Asthma develops when a combination of inherited genetic susceptibility and environmental exposures triggers chronic inflammation in the airways. There is no single cause. Instead, the condition results from an interplay between your genes, your immune system’s early programming, the allergens you breathe, and the air quality where you live. Globally, asthma affects over 260 million people and causes more than 450,000 deaths each year, most of which are preventable.
What Happens Inside the Airways
In a person with asthma, the lining of the airways becomes chronically inflamed. The immune system overreacts to triggers that wouldn’t bother a healthy lung. A key part of this process involves a branch of the immune system driven by a type of white blood cell called a T-helper 2 (Th2) cell. When the airway lining is irritated, it releases signaling molecules that activate these Th2 cells, which then set off a cascade of inflammation. This cascade recruits eosinophils, another type of immune cell, into the airway walls and ramps up production of an antibody called IgE, which is the same antibody responsible for allergic reactions.
Over time, two structural problems develop. First, the smooth muscle wrapped around the airways becomes dysfunctional and hyperresponsive, meaning it contracts too easily and too forcefully in response to minor irritants. Second, the airways undergo “remodeling,” where the walls thicken and scar tissue forms, permanently narrowing the passages through which air flows. These changes explain why asthma is a chronic condition rather than something that simply comes and goes.
Genetics Set the Foundation
Asthma runs strongly in families. Studies estimate that genetics account for 55 to 74 percent of asthma risk in adults and as high as 90 percent in children. That doesn’t mean a specific “asthma gene” exists. Researchers have identified dozens of gene regions linked to susceptibility, and the combined effect of all identified genes explains about 38 percent of childhood asthma cases. Many of these genes are involved in immune signaling and inflammation, particularly the pathways that control how the body produces inflammatory molecules and responds to allergens.
Having a genetic predisposition doesn’t guarantee you’ll develop asthma. It means your immune system is more likely to tip toward the kind of allergic, airway-centered inflammation that defines the disease, if the right environmental triggers come along.
Early Life and the Microbiome
Some of the most important causes of asthma trace back to the first months of life, before a child ever wheezes. The composition of bacteria in an infant’s gut appears to play a surprisingly large role in whether the immune system develops normally or tilts toward allergic inflammation.
Babies with lower gut microbial diversity in early life have a higher risk of persistent wheezing and later asthma. Specific beneficial bacteria, including Bifidobacterium and Faecalibacterium, along with the short-chain fatty acids they produce, are associated with lower risk. On the other hand, certain bacterial metabolites found in elevated levels in newborn stool have been shown to interfere with the immune system’s ability to build tolerance, potentially setting the stage for asthma.
What shapes this early microbiome matters too. Babies born vaginally tend to be colonized with a more diverse set of gut bacteria compared to those born by cesarean section. Meta-analyses show that children born by C-section have about a 17 percent higher risk of developing asthma and wheezing after age two. Having older siblings also shifts the infant gut microbiome toward a more protective profile, with higher levels of beneficial bacteria. This helps explain the long-observed pattern that younger siblings and children raised on farms, who encounter more microbes early on, tend to develop asthma less often.
Even a mother’s diet during pregnancy can influence her child’s asthma risk. High-fiber diets boost production of acetate, a short-chain fatty acid that can cross the placental barrier and promote the development of regulatory immune cells in the fetal lungs, reducing the child’s susceptibility to asthma after birth.
Allergens and Sensitization
For people with the genetic predisposition, repeated exposure to specific allergens can trigger the immune system to develop a lasting allergic response, a process called sensitization. Once sensitized, each subsequent exposure provokes the airway inflammation that drives asthma symptoms.
Indoor allergens are the most significant. In humid climates, dust mite allergens are by far the most important cause of sensitization, with a clear dose-response relationship: the more dust mite allergen in the home, the higher the likelihood of becoming sensitized. Cockroach allergens follow a similar pattern. Pet dander, mold spores, and rodent proteins also contribute.
The timing of when sensitization happens is still debated. Objective signs of an immune response to allergens typically don’t appear until after age two, and some children don’t show positive allergy tests or symptoms until after age five. This makes it difficult to pinpoint a single critical window, though cumulative exposure during the first several years of life clearly matters.
Air Pollution as a Direct Cause
Outdoor air pollution is now recognized as a cause of new asthma cases, not just a trigger for existing ones. Fine particulate matter (PM2.5), the tiny particles produced by vehicle exhaust, industrial emissions, and wildfires, has the strongest evidence behind it.
For every 10 microgram-per-cubic-meter increase in long-term PM2.5 exposure, the risk of childhood asthma rises by 21.4 percent. In adults, the same increase raises risk by 7.1 percent. Children are more vulnerable because their lungs are still developing and they breathe more air relative to their body size. A 2024 global analysis estimated that nearly one-third of all asthma cases worldwide in 2019 were associated with PM2.5 exposure, making air pollution one of the largest modifiable causes of the disease.
Nitrogen dioxide, ozone, and traffic-related pollution also contribute, though PM2.5 has the most robust data linking it to new diagnoses rather than just worsening of existing symptoms.
Occupational Asthma in Adults
Adults who develop asthma for the first time often trace the onset to something in their workplace. Occupational asthma accounts for a meaningful share of adult-onset cases and is caused by repeated inhalation of specific sensitizers or irritants on the job.
The list of known workplace triggers is extensive. Flour dust is one of the most common, affecting bakers and millers. Hardwood dusts from woodworking cause asthma across roughly 40 tree species. Chemical disinfectants like glutaraldehyde, used to sterilize medical instruments, are a well-documented cause in healthcare settings. Other triggers include:
- Isocyanates in spray paints, foams, and coatings
- Platinum salts in metal refining and catalyst manufacturing
- Chromium compounds in welding fume and cement
- Enzyme additives like amylase in baking and detergent production
- Animal proteins from laboratory work, farming, or seafood processing
The pattern is usually the same: symptoms improve on days off and vacations, then return with workplace exposure. Removing the person from the exposure is the most effective intervention, and early recognition makes a significant difference in long-term outcomes.
Why Some People Get Asthma and Others Don’t
The frustrating reality is that two people can share the same genetic risk, grow up in the same neighborhood, and breathe the same air, yet only one develops asthma. The current understanding is that asthma requires multiple hits. Genetic susceptibility loads the gun. Early-life microbial exposure (or lack of it) shapes the immune system’s tendencies. Then ongoing allergen exposure, air pollution, respiratory infections, or occupational chemicals provide the final push.
Obesity, tobacco smoke exposure (especially in utero or during childhood), and respiratory viral infections in infancy, particularly with respiratory syncytial virus, also increase risk independently. Each of these factors nudges the immune system further toward the pattern of chronic airway inflammation that defines asthma. The more risk factors that overlap in one person, the more likely they are to cross the threshold from healthy airways to disease.