Bronchitis is caused by inflammation of the airways leading to your lungs, and the trigger depends on whether it’s acute or chronic. Acute bronchitis, the kind that comes on suddenly and clears up within a few weeks, is caused by a viral infection in 85% to 95% of cases in healthy adults. Chronic bronchitis, defined as a productive cough lasting at least three months in two consecutive years, is most often caused by cigarette smoking or long-term exposure to airborne irritants.
Viruses Are the Primary Cause of Acute Bronchitis
The same viruses that give you a cold or the flu are responsible for the vast majority of acute bronchitis cases. Rhinovirus, the most commonly identified culprit, accounts for roughly 21% of cases where a specific virus is found. Influenza A and B, respiratory syncytial virus (RSV), adenovirus, coronaviruses, and enteroviruses round out the list. These viruses infect the lining of the bronchial tubes, triggering inflammation that produces swelling, excess mucus, and the persistent cough that defines the illness.
Seasonal patterns matter. Influenza, RSV, and coronaviruses peak in winter. Rhinovirus hits hardest in spring and fall. Enteroviruses are more of a summer problem. This is why bronchitis tends to follow the same seasonal waves as cold and flu season, and why a bout of bronchitis often starts as what feels like an ordinary upper respiratory infection before the cough deepens and lingers.
Bacteria Play a Smaller Role
Bacterial infections cause roughly 10% or fewer of acute bronchitis cases. The bacteria most commonly involved include Mycoplasma pneumoniae, Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, and Bordetella pertussis (the bacterium behind whooping cough). Bacterial bronchitis is more likely in people with underlying health problems or weakened immune systems rather than in otherwise healthy adults. This distinction is important because it means antibiotics are rarely useful for a typical case of acute bronchitis, since antibiotics do nothing against viruses.
How Inflammation Builds Inside the Airways
Whether triggered by a virus, bacteria, or an irritant, the core process is the same. Your bronchial tubes become inflamed, and the cells lining them shift into overdrive. The mucus-producing glands in the airway walls can swell to several times their normal size. At the surface level, certain mucus proteins can spike to 40 to 200 times their normal concentrations during inflammation.
Mucus itself changes character. Healthy mucus is thin and easily cleared by the tiny hair-like structures (cilia) that sweep debris out of your lungs. Inflamed airways produce mucus that is thicker, more concentrated, and harder to move. This heavier mucus accumulates and can form plugs within the airway, which narrows the passages and makes breathing harder. The combination of swollen airway walls and clogged, sticky mucus is what produces the hallmark symptoms: a deep, productive cough, chest tightness, and sometimes wheezing.
Smoking and Chronic Bronchitis
Cigarette smoking is the single most important risk factor for chronic bronchitis. The repeated exposure to smoke keeps the airways in a constant state of irritation, leading to persistent overproduction of mucus throughout the bronchial tree. Over time, the mucus becomes increasingly concentrated and difficult to cough up, forming plugs that block smaller airways. These plugs create a cycle: blocked airways trap bacteria, leading to repeated infections, which cause more inflammation, which produces more mucus.
Chronic bronchitis is not exclusively a smoker’s disease, though. Prevalence estimates among nonsmoking adults range from 3% to 22% depending on geographic location, suggesting that other environmental factors contribute significantly. Secondhand smoke exposure carries its own risk, and people with a family history of chronic obstructive pulmonary disease (COPD) are more vulnerable regardless of their smoking status.
Air Pollution and Occupational Exposures
Long-term exposure to polluted air is a well-documented cause of bronchitis, particularly in childhood. A study following over 1,300 people in southern California found that higher average exposure to particulate matter and nitrogen dioxide between birth and age 17 was associated with a 69% and 51% increased risk of bronchitic symptoms, respectively, per standard deviation increase in pollution levels. The pollution concentrations in the study communities exceeded World Health Organization annual guidelines, but they were within the range found in many urban areas worldwide.
Workplace exposures carry similar risks. People who regularly breathe in dust, vapors, gases, or chemical fumes face elevated rates of chronic bronchitis. Mineral dust and certain metals like vanadium have been specifically linked to bronchial inflammation. Jobs in mining, construction, manufacturing, and agriculture are common settings for this kind of exposure. The combination of occupational dust and chemical fumes together appears to carry a higher risk than either one alone.
Acid Reflux as a Hidden Trigger
Gastroesophageal reflux disease (GERD) can cause or worsen bronchitis through two distinct pathways. First, tiny amounts of stomach acid can travel up the esophagus and get aspirated into the airways, directly irritating the bronchial lining and triggering inflammation. This micro-aspiration accounts for 10% to 15% of cases of unexplained chronic cough and can produce bronchial inflammation without the classic heartburn symptoms that most people associate with reflux.
Second, acid in the lower esophagus can stimulate a nerve reflex (through the vagus nerve) that increases airway sensitivity. This doesn’t require any acid to actually reach the lungs. The nerve signal alone can make the bronchial tubes more reactive to other irritants, lowering the threshold for coughing and inflammation. For people with both GERD and bronchitis, treating the reflux often improves respiratory symptoms.
Who Is Most Susceptible
Several factors increase your likelihood of developing bronchitis beyond the direct causes listed above. Older adults are more vulnerable because the immune system’s ability to fight off respiratory viruses declines with age, and years of cumulative exposure to irritants take a toll on airway defenses. People with existing respiratory conditions like asthma, cystic fibrosis, or bronchiectasis have airways that are already inflamed or structurally compromised, making them more susceptible to both acute and chronic bronchitis.
Genetics play a role as well. Alpha-1 antitrypsin deficiency is an inherited condition in which the body produces insufficient amounts of a protein that protects the lungs from damage caused by immune cells. Without enough of this protein, the normal activity of white blood cells can erode lung tissue over time. The most common disease-causing variant is the Z allele, and people who carry two copies (ZZ) have a significantly disrupted balance between protective and destructive proteins in their lungs. Smoking dramatically worsens outcomes for carriers, because cigarette smoke inactivates whatever protective protein remains. Never-smokers who carry just one copy of the Z allele (MZ) do not appear to have an increased risk of COPD, suggesting that the genetic vulnerability primarily matters when combined with environmental damage.