What Causes Asthma?
Medically reviewed by Daniel More, MD
Asthma is a chronic lung disease that affects the airways, causing them to become inflamed and narrowed. Asthma symptoms, such as shortness of breath, wheezing, and chest tightness, can be triggered by exercise, exposure to allergens and irritants, and respiratory infections. While the exact cause of asthma is unknown, researchers have identified several factors that increase the risk of the disease, including genetics, allergies, exposure to lung irritants, and immune system dysfunction.
In this article, we'll delve into the biological processes that lead to asthma symptoms and explore potential causes and triggers of the disease.
Theories
The cause of asthma likely varies from person to person, but it occurs most often in response to an initial trigger. People with asthma have hyperresponsive (sensitive) airways that become inflamed, swollen, and narrowed when they are exposed to their triggers. Inflammation in the airways causes the surrounding muscles to contract, restricting the airway passages and causing shortness of breath, wheezing, coughing, and chest tightness. Inflammation can also lead to excess mucus production and worsening symptoms.
Researchers are still working to find the exact cause(s) of asthma, but it is thought to be caused by a combination of genetic and environmental factors.
Exposure to Allergens
Allergies and asthma often go hand-in-hand, and people with allergies are more likely to develop asthma than those who do not have allergies. When people with allergies are exposed to an allergen—such as pollen, pet dander, mold, and dust—the immune system thinks it is being attacked and overreacts in response. When asthma symptoms occur, such as shortness of breath and chest tightness, it is known as allergic asthma.
Viral Respiratory Infections
Viral respiratory infections, such as the common cold and influenza, may be a possible cause of asthma. When you develop a viral infection, your body's immune system produces certain immune cells and releases chemicals to fight the virus. In some people, this immune response can trigger inflammation and narrowing of the airways, leading to asthma symptoms.
Research shows that some children who experienced viral and bacterial respiratory infections develop chronic asthma. This may be because the immune system and lungs are still developing in infancy and early childhood, making them more susceptible to damage and inflammation from respiratory viruses.
Exposure to Lung Irritants
Exposure to lung irritants may play a role in the development of asthma. The effects of exposure to lung irritants vary, depending on the type of irritant and how long you are exposed, as well as individual factors such as genetics and underlying health conditions. For example, people exposed to chemicals or dust particles in the air at work may have an increased risk of asthma due to chronic exposure to these irritants. When these irritants are inhaled, they can cause inflammation and constriction of the airways.
Lung irritants include:
Air pollution
Cigarette smoke
Certain dusts (e.g., industrial or wood)
Chemical fumes and vapors
Mold
Is Asthma Hereditary?
It is well known that asthma runs in families, which suggests the condition is strongly linked to genetics. People with family members with asthma are more likely to develop the condition, and you have a 3-6 times higher risk if one of your parents has asthma.
Researchers have identified over 100 genes linked to asthma, and many of those genes play a role in how the immune system and lungs function. Asthma shares genetic risk factors with other conditions associated with an overactive immune system, including allergic rhinitis (hay fever) and eczema. These genes may explain the airways of people with asthma are more sensitive to allergens and environmental irritants than people without asthma.
A genetic predisposition for asthma does not necessarily mean you will develop it. Research suggests that many factors contribute to its development, and the way certain genes interact with one another combined with environmental factors is likely what causes asthma.
Who Gets Asthma?
Asthma is very common and can affect people of all ages, genders, and ethnicities. However, some people are more likely to develop the condition than others.
Age: Asthma can develop at any age but most commonly begins in childhood. According to the Centers for Disease Control and Prevention (CDC), asthma is more common in children than adults, with the highest rates of asthma occurring in children ages 5-14.
Sex: Asthma affects both males and females, but research shows that boys are more likely to have asthma than girls. This reverses in adulthood, with women more likely to be affected than men.
Ethnicity: Asthma prevalence varies across different ethnic groups. In the U.S., asthma is more common in African American and Hispanic populations—especially people of Puerto Rican descent—compared to white populations. The reasons for these disparities are not entirely clear but may be related to differences in environmental exposures, genetic factors, and access to healthcare.
Until recently, most research on potential asthma causes focused on white people of European descent. As a result, certain ethnic groups and races are underrepresented in the research data on asthma, making it difficult to understand why certain groups are more affected.
Risk Factors
Several risk factors can increase the likelihood of developing asthma. Having one or more risk factors does not necessarily mean you will develop asthma. Being aware of your risk factors can help ensure you seek a diagnosis and treatment if you develop symptoms.
Family History
Having a family member or members with a history of asthma increases your risk of developing the condition. For example, if one parent has asthma, your risk of asthma increases by 25%. If both parents do, your risk is 50% higher than those who do not have a parent with asthma.
Allergies
People with allergies, such as hay fever or atopical dermatitis (eczema), are more likely to develop asthma. This may be because allergens can trigger an immune response that leads to inflammation and constriction of the airways.
Occupational Exposure
Exposure to certain lung irritants, such as chemicals, vapors, and dusts, can increase the risk of asthma or worsen existing symptoms. Long-term exposure to these irritants can inflame and damage the airways.
Smoking
Smokers, people whose mothers smoked during pregnancy, and those exposed to secondhand smoke have a high risk of asthma. Cigarette smoke irritates the airways and causes inflammation and excess mucus production—key characteristics of asthma.
Air Pollution
Exposure to air pollution, such as smog, raises the risk of asthma. People living in cities and areas with dense smog are more likely to develop the condition.
Premature Birth
Children born prematurely (before 37 weeks gestation) are more than 1.64 times more likely to develop asthma than those born full-term. Children born at a low birth weight also have a slightly increased risk of the disease.
Obesity
People who are overweight or have obesity have a higher risk of developing asthma. The exact reason for this is unclear, but some evidence suggests that the chronic low-grade inflammation associated with extra body weight may contribute to the development of asthma.
Stress
It is well-established that stress and strong emotions can trigger asthma symptoms in some people, but new research suggests that chronic stress may contribute to developing the disease. In addition, stressful and traumatic events in childhood and adolescence have also been linked to an increased risk of adult-onset asthma.
A Quick Review
Asthma is a common lung disease that affects an estimated 226 million people worldwide. Though the specific cause(s) of asthma are not yet fully understood, researchers believe that a combination of genetic and environmental factors plays a role in developing the disease. Certain risk factors can increase the likelihood of asthma, including family history, allergies, exposure to irritants and pollution, smoking, and premature birth.
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