The bronchitis infection can be caused by either a virus or bacteria, although viral bronchitis is much more common.
In most cases, bronchitis is caused by the same viruses that cause the common cold or flu. The virus is contained in the millions of tiny droplets that come out of the nose and mouth when someone coughs or sneezes.
These droplets typically spread about 1m (3ft). They hang suspended in the air for a while, then land on surfaces where the virus can survive for up to 24 hours. Anyone who touches these surfaces can spread the virus further by touching something else. Everyday items at home and in public places, such as door handles and keyboards, may have traces of the virus on them. People usually become infected by picking up the virus on their hands from unclean objects, and then placing their hands near their nose or mouth. It is also possible to breathe in the virus if it is up in the air in aerial droplets.

Breathing in nuisance substances

Bronchitis can also be caused by breathing in irritant substances, such as smog, chemicals in household products or tobacco smoke.

Smoking is the main cause of long-term (chronic) bronchitis, and it can affect people who inhale second-hand smoke, as well as smokers themselves.

Occupational exposure

You may also be at risk of bronchitis and other forms of chronic obstructive pulmonary disease (COPD) if you are often exposed to materials that can damage your lungs, such as:

  • grain dust
  • textiles (fabric fibers)
  • ammonia
  • strong acids
  • chlorine

This is sometimes mentioned to as “occupational bronchitis”, and usually comforts once you are no longer exposed to the irritant substance.

Causes of Acute and Chronic Bronchitis

Acute Bronchitis

Infections or lung irritations cause acute bronchitis. Acute bronchitis is normally caused by viruses, and usually those that cause colds and flu. The same viruses that cause colds and the flu are the most common cause of acute bronchitis. Sometimes bacteria can also cause the condition. Bacteria and exposure to substances that irritate the lungs – tobacco smoke, dust, fumes, vapors and air pollution can also cause it.

Certain substances can irritate your lungs and airways and raise your risk for acute bronchitis. For example, inhaling or being exposed to tobacco smoke, dust, fumes, vapors, or air pollution raises your risk for the condition. These lung irritants also can make symptoms worse. Being exposed to a high level of dust or fumes, such as from an explosion or a big fire, also may lead to acute bronchitis.

Chronic Bronchitis

Chronic bronchitis is caused by the repeated irritation and damage of the lung and airway tissue. Repeatedly breathing in fumes that irritate and damage lung and airway tissues causes chronic bronchitis. Smoking is the major cause of the condition. Smoking most commonly causes it, but can also be caused by long-term exposure to air pollution, dust and fumes from the environment, as well as repeated episodes of acute bronchitis.

Breathing in air pollution and dust or fumes from the environment or workplace also can lead to chronic bronchitis. People who have chronic bronchitis go through periods when symptoms become much worse than usual. During these times, they also may have acute viral or bacterial bronchitis.