Asthma Causes and Triggers

The exact cause of asthma is unknown, and the causes may be different from person to person. However, asthma often happens when the immune system strongly reacts to a substance in the lungs.

How asthma affects the lungs

Normally, the body’s immune system helps fight infections. But it may also respond to other things you breathe in, such as pollen or mold. In some people, the immune system reacts strongly by creating inflammation .

When this happens, the airways swell, narrow, and may create more mucus. The muscles around the airways may also tighten. This can make it even harder to breathe. Over time, the airway walls can become thicker.

Can I prevent asthma?

Because the exact cause of asthma is unknown, you may not be able to prevent asthma in yourself or your children. You or your child may develop asthma when the body’s immune system is still developing.

Research suggests that you may be able to take some steps to help prevent asthma from developing. They include doing your best to keep your home free of dampness and mold, avoiding air pollution as much as possible, and making a healthy weight a priority for you and your children.

Research for your health

NHLBI-supported research on asthma has led to more and better treatment options to improve the health of people who have asthma. Through our current research, we hope to better understand how our genes and the environment we live in affect our risk for developing asthma.

What causes asthma?

Asthma often starts during childhood when your immune system is still developing. Multiple factors may work together to cause it, such as:

  • Things in the environment (called allergens) that affected you as a baby or young child, which may include cigarette smoke or certain germs
  • Viral infections that affect breathing
  • Family history, such as a parent who has asthma (especially your mother)

These can affect how your lung develops or how your body fights germs. Other things that may raise the risk of developing asthma include the following.

  • Allergies: Asthma is usually a type of allergic reaction. People who have asthma often have other types of allergies, such as food or pollen.
  • Obesity: This condition raises your chances of developing asthma or making your asthma symptoms worse.
  • Race or ethnicity: African Americans and Puerto Ricans are at higher risk of asthma than people of other races or ethnicities are. African American and Hispanic children are more likely than non-Hispanic white Americans to die from asthma-related causes.
  • Sex: More boys than girls have asthma as children, while asthma is more common among women in teens and adults.
  • Occupational hazards: Breathing in chemicals or industrial dusts in the workplace can raise your risk of developing asthma.
Asthma in Our Communities fact sheet
Fact sheet
Asthma in Our Communities

Learn how asthma disproportionately affects different communities and steps for managing asthma.

Asthma triggers

Asthma triggers are things that set off or make asthma symptoms worse. Common triggers for asthma include:

  • Indoor allergens, such as dust mites, mold, and pet dander or fur
  • Outdoor allergens, such as pollens and mold
  • Emotional stress, such as intense anger, crying, or laughing
  • Physical activity, although with treatment, you or your child should still be able to be active
  • Infections, such as colds, influenza (flu), or COVID-19
  • Certain medicines, such as aspirin, which may cause serious breathing problems in people with severe asthma
  • Poor air quality or very cold air

Visit the Managing Asthma page to learn about avoiding triggers.

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