. Puerto Ricans have twice the asthma rate of the overall Hispanic American population. The asthma rate among African American children is about twice that of white American children. Asthma rates also are higher among low-income children. There are 3,500–5,000 deaths from asthma annually in the United States. African Americans and Puerto Ricans have significantly higher death rates from asthma compared to non-Hispanic white Americans.
Asthma attacks are usually caused by inhaling an allergen to which an individual is hypersensitive. Exposure to certain childhood infections or irritants, such as air pollution or secondhand tobacco smoke, can make the airways more sensitive. Differences in the gut microbiome, the trillions of microorganisms that inhabit the human gastrointestinal tract, are also implicated in asthma. A 2015 study reported that infants at risk for asthma had lower levels of certain types of gut bacteria during their first 100 days of life. It has been suggested that improved hygiene and widespread use of antibiotics may play a role in the increase in asthma, because exposure to environmental microbes normally trains children's developing immune systems not to overreact to foreign substances. For example, children who grow up on dairy farms are much less likely than other children to develop asthma.
In addition to allergens and exercise, conditions such as acid reflux (heartburn, which occurs when acid from the stomach backs up into the esophagus), inhaling cold air (cold-induced asthma), anxiety or stress, or even crying or laughing can trigger asthma attacks in some people. Sinus inflammation or allergy flare-ups that cause rhinitis (inflammation of the lining of the nose) can worsen asthma.
Symptoms of asthma attacks include wheezing and shortness of breath, the most obvious indications that the airways are narrowing. Sometimes the wheezing is mild and can only be heard by a physician using a stethoscope. At other times, the wheezing is loud, particularly when the person tries to expel air. During an asthma attack, people often report coughing and a feeling of tightness in their chests. Severe asthma attacks require emergency treatment because the person may be unable to breathe in adequate oxygen. Their skin may turn bluish, and they may become confused.
Asthma can be diagnosed from personal and family histories of asthma and allergies and by listening to the patient's chest for wheezing. Physicians also look for chest expansion while the patient breathes in air. A test called spirometry measures how rapidly a person can exhale air and how much air the lungs retain. EIA may be diagnosed by peak flow measurements that compare how quickly air can be exhaled before and after exercise. Physicians may have patients exercise on a treadmill before listening for wheezing.
Asthma requires ongoing management, and patients must work closely with their health care providers to keep it under control. Managing asthma includes avoiding triggers, taking medications, and maintaining a healthy lifestyle.
There are many different asthma medications. Asthma control medicines help prevent attacks and keep the asthma under control. These include:
Short-acting or quick-relief medications are inhaled during acute asthma attacks. A short-acting bronchodilator, usually containing a beta-2 agonist, can relieve symptoms quickly. Inhaled steroids temporarily relax muscles that surround narrowed airways. Albuterol is the most common inhaled bronchodilator in the United States. During exercise or athletic activities, people with asthma usually keep an inhaler with them in the event of an asthma attack, however, short-acting bronchodilators may be ineffective if a person is very short of breath and cannot perform normal activities. A severe asthma attack can be a medical emergency that requires supplemental oxygen and, rarely, mechanical ventilation.
More than half of children with asthma have no symptoms by the time they reach adulthood. If children are exposed to chronic infections, air pollution, cigarette smoke, or allergens, however, they may continue to have asthma symptoms as adults. People with a family history of allergies and asthma are more likely to have the disease throughout life. Most people with asthma can live normal lives by working with their health care providers to manage the disease through a combination of control and acute medications, healthy lifestyles that include appropriate levels of fitness, and avoidance of triggers. Although deaths from asthma are relatively rare, sometimes the disorder progressively worsens, eventually leading to respiratory failure.
There is no proven strategy for preventing asthma, although early childhood exposure to microbes, especially from indoor/outdoor pets and livestock, appears to be somewhat protective by altering the balance of microbes in the human gut and respiratory system. Although people with asthma cannot change their family histories or previous childhood exposures, they can take steps to prevent attacks, including using control medications. Avoiding triggers may include keeping animal dander to a minimum and reducing exposure to dust mites, humidity, and chemicals, depending on one's specific sensitivities. Avoiding exposure to tobacco, wood smoke, and other environmental pollutants is particularly important. People with asthma may need to exercise indoors when pollution levels are high. Although indoor air also can contain pollutants, filters can help keep the air clean.
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American Academy of Allergy, Asthma, & Immunology, 555 E. Wells St., Ste. 1100, Milwaukee, WI, 53202-3823, (414) 272-6071, firstname.lastname@example.org, http://www.aaaai.org .
Asthma and Allergy Foundation of America, 8201 Corporate Dr., Ste. 1000, Landover, MD, 20785, (800) 7-ASTHMA (727-8462), email@example.com, http://www.aafa.org .
Teresa G. Odle, BA, ELS
Revised by Margaret Alic, PhD