Asthma is caused by a combination of inherited genetic traits and environmental exposures that together make the airways overly reactive and prone to inflammation. No single factor explains it. Instead, genes load the gun and the environment pulls the trigger, with dozens of possible contributing factors ranging from childhood infections to air pollution to obesity. Globally, asthma affected an estimated 363 million people in 2023 and caused 442,000 deaths.
Genetics Set the Stage
Twin and family studies estimate that asthma’s heritability falls somewhere between 25 and 80 percent, meaning genetics account for a significant but highly variable share of your overall risk. If one or both of your biological parents have asthma, you’re substantially more likely to develop it yourself. But there’s no single “asthma gene.” Dozens of genetic variants each contribute a small amount of risk, and they interact with environmental factors in ways researchers still don’t fully understand. Genetic testing for asthma susceptibility isn’t recommended because so many of these interacting factors remain unidentified.
What your genes primarily influence is how your immune system behaves. Some people inherit an immune system that’s more likely to overreact to harmless substances like pollen or dust mites, producing an exaggerated inflammatory response in the airways. Others inherit airways that are structurally more prone to narrowing. These tendencies don’t guarantee asthma, but they make it far more likely when combined with the right environmental conditions.
What Happens Inside Your Airways
In a person with asthma, the airways are in a state of chronic low-level inflammation even when symptoms aren’t present. When triggered, the immune system ramps up this inflammation dramatically. The lining of the airways swells, the muscles wrapped around the airways tighten, and the body produces excess mucus. All three of these responses narrow the space air has to pass through, which is what creates that characteristic wheezing, chest tightness, and shortness of breath.
The most common form of asthma, responsible for the vast majority of cases, involves a specific branch of the immune system driven by a set of signaling molecules that promote allergic-type inflammation. This type features elevated levels of certain white blood cells called eosinophils that accumulate in the airway walls. A smaller subset of asthma cases don’t follow this allergic pattern and tend to respond poorly to standard treatments like inhaled steroids. These cases are less well understood and are often defined simply by the absence of the typical allergic markers.
Early Childhood Infections
What happens in the first years of life plays an outsized role in whether asthma develops. Respiratory syncytial virus (RSV), a near-universal childhood infection, is one of the strongest early risk factors. Research from the NIH found that infants who avoided RSV infection during their first year of life had a 26% lower risk of developing asthma by age 5. Among children who caught RSV in infancy, 21% had asthma by age 5, compared with 16% of those who didn’t. The researchers estimated that roughly 15% of childhood asthma cases could be prevented by avoiding RSV infection in infancy alone.
Severe respiratory infections in early life appear to cause lasting changes to the airways and immune system during a critical window of development. The damage and inflammation from these infections may reshape how the airways grow and how the immune system responds to future irritants, setting up the chronic inflammatory cycle that defines asthma.
The “Too Clean” Problem
One of the more counterintuitive findings in asthma research is that growing up in extremely clean environments may actually increase your risk. The hygiene hypothesis, supported by epidemiological studies, suggests that a newborn’s immune system needs exposure to certain germs to develop properly. Without that exposure, the immune system essentially gets a poor education and is more likely to overreact to harmless triggers later in life.
The evidence shows that allergic diseases and asthma are more common in homes with low levels of bacterial molecules called endotoxins. These molecules normally activate a molecular switch on immune cells that helps train them to respond appropriately to threats. When this training is weak or absent, the immune system’s T-cells, which are critical for fighting infections, can inadvertently trigger asthma-like responses instead of clearing infections efficiently. This helps explain why asthma rates are higher in developed countries and why children raised on farms, where microbial exposure is abundant, tend to have lower asthma rates.
Environmental Triggers and Allergens
For people with the underlying predisposition, a wide range of environmental exposures can both initiate and worsen asthma. Common allergens include dust mites, mold spores, pet dander, cockroach droppings, and pollen. These are sometimes called “triggers” because they provoke symptoms in someone who already has asthma, but chronic exposure to high levels of allergens during childhood may also contribute to developing the condition in the first place.
Air pollution is another major factor. Nitrogen dioxide from vehicle exhaust, fine particulate matter from industrial sources, and ground-level ozone all irritate the airways and promote the kind of inflammation that underlies asthma. Children who grow up near busy roads or in areas with poor air quality develop asthma at higher rates.
Climate change is compounding the problem. Rising temperatures, increased atmospheric carbon dioxide, and shifting precipitation patterns are extending pollen seasons, increasing the amount of pollen plants produce, and potentially making that pollen more potent as an allergen. The CDC notes that these changes may heighten health effects for a growing number of people.
Workplace Exposures
Adults can develop asthma for the first time from repeated exposure to irritants at work, a condition called occupational asthma. The CDC lists numerous industrial chemicals known to cause it, including diisocyanates (found in polyurethane production, spray painting, and foam manufacturing), wood dusts from western red cedar and other species, metal dusts and metalworking fluids, acid anhydrides used in epoxy resins and dyes, and industrial enzymes. Even acrylic monomers found in adhesives and complex platinum salts can sensitize the airways over time. Occupational asthma typically improves when the exposure stops, but in some cases, the airway changes become permanent.
Obesity and Asthma
Carrying excess weight is both a risk factor for developing asthma and a factor that makes existing asthma harder to control. The relationship works through multiple pathways. Fat tissue is not inert storage; it actively produces signaling molecules that promote low-grade inflammation throughout the body, including in the airways. Excess abdominal weight also physically compresses the lungs, reducing lung volumes and making the airways more prone to narrowing.
Research published in the European Respiratory Journal describes how excess fat disrupts the normal communication between different types of immune cells, amplifying the inflammatory cascade in the airways. This means that for some people, weight loss can meaningfully improve asthma control, sometimes as much as adding another medication.
Other Contributing Factors
Several additional factors raise asthma risk. Tobacco smoke exposure, particularly during pregnancy and early childhood, is one of the most preventable causes. Children whose mothers smoked during pregnancy have higher asthma rates, and secondhand smoke exposure continues to damage developing airways after birth.
Hormonal factors also play a role. Before puberty, boys are more likely to have asthma than girls. After puberty, the pattern reverses, with women having higher rates than men through adulthood. Hormonal fluctuations during menstrual cycles, pregnancy, and menopause can all influence asthma severity, suggesting that sex hormones interact with airway inflammation in ways that aren’t fully mapped out.
Stress and strong emotions can worsen airway inflammation and trigger symptoms, though they don’t cause asthma on their own. Gastroesophageal reflux, where stomach acid backs up into the esophagus, is another common aggravating factor. The acid can reach the airways and trigger inflammation, creating a cycle where asthma and reflux each make the other worse.