Bronchitis is characterized by inflammation of the bronchial tubes i.e. the airways tubes that extend from the windpipe into the small airways and alveoli in the lungs.
More than 10 million people suffer from chronic bronchitis in India alone. The main symptom is a cough, which may bring up yellow-grey mucus (phlegm).
Air pollution is one the leading cause and risk factor contributing to this ailment.
What is Air pollution?
Air pollution in the broader sense means the presence of unwanted chemicals or compounds in the air that can lower/destroy the quality of the air or cause harmful changes to life.
Air pollution has been a serious environmental problem and a major concern for public health worldwide. It continues to be a major environmental health risk, particularly in developing countries where the motor vehicle traffic and industrialization increase quickly.
The respiratory system (from nose to lung) is at risk from air pollutants, including ozone (O3), carbon monoxide (CO), nitrogen dioxide (NO2), sulphur dioxide (SO2), and particulate matters (PM2.5, PM10).
Several studies have shown that air pollutants could induce oxidative stress and inflammation, resulting in airway injury and dysfunction.
What is Chronic Bronchitis?
Chronic bronchitis is a disease of the lung characterized by a persisting infection and inflammation of the larger airways of the lungs called the bronchi. There are two main bronchi in the lungs the right and left.
Chronic bronchitis occurs as part of a complex disease known as the chronic obstructive pulmonary disease (COPD) which also includes emphysema and small airways disease.
Chronic bronchitis specifically refers to a chronic cough and daily mucus production for at least three months of two or more consecutive years.
The Link between Air Pollution and Chronic Bronchitis
Polluted air contains tiny particles called “irritants” that damage the lungs and for people who already suffer from bronchitis, irritants can be very harmful.
It can make their symptoms worse and cause chronic bronchitis (also called “exacerbations”) to happen more often.
Recent studies suggest a possible relationship between classically defined chronic bronchitis and long-term exposure to the criteria pollutants PM2.5, PM10 (particulate matter < 2.5 μm and < 10 μm in diameter, respectively), and nitrogen dioxide (NO2).
Several processes can be initiated due to exposure to the various air pollutants, like the development of direct damage to the lung tissue due to toxic particles in the smoke and thermal injury.
Secondly, direct activation of macrophages, neutrophils and other immune cells by particulate matters (PM) and/or by the initiation of inflammation processes and oxidative stress.
1. Air pollution causes inflammation response in lung tissue leading to bronchitis
Ultra-fine particulate matters in the air pollution can travel down through the windpipe and finally into the air sacs in the lungs. It is air sacs that let the air in and out of the lungs while breathing.
Several studies have shown that chronic bronchitis patients are often associated with the continuous cycle of inflammation and oxidative stress identified by increased circulatory levels of inflammatory biomarkers.
Mucous metaplasia, a process in which mucus is overproduced in response to inflammatory signals, is the foundation for CB. The primary mechanisms responsible for excessive mucus in COPD are overproduction and hypersecretion by goblet cells and decreased the elimination of mucus. 
Two main theories have been suggested explaining this inflammation response: 1) an imbalance in the oxidant and antioxidant system, and 2) an imbalance between the action of proteases and anti-proteases.
Inflammatory cells contribute to tissue damage by release of active proteases (chemicals that can break down proteins) enzyme and reactive oxygen species (ROS). The accumulation of macrophages (a type of immune cells) in the alveoli, bronchioli and small airways is associated with the development of mucus.
Proteases also mutate the mucin gene that is responsible for the production of mucus in humans. It sends the mucin gene into overdrive thus leading to an overproduction of mucus that is more than the normal rate.
Summary: Studies shows an enhanced or abnormal inflammatory response to inhaled particulate matters or gases, has the potential to damage lungs tissue, a characteristic feature of emphysema.
2. Oxidative stress is another driving mechanism in chronic bronchitis
Oxidative stress plays a key role in the development of bronchitis. Excess of oxidants are generated either from cigarette smoke or air pollutants or from activated neutrophils and macrophages (inflammatory response) releasing ROSs.
Oxidative stress is both a by-product and causing factor of inflammation. ROS also induces direct damage to various macromolecules, such as proteins, lipids, and nucleic acids.
Several studies prove that oxidative stress gives rise to an inflammatory process that in turn aids in the above-mentioned mechanism involved in the development and progression of bronchitis.
The increased oxidation leads to the cell-death by the means of two processes 1) apoptosis, and 2) autophagy.
ROS are also associated with another process called cellular senescence (loss of a cell’s power of division and growth), that can stop tissue repair, increase chronic inflammation, and increased weakness to infection, which are the typical features of bronchitis and COPD.
This imbalance in increased cell-death versus decreased cell renewal or tissue repair leads to the destruction of air-ways or wind-pipe (trachea).
Summary: Emphysema is a dynamic phenomenon involving not only the gradual destruction of lung tissue by an excess of proteases, but also cell-death by various process namely; apoptosis, autophagy, and senescence.
Air pollution can increase the rate of mortality in Emphysema patients
Patients with chronic bronchitis symptoms had a threefold increased risk of developing new COPD compared to those who show no signs or symptoms. Chronic bronchitis increases mortality.
Long-term air pollution exposure was reported to increase mortality by any cause. Air pollution is the cause and aggravating factor of many respiratory diseases like emphysema, COPD, lung cancer etc. 
Globally, seven million deaths were caused due to the joint effects of household and outdoor air pollution. In 2016, the Global Burden of Disease Study reported a prevalence of 251 million cases of COPD globally.
Around 90% of COPD deaths that includes both emphysema and bronchitis occur in low and middle-income countries.
How Indoor Air Pollution can cause Chronic Bronchitis?
Indoor air pollution is one of the most dangerous but often disregarded dangers. The offices, schools, and homes are mostly centers of indoor air pollution.
Three billion people worldwide are exposed to toxic amounts of indoor air pollution (IAP) every day. IAP accounts for up to four million deaths annually.
Many cleaning compounds, certain construction materials, and even air fresheners emit hazardous gases. These remain highly concentrated in poorly ventilated or AC rooms.
Smoke from burning fuels, smoking tobacco, cigarette or cigars, even being exposed to second-hand smoke over time can cause someone who has never smoked to develop emphysema leading to COPD.
The most vulnerable sections of people exposed to these respiratory dangers are women, children, elderly people and those having a history of respiratory illnesses. These people generally spend most of their time within the four walls.
The indoor air pollutants not only weaken our lungs but also invite infections that can lead to many more respiratory diseases and can aggravate the symptoms of existing illness.
There is significant evidence linking air pollution and lung diseases including chronic bronchitis, COPD, emphysema, asthma, etc.
Population-based studies have demonstrated that both short- and long-term exposure to indoor- and outdoor air pollution increases the risk of stroke.
Chronic bronchitis has a significant impact on morbidity and quality of life. Smoking is a major contributor hence smoking cessation needs to form an important strategy in prevention.
Alteration in lifestyle and regular vaccination for influenza as well as pneumonia can reduce the morbidity in those who have chronic bronchitis.
Pulmonary rehabilitation is an important part of treatment for chronic bronchitis is pulmonary rehabilitation which consists of education, lifestyle modification, regular physical activity and avoidance of exposure to known pollutants either at work or living environment
Air pollution should be recognized more widely as one of the most important modifiable risk factors for the prevention and management of diseases it causes.
Healthcare professionals have an important role in promoting the awareness of this information, not just to improve the health of individual patients, but also to place pressure on policymakers for air pollution to be a public health priority.