Skip to Content



Victor E. Ortega

, MD, PhD, Center for Genomics and Personalized Medicine Research, Wake Forest School of Medicine;

Frank Genese

, DO, Wake Forest School of Medicine

Last full review/revision August 2019 by Victor E. Ortega, MD, PhD; Frank Genese, DO

Asthma is a condition in which the airways narrow—usually reversibly—in response to certain stimuli.

  • Coughing, wheezing, and shortness of breath that occur in response to specific triggers are the most common symptoms.
  • Doctors confirm the diagnosis of asthma by doing breathing (pulmonary function) tests.
  • To prevent attacks, people should avoid substances that trigger asthma and should take drugs that help keep airways open.
  • During an asthma attack, people need to take a drug that quickly opens the airways.

(See also Asthma in Children, Wheezing in Infants and Young Children, and Asthma During Pregnancy.)

Asthma affects more than 25 million people in the United States, and it is becoming more common. The reason for the increase in asthma is not known.


The causes of asthma are unknown, but asthma likely results from complex interactions between many genes, environmental conditions, and nutrition. Environmental conditions and circumstances around pregnancy, birth, and infancy have been associated with the development of asthma in childhood and later in adulthood. Risk appears to be higher if a person's mother became pregnant at a young age or had poor nutrition during pregnancy. Risk may also be higher if someone is born prematurely, had a low weight at birth, or was not breastfed. Environmental conditions such as exposures to household allergens (such as dust mites, cockroaches, and pet dander) and other environmental allergens have also been associated with the development of asthma in older children and adults. Diets low in vitamins C and E and in omega–3 fatty acids have been also linked to asthma, as has obesity; however, there is no evidence that dietary supplements of these substances prevent the development of asthma.

In developed countries, having smaller families with fewer children, cleaner indoor environments, and use of vaccinations and antibiotics may in early life decrease the body's ability to develop resistance to allergens in the environment and may partly explain the increase in asthma in these countries (the hygiene hypothesis).

Narrowing of the airways is often caused by abnormal sensitivity of cholinergic receptors, which cause the muscles of the airways to contract when they should not. Certain cells in the airways, particularly mast cells, are thought to be responsible for initiating the response. Mast cells throughout the bronchi release substances such as histamine and leukotrienes, which cause the following:

  • Smooth muscle to contract
  • Mucus secretion to increase
  • Certain white blood cells to move to the area

Eosinophils, a type of white blood cell found in the airways of people with asthma, release additional substances, contributing to airway narrowing.

In an asthma attack (sometimes called a flare-up or an exacerbation), the smooth muscles of the bronchi contract, causing the bronchi to narrow (called bronchoconstriction) The tissues lining the airways swell due to inflammation and mucus secretion into the airways. The top layer of the airway lining can become damaged and shed cells, further narrowing the airway. A narrower airway requires the person to exert more effort to breathe. In asthma, the narrowing is reversible, meaning that with appropriate treatment or on their own, the muscular contractions of the airways stop, inflammation resolves so that the airways widen again, and airflow into and out of the lungs returns to normal.

How Airways Narrow

During an asthma attack, the smooth muscle layer goes into spasm, narrowing the airway. The middle layer swells because of inflammation, and excessive mucus is produced. In some segments of the airway, mucus forms plugs that nearly or completely block the airway.

How Airways Narrow

Asthma triggers

In people who have asthma, the airways narrow in response to stimuli (triggers) that usually do not affect the airways in people without asthma. Such triggers include

  • Allergens
  • Infections
  • Irritants
  • Exercise (called exercise-induced asthma)
  • Stress and anxiety
  • Aspirin

Many inhaled allergens, including pollens, particles from dust mites, body secretions from cockroaches, particles from feathers, and animal dander, can trigger an asthma attack. These allergens combine with immunoglobulin E (IgE, a type of antibody) on the surface of mast cells to trigger the release of asthma-causing chemicals. (This type of asthma is called allergic asthma.) Although food allergies induce asthma only rarely, certain foods (such as shellfish and peanuts) can induce severe attacks in people who are sensitive to these foods.

