Asthma is caused by a combination of inherited genes and environmental exposures that together trigger chronic inflammation in the airways. No single factor explains it. Around 262 million people worldwide have asthma, and the condition develops when genetically susceptible airways encounter specific triggers that push the immune system toward an overreactive pattern.
Genetics Set the Foundation
Asthma runs in families, and the strength of that connection depends on how closely you’re related to someone who has it. If neither of your parents has asthma, your baseline risk sits around 5%. With one parent affected, that jumps to about 25%. If both parents have asthma, the risk reaches roughly 50%. Identical twins share a 75% concordance rate, meaning if one twin has asthma, three out of four times the other will too.
Several specific genes contribute. One of the first identified was a gene on chromosome 20 that’s active in the smooth muscle cells lining the airways and in the connective tissue of the lungs. This gene influences how the airways remodel themselves over time and how sensitive they become to irritants. But genetics alone don’t determine whether you develop asthma. They create a predisposition that environmental factors then activate.
What Happens Inside the Airways
In asthma, the immune system tips toward a specific type of inflammatory response. A subset of immune cells produces signaling molecules that recruit eosinophils (a type of white blood cell) into the airway walls and ramp up production of an antibody called IgE. IgE is the same antibody responsible for allergic reactions, which is why asthma and allergies so often overlap.
This inflammatory process does three things. It swells the airway lining, tightening the space air can move through. It triggers the smooth muscles wrapping around the airways to contract, narrowing them further. And over time, it remodels the airway walls, making them thicker and permanently more reactive. That heightened reactivity, called bronchial hyperresponsiveness, is why people with asthma can have symptoms triggered by stimuli that wouldn’t bother someone without the condition, like cold air or exercise.
Allergens That Trigger and Sustain It
Indoor allergens are among the most persistent asthma triggers because exposure happens for hours every day. Dust mites top the list. Proteins from dust mite waste are unusually effective at provoking immune reactions because they can physically break through the protective barrier of the airway lining, giving the immune system direct access to the irritant. One specific mite protein even mimics a natural human immune molecule, essentially tricking the body’s defenses into activating when they shouldn’t.
Mold is another major indoor trigger. Outdoor species like Alternaria and Cladosporium are linked not just to everyday symptoms but to severe, life-threatening asthma flare-ups. Indoor mold species, particularly Aspergillus, can cause a specific form of allergic lung disease in sensitized people. Pollen from grasses, birch trees, and olive trees are common outdoor triggers, with exposure patterns shifting by season and geography.
Air Pollution and Traffic Exhaust
Living near heavy traffic raises the risk of developing asthma, particularly in children. A prospective study tracking children across 11 Southern California communities found that higher levels of nitrogen dioxide (a gas produced by car and truck engines) near a child’s home significantly increased their chance of new asthma onset. Children exposed to moderately higher levels of this pollutant had about a 29% greater risk of developing asthma. At the highest exposure differences measured, the risk more than tripled.
These effects held across seasons, though fall and winter exposures showed slightly stronger associations, likely because temperature inversions trap pollutants closer to the ground during colder months.
Early Childhood Exposures Matter Most
The first years of life represent a critical window when the immune system is learning what to react to and what to ignore. Two competing forces shape asthma risk during this period: infections that damage the airways and microbial exposures that train the immune system properly.
Respiratory syncytial virus, or RSV, is one of the clearest examples. Infants who avoided RSV infection during their first year had a 26% lower risk of developing asthma by age 5. About 21% of children infected with RSV in infancy had asthma by age 5, compared with 16% of those who weren’t infected. Researchers estimate that roughly 15% of early childhood asthma cases could be prevented by avoiding RSV infection in the first year of life.
On the protective side, the “hygiene hypothesis” explains why children raised in overly clean environments develop asthma at higher rates. Babies need exposure to certain bacteria to train their immune systems. Bacterial molecules flip a molecular switch on immune cells into the “on” position, teaching the system to respond proportionally to threats rather than overreacting. Without that early education, the immune system is more likely to develop the kind of allergic, overreactive pattern that drives asthma. Studies consistently show that asthma and allergies are more common in homes with lower levels of bacterial exposure.
The Gut Connection in Infancy
The bacteria living in an infant’s gut play a surprisingly important role in lung health. Specific beneficial bacteria, including Bifidobacterium, Faecalibacterium, and Lactobacillus species, help expand a population of regulatory immune cells that dial down excessive inflammation throughout the body, including the lungs. When these bacteria are absent or reduced in infancy, the risk of developing asthma goes up.
Breastfeeding appears protective, likely because it promotes a healthier balance of gut bacteria compared to formula feeding. Conversely, early antibiotic use disrupts gut bacteria at a critical developmental stage, and multiple studies link infant antibiotic exposure to increased asthma rates in early childhood. Short-chain fatty acids produced by gut bacteria from dietary fiber have been shown to suppress the exact type of immune overreaction (the allergic, eosinophil-driven response) that characterizes asthma.
Obesity Fuels Airway Inflammation
Obesity has a causal link to adult-onset asthma, not just an association. Excess fat tissue, particularly around the abdomen, acts as an active immune organ that pumps out inflammatory signaling molecules. Two of the most important are leptin and a pro-inflammatory cytokine called IL-6, both of which are elevated in people with asthma and obesity compared to people with either condition alone.
The connection goes beyond inflammation. Central obesity and insulin resistance directly impair lung function by reducing the space available for the lungs to expand and by altering the metabolic environment of the airways. The dietary patterns associated with obesity, specifically high sugar, high fat, and low fiber, independently increase airway inflammation. A European study of 202 participants found that obesity and asthma together had an additive effect, worsening inflammation more than either condition would on its own and altering the bacterial communities in the gut, lungs, nose, and mouth.
Workplace Chemicals and Dusts
Occupational asthma accounts for a meaningful share of adult-onset cases and develops through two distinct pathways. The first involves immune sensitization, where repeated exposure to a substance gradually trains the immune system to react to it. High-molecular-weight sensitizers include animal proteins, plant materials, bacteria, and fungi encountered in farming, laboratories, and food processing. Low-molecular-weight sensitizers include diisocyanates (common in painting and industrial coatings), wood dusts, cleaning chemicals, and certain compounds used in hairdressing.
The second pathway is irritant-induced asthma, which can develop after a single high-intensity exposure. Emergency responders at the World Trade Center collapse are a well-documented example, where massive exposure to irritating dust, gases, and fumes triggered asthma without the gradual sensitization process. Agricultural workers face a uniquely complex exposure profile that includes bacteria, fungi, particulates, gases, pesticides, and allergens all at once. Organophosphorus pesticides have been identified as independent risk factors for workplace-related asthma among exposed adults.