The cause of asthma is not well understood, and it may vary for different people with asthma. Most researchers believe the development of asthma is due to a combination of genetic and environmental risk factors. Asthma is often an allergic reaction related to an overreaction by the immune system in response to a substance that enters the lungs.

In people with asthma, the small airways of the lungs narrow, obstructing the flow of air and causing symptoms such as wheezing, chest tightness, shortness of breath, and coughing. Symptoms of asthma can range from mild and intermittent to severe and persistent and even life-threatening.

Risk Factors for Asthma

It is important to note that while science is good at finding correlations, or apparent relationships, between factors and disease, correlation does not prove the factor causes the disease. Many risk factors for asthma have been identified and are being studied, but none have been pinpointed as the cause of asthma.

Age

Asthma can affect all age groups, but it often starts in childhood — usually before age 4. Different types of asthma tend to develop at different ages. Asthma in young children is usually allergic asthma, while asthma that develops in older adults is more likely to be nonallergic.

Hereditary Factors

Asthma runs in families. If one of your parents has asthma, you are more likely (but not guaranteed) to develop it. Some studies suggest having one parent with asthma presents a 25 percent risk of a child having asthma too. If both parents have asthma, the risk may rise to 50 percent. However, some research indicates that having a mother with asthma — especially when the asthma is not well controlled during pregnancy — raises the risk more than having a father with asthma. Genes inherited from parents contribute differently to the risk for developing different types of asthma.

Researchers have identified eight genes so far that are related to asthma risk, with more than 100 other genes still under study. Some of these genes may influence how the body responds to vitamin D levels or respiratory infections. Others may influence allergic responses or the production of mucus.

Sex

Among children, boys are more likely than girls to have asthma. During adolescence and adulthood, more women have asthma than men.

Asthma and Race

In the United States, asthma disproportionately affects racial and ethnic minorities. People with Black and Puerto Rican heritage are more likely to have asthma, and children with these backgrounds are more likely to die from asthma than white children. Research by the Asthma and Allergy Foundation of America shows that in the U.S.:

• Puerto Ricans have the highest rate of asthma prevalence.
• Black people are nearly three times more likely to die from asthma than white people.
• Black people make five times more trips to the emergency room for asthma than white people.

The reasons for these disparities involve complex associations between economic, social, biological, and behavioral factors. Research to understand these factors and improve health outcomes is ongoing.

Environmental Factors

Risk factors that are not related to inherited genes are known as environmental factors. There are many environmental risk factors that lead to an increased risk for developing asthma. These include:

• A child’s exposure to cigarette smoke or certain cleaning products while in the womb or during early life
• Air pollution from vehicle traffic, dust, pollen, or allergens
• Exposure to microbes, such as bacteria, viruses, and fungi
• Use of antibiotics within the first year of life
• Stress in the child during early life; stress or depression in the mother during pregnancy

Occupational Factors

People working in certain industries are more likely to develop occupational asthma, potentially caused by substances they encounter in the workplace. Those who work in the following fields are more likely to be diagnosed with occupational asthma:

• Agricultural workers, such as farmers or those who work in grain elevators
• Bakers and millers
• Manufacturers of detergents or pharmaceuticals
• Laboratory workers, especially where animals are involved
• Those who work with metal, plastic, or wood

Other Health Conditions

People with other health conditions — including allergies, obesity, and viral respiratory infections — are more likely to have asthma. There is a strong connection between asthma, allergies, and eczema (also known as atopic dermatitis) that extends to a family history of these conditions. Other lung diseases, including chronic obstructive pulmonary disease (COPD), can make asthma symptoms worse. Read more about conditions related to asthma.

Triggers for Asthma Attacks

Once someone has developed asthma, certain irritants and situations may trigger asthma attacks. Asthma triggers differ from person to person. Common asthma triggers include:

• Tobacco smoke
• Furry pets
• Allergens, such as dust mites, mold, or pollen
• Air pollution
• Pests, such as cockroaches or mice
• Fumes from detergents, cleaning products, or disinfectants
• Exercise or physical activity
• Viral infections
• Strong emotions
• Certain foods
• Acid reflux
• Cold air and changes in weather
• Certain drugs, such as beta blockers and migraine medications
• Food additives, including sulfites

Can Asthma Be Prevented?

Unfortunately, there is no way to prevent asthma from developing. Some risk factors, such as inherited genes, are beyond anyone’s control. However, researchers have identified a few factors associated with a lower risk for asthma that may have a protective effect. Potentially protective aspects include:

• Attending day care during the first six months of life
• Growing up on a farm
• Mothers taking vitamin D or fish oil supplements during pregnancy
• Mothers with asthma keeping their condition well-controlled with asthma treatments during pregnancy

Around the world, some countries have conducted programs or clinical trials aimed at preventing asthma. Smoking-cessation campaigns and bans on public smoking are often prompted in part by an effort to prevent asthma. Some clinical trials have been designed to identify babies or pregnant mothers with a high risk for asthma and supplement their diets with vitamin D or fish oil. Other trials are aimed at modulating the microbiome — the community of bacteria and other microorganisms living in the human body — with vaccinations or probiotic supplements. So far, results are mixed. Researchers are hopeful that one day it may be possible to prevent asthma.

If you are worried about your risk or your child’s risk for developing asthma, you may be able to lower your risk. Consider taking steps to change these environmental factors:

• Quit smoking and avoid secondhand smoke.
• Use air purifiers in the home, such as these products certified “asthma and allergy friendly” by the Asthma and Allergy Foundation of America.
• Avoid allergen triggers.
• Use protective equipment, such as respirators, if your job involves airborne particles.

Talk to your allergist or health care team about what you can do to reduce your risk factors for asthma.

Condition Guide

• Asthma — An Overview
• Symptoms of Asthma
• Asthma — The Path to Diagnosis
• Conditions Related to Asthma
• Types of Asthma
• Treatments for Asthma