Infectious triggers are usually viral respiratory infections, such as colds, bronchitis, and, less commonly, pneumonia.

Irritants that can provoke an asthma attack include smoke from tobacco, marijuana, or cocaine; fumes (such as from perfumes, cleaning products, or air pollution); cold air; and stomach acid in the airways caused by gastroesophageal reflux disease (GERD). Air pollution has been linked to asthma attacks.

Some people who have asthma can develop airway narrowing when exercising. This type of airway narrowing may be due to breathing drier, colder air through the mouth while exercising.

Stress and anxiety can trigger mast cells to release histamine and leukotrienes and stimulate the vagus nerve (which connects to the airway smooth muscle), which then contracts and narrows the bronchi.

Crying or hearty laughing may trigger symptoms in some people.

Aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs) are triggers for almost 30% of people with severe asthma, but they are triggers in less than 10% of people with asthma overall.

Reactive airways dysfunction syndrome

Reactive airways dysfunction syndrome (RADS) is a rapid onset and persistent asthma-like disorder that occurs in people with no history of asthma. It is a form of environmental lung disease caused by a single large exposure to nitrogen oxide or volatile organic compounds (such as those in certain bleaches and cleaning products). People have symptoms similar to those of asthma, including cough, wheezing, and shortness of breath. Treatment is similar to usual treatment for asthma.


Asthma attacks vary in frequency and severity. Some people who have asthma are symptom-free most of the time, with only an occasional brief, mild episode of shortness of breath. Other people cough and wheeze most of the time and have severe attacks after viral infections, exercise, or exposure to other triggers.

Wheezing is a musical sound that occurs when the person breathes out. Coughing may be the only symptom in some people (cough-variant asthma). Some people with asthma produce a clear, sometimes sticky (mucoid) phlegm (sputum).

Peak Flow Meter
Peak Flow Meter

Education about how to prevent and treat asthma attacks is beneficial for all people who have asthma and often for their family members. Proper use of inhalers is essential for effective treatment. People should know

  • What can trigger an attack
  • What helps to prevent an attack
  • How to use drugs properly
  • When to seek medical care

Monitoring Asthma at Home

Some people use a handheld peak flow meter to evaluate their breathing and determine when they need intervention, before their symptoms become severe. People who experience frequent, severe asthma attacks should know how to reach help quickly.

Peak expiratory flow (the fastest rate at which air can be pushed out of the lungs) can be measured using a small handheld device called a peak flow meter. This test can be used at home to monitor the severity of asthma. Usually, peak flow rates are lowest between 4 AM and 6 AM and highest at 4 PM. However, more than a 30% difference in rates at these times is considered evidence of moderate to severe asthma. People with moderate to severe asthma, particularly those who need daily treatment to control symptoms, often use a peak flow meter to take measurements and compare them to their personal best to help identify signs of worsening asthma or the onset of an asthma attack.

All people with asthma should have a written treatment action plan that was devised in collaboration with their doctor. Such a plan allows them to take control of their own treatment and has been shown to decrease the number of times people need to seek care for asthma in the emergency department.

Treating Asthma Attacks

An asthma attack can be frightening, both to the person experiencing it and to others around. Even when relatively mild, the symptoms provoke anxiety and alarm. A severe asthma attack is a life-threatening emergency that requires immediate, skilled, professional care. If not treated adequately and quickly, a severe asthma attack can cause death.

An acute attack in a person whose asthma has been controlled by drugs is called an exacerbation or flare-up.

Mild attacks

People who have a mild asthma attack are usually able to treat it without assistance from a health care practitioner. Typically, they use an inhaler to deliver a dose of a short-acting beta-adrenergic drug such as albuterol, move into fresh air (away from cigarette smoke or other irritants), and sit down and rest. They can use the inhaler 3 times 20 minutes apart if needed. An attack usually subsides in 5 to 10 minutes. An attack that does not subside after using an inhaler 3 times or that gets worse is likely to require additional treatment supervised by a doctor.

