Bronchitis happens when the airways leading to your lungs (the bronchial tubes) become inflamed and swollen, producing excess mucus that triggers a persistent cough. The cause depends on whether the bronchitis is acute or chronic. Acute bronchitis is almost always caused by a viral infection, while chronic bronchitis develops from long-term exposure to irritants like cigarette smoke or polluted air.
Acute Bronchitis: A Viral Infection in Most Cases
At least 90% of acute bronchitis cases are caused by viruses, the same kinds responsible for colds and the flu. The most common culprits include rhinovirus (the typical cold virus), influenza A and B, parainfluenza, respiratory syncytial virus (RSV), coronavirus, and human metapneumovirus. In practice, doctors can only pin down the specific virus in a minority of cases because the symptoms overlap so much and testing rarely changes the treatment.
The infection starts in the upper respiratory tract, your nose and throat, then spreads downward into the bronchial tubes. Once the virus reaches the airway lining, it triggers inflammation that causes the walls of the tubes to swell. The inflamed lining produces extra mucus, which narrows the airways and sets off the cough your body uses to clear them. This whole process is self-limiting, meaning your immune system handles it without antibiotics. The cough can linger for two to three weeks even after the infection clears, because the airway lining takes time to heal.
Roughly 5% of the general population develops acute bronchitis in any given year, generating about 10 doctor visits per 1,000 people annually. Despite being overwhelmingly viral, bronchitis remains one of the most common reasons antibiotics are prescribed unnecessarily.
Can Bacteria Cause It?
Bacteria account for a small fraction of acute bronchitis cases. When they are involved, the infection is typically caused by organisms like Mycoplasma pneumoniae, Chlamydophila pneumoniae, or Bordetella pertussis (the whooping cough bacterium). These are more likely in specific situations: outbreaks in close-contact settings, people with weakened immune systems, or cases where a viral infection damages the airway lining enough that bacteria take hold as a secondary infection.
Bacterial bronchitis doesn’t look dramatically different from viral bronchitis at first. Both cause cough, mucus production, and chest discomfort. Green or yellow mucus alone doesn’t confirm a bacterial cause, which is a common misconception. Mucus color changes simply because of immune cells responding to inflammation, regardless of what triggered it.
How Chronic Bronchitis Develops
Chronic bronchitis is a different condition with a different cause. It’s defined as a cough that produces mucus for at least three months in two consecutive years. Rather than a single infection, it results from repeated, long-term irritation of the bronchial lining.
Cigarette smoke is the dominant cause. When smoke is inhaled repeatedly over months and years, it triggers a cascade of damage: persistent inflammation, oxidative stress (where harmful molecules overwhelm the body’s ability to repair cells), and destruction of the structural tissue supporting the airways. Over time, the airway walls thicken because the normal balance between cell death and cell replacement breaks down. Cells lining the bronchial tubes that normally sweep mucus upward are destroyed and replaced by mucus-producing cells, which leads to chronic overproduction of mucus and a narrowed airway that’s harder to breathe through. These changes are largely permanent. Chronic bronchitis is considered a form of chronic obstructive pulmonary disease (COPD).
Environmental and Occupational Irritants
Smoking isn’t the only long-term irritant that causes chronic bronchitis. Occupational exposure to vapors, gases, dust, and fumes poses a real risk, particularly in industries like mining, manufacturing, agriculture, and construction. Workers exposed to substances like metal dust, grain dust, and chemical fumes such as vanadium compounds show higher rates of chronic bronchitis. These irritants damage the airway lining through the same inflammatory pathways that cigarette smoke does, just from a different source.
Outdoor air pollution also plays a measurable role. A large study published in Thorax found that exposure to nitrogen dioxide and black carbon (a component of soot from diesel engines and burning fossil fuels) was associated with a 5 to 7% increase in the odds of developing chronic bronchitis per unit increase in pollution concentration. The association was slightly stronger in women, younger adults, and people who had never smoked, suggesting that air pollution acts as an independent cause rather than simply adding to smoking-related damage.
Vaping and Bronchial Irritation
E-cigarettes introduce a different set of chemicals into the airways. Several compounds in e-liquids are known to damage bronchial tissue. Diacetyl, a flavoring additive, causes inflammation and can lead to permanent scarring in the smallest airways. Formaldehyde, produced when e-liquid is heated, is directly toxic to lung tissue. Acrolein, a chemical also used as a herbicide, damages the airway lining. Vitamin E acetate, used as a thickening agent in some products, acts as an irritant when inhaled and has been linked to severe lung injury.
These substances can trigger acute bronchial inflammation that mimics traditional bronchitis, and there is growing concern that long-term vaping may lead to chronic airway changes similar to those seen in smokers. Inhaling oily substances from e-liquids can also spark a condition called lipoid pneumonia, where fat droplets reach deep into the lungs and cause an inflammatory response.
Acid Reflux as a Contributing Factor
Gastroesophageal reflux disease (GERD) can irritate the airways and worsen bronchitis symptoms through two distinct mechanisms. The first is direct: stomach acid that refluxes high enough can be aspirated into the lower respiratory tract, where it irritates the bronchial lining and triggers coughing. The second is indirect. When acid contacts the lower esophagus, it stimulates a nerve reflex (the esophageal-bronchial reflex) that causes the airways to constrict and produce mucus, even though no acid actually reaches the lungs. Both pathways can sustain a chronic cough that overlaps with or aggravates existing bronchitis, particularly in people who already have COPD.
What Makes Some People More Vulnerable
Not everyone exposed to the same virus or irritant develops bronchitis. Several factors increase susceptibility:
- Weakened immune function: Older adults, young children, and people with chronic illnesses are more likely to develop acute bronchitis after a respiratory infection and less likely to clear it quickly.
- Asthma or allergies: Pre-existing airway inflammation makes the bronchial tubes more reactive to viral infections and environmental irritants.
- Secondhand smoke exposure: Living with a smoker increases your risk of both acute and chronic bronchitis, even if you’ve never smoked yourself.
- Genetic variation: Individual differences in how efficiently the body repairs damaged airway tissue and manages inflammation help explain why some long-term smokers develop chronic bronchitis while others don’t.
Repeated bouts of acute bronchitis can also prime the airways for chronic problems. Each episode of inflammation causes minor damage, and if the lining doesn’t fully recover before the next insult, the cumulative effect can push the airways toward the thickened, mucus-heavy state that defines chronic bronchitis.