Severe attacks

People who have severe symptoms should typically go to an emergency department. For severe attacks, doctors give frequent (or sometimes continuous [without pausing]) treatment) using inhaled beta-adrenergic bronchodilator drugs delivered by a device called a nebulizer. Doctors sometimes give these bronchodilator drugs in combination with anticholinergic drugs. People are also given a corticosteroid, such as prednisone, by mouth or by vein (intravenously). Supplemental oxygen may be given during attacks.

Generally, people who have a severe asthma attack are admitted to the hospital if their lung function does not improve after they have received an inhaled beta-adrenergic drug and corticosteroids by mouth or vein. People also are hospitalized if they have a seriously low blood oxygen level or a high blood carbon dioxide level.

Antibiotics may be needed if a doctor suspects a bacterial lung infection. However, most such infections are due to viruses for which (with a few exceptions) no treatment exists.

People experiencing very severe asthma attacks may need to have an artificial airway passed through their mouth and throat (intubation) and be placed on a mechanical ventilator.

Preventing Asthma Attacks

Asthma is a chronic condition that cannot be cured, but individual attacks can often be prevented. Prevention efforts depend on the frequency of attacks and the stimuli that trigger the attacks.

Identifying and eliminating or avoiding stimuli that trigger asthma attacks may commonly prevent them.

  • Irritating fumes: People who have asthma should avoid cigarette smoke and other irritating fumes and try to avoid exposure to people with upper respiratory infections.
  • House dust mites: When dust and allergens are triggers, air filters and barriers (such as mattress covers, which reduce the amount of particles from dust mites that are in the air) can help considerably. Exposure to house dust mites can be reduced by removing wall-to-wall carpets and curtains and using air conditioning to keep the relative humidity low (preferably below 50%) in the summer.
  • Animal dander: Animals with fur or hair, most commonly cats and dogs, often must be given away to decrease the overall exposure to animal dander. Other measures that can help include limiting the family pet to certain rooms of the house or, if possible, keeping it out of the house. Washing the pet weekly can also help.
  • Drugs: Avoiding aspirin and NSAIDs helps prevent attacks in people whose asthma is triggered by these drugs. Drugs that block the beneficial effects of beta-adrenergic drugs (called beta-blockers) may worsen asthma. Tartrazine, a yellow coloring used in some drug tablets and food, may also bring on an attack.
  • Exercise: Often, attacks triggered by exercise can be blocked by taking asthma drugs beforehand.
  • Cold: For outdoor activity in cold weather, people with asthma can wear a ski mask or scarf that covers the nose and mouth to help keep the air being breathed in warm and moist.
  • Sulfites: Sulfites—commonly added to foods as a preservative—may trigger attacks after a susceptible person eats a certain food or drinks beer or red wine. Sulfites can be avoided by careful attention to diet choices.

Allergen desensitization through the use of allergy shots may help prevent attacks in people whose asthma is triggered by allergies. A doctor-supervised desensitization program may also be used for people whose asthma is triggered by aspirin or NSAIDs.

Drugs, such as inhaled or oral corticosteroids, leukotriene modifiers, long-acting beta-adrenergic drugs, methylxanthines, antihistamines, or mast cell stabilizers are used to prevent attacks in most people with asthma. A minority of people with asthma have severe disease that remains uncontrolled, causing repeated attacks despite treatment with a combination of therapies. These people may benefit from treatment with immunomodulator drugs that block substances that cause allergic inflammation.


Many children outgrow asthma, but wheezing may persist into adulthood or asthma may return in later years. Female sex, smoking, earlier age of onset, and allergy to household dust mites increase the risk that asthma will persist or return.

Although people may die as a result of a severe asthma attack, most of these deaths are preventable with treatment. Thus, the prognosis is good with adequate access and adherence to treatment.

More Information

  • Allergy and Asthma Network
  • American Academy of Allergy, Asthma and Immunology
  • Asthma & Allergy Foundation of America

Drugs Mentioned In This Article

Generic Name Select Brand Names
prednisone RAYOS
Aspirin No US brand name

© 2020 Merck Sharp & Dohme Corp., a subsidiary of Merck & Co., Inc., Kenilworth, NJ, USA. Merck Manual Disclaimer