Indoor & Outdoor Air Pollution and Asthma

A record number of people are asthma patients, and experts blame the growing air pollution and a lack of basic care.

There is increasing evidence of the association between air pollution and an increase in the severity of asthma in children, while some studies also suggesting a contribution to new onset of asthma.

Between 120 and 150 million people around the world suffer from asthma and this number is rising. Worldwide, deaths from this condition have reached over 180,000 annually.[1]

India has an estimated 20 million asthma patients of which 10% of these are children between the ages of 5 to 11.[2]

What is Air Pollution?

The United States Environmental Protection Agency (EPA) defines air pollution as “any visible or invisible particle or gas found in the air that is not part of the natural composition of air.”

Air pollution comes from many different sources – some are man-made and some are naturally occurring. Air pollution includes gases, smoke from fires, volcanic ash, and dust particles.

Research shows that environmental factors including n air pollution can worsen various respiratory problems including asthma.

Read our detailed article on how air pollution causes respiratory issues here>

For example, an incident in London, UK, in June 1994 recorded 640 people rushed to emergency departments due to the severe asthma attacks.[3]

What is Asthma?

Asthma is a chronic disease of the airways that makes breathing difficult.

People who suffer from asthma have inflammation of the air passages that results in a temporary narrowing of the airways that carry oxygen to the lungs.[4]

This inflammation is the cause of the symptoms like including coughing, wheezing, shortness of breath, and chest tightness. Severe asthma can result in decreased activity and inability to talk.[5]

Unfortunately, there is no cure for asthma but with proper precautions and some treatments, its symptoms can be controlled.

An asthma episode, or an asthma attack, is when symptoms are worse than usual. They can come on suddenly and can be mild, moderate or severe.

An asthma attack does allow enough air to get into the lungs, but it does not let the carbon dioxide to leave the lungs as fast as it should.

Carbon dioxide is poisonous if not expelled and can build up in the lungs during a prolonged attack. Thus, lowering the amount of oxygen getting into your bloodstream.

Air Pollution and Asthma

air pollution & asthma

A European study suggests that an exposure to air pollution early in life may contribute to the development of asthma in childhood and teenage years.

Long-term air pollution exposure in infants and developing foetuses can develop childhood asthma.[6]

Asthma is directly linked to the lung function and its health. Lower the lung function higher the chances of developing asthma.[7]

It is well established that air pollution has a negative impact on the lung function and is responsible for many deformities in the lung tissue.

In this article, we’ll relate air pollution to the development of new asthma as well as worsening of the existing disease.

1. Air pollution causes oxidative stress in airways and lungs that plays a critical role in asthma

Many air pollutants like ozone and traffic-related air pollution especially nitrogen oxides and sulphur oxides stimulate both oxidative and inflammatory response.

Oxidative stress is both the causative factor and a side-effect of chronic inflammation in lungs and airways.

A large amount of population and clinical studies support the relationship between increased Reactive Oxygen Species (ROS) and the development of bronchial asthma.

Many research suggested air pollution as a causative factor for more severe asthma and more frequent asthma attacks.[8]

Increased oxidative stress in asthma patients is also related to suppressed lung function.

In bronchial asthma, oxidative stress worsens airway inflammation by inducing diverse pro-inflammatory proteins and cells, enhancing bronchial spasms (contraction of the bronchioles or small airways), and increasing mucus secretion.[9]

In a study using a mouse model suffering from asthma showed that increased oxidative stress plays a critical role in escalating an immune response.

Therefore, it is strongly suggested that an increased level of ROS acts as a critical contributor to the induction of allergic airway inflammation.[10] [11]


Oxidative stress caused due to the inhalation of various air pollution leads to the airway inflammation and stimulation of mucus secretion, all of which are believed to play a key role in the development and progression of asthma.  

2. Chronic inflammation due to air pollution can lead to an onset of asthma

Asthma represents a chronic inflammatory process of the airways followed by healing, the end-result of which is an altered structure and function referred to as a “remodeling of the airways”.

Repair usually involves 2 distinct processes: regeneration that is the replacement of injured tissue by the same type of tissue and replacement by connective tissue that eventually forms into a scar tissue.

Air pollution especially Particulate matters (PM) has been very well linked to chronic inflammation and airways remodeling.

PM penetrates into and is stored in the walls of small airways, and that, even in non-smokers, long-term exposure to high levels of PMs is associated with small airway remodeling.[12] [13]

In asthma, the processes of regeneration and formation of connective tissue can sometimes lead to a complete or altered renewal of airway structure and functions.

This alteration is usually associated with increased in mucus secretion and scarring of the lung tissue termed as “fibrosis”.[14]

The presence of increased mucous secretion and inflammatory cells, not only blocks the airway passages but also causes an increased surface tension that favors airway closure.

Constriction or blockage of airways is an important hallmark in asthma development and progression.


Long-term exposure to particulate matters results in a chronic inflammation in the airway passage that can cause restricted airflow into the lungs thus resulting in at the beginning of asthma.

3. Air Pollutants can increase and worsen the asthma attacks

When various air pollutants are inhaled, they irritate the linings of the airway from the nose and throat all the way down to the air sacs (alveoli) inside the lungs.

The exact mechanisms of irritation and the body’s reactions depend on the type of pollutant. In general, they cause oxidative stress, airway inflammation and hyper-responsiveness in the airway.

There are several ways in which an air pollutant may affect the airways of asthma patients;

  1. The pollutants may act as triggers also known as ‘inciters’ when the airways are hyper-responsive, resulting in temporary airway narrowing.
  2. The pollutant may act as an inducer thus increasing the airway inflammation and other symptoms like airway hyper-responsiveness.
  3. The pollutant may have a direct toxic effect on the airways, leading to asthma-like symptoms in normal healthy individuals.

There are three documented episodes of major air pollution associated with cold weather and little wind, where the polluted air became trapped as a result of temperature inversion, resulting in high concentrations of SO2 (over 1 p.p.m.), sulphuric acid and particulate matter.

These occurred in the Meuse Valley, Belgium in 1930, Dondora, USA in 1948, and in London, UK in 1952. In each episode, there was evidence that asthmatic patients showed severe symptoms with increased hospital admissions.[15]


Air pollutants may, therefore, cause exacerbation of pre-existing asthma or may increase the risk of developing asthma.   

4. Indoor air pollution is a major contributor to asthma

There are many sources of indoor air pollution inside of a home environment.

Air pollution inside homes consists of a complex mixture of agents either penetrating from outdoor air and/or substances generated by indoor sources.

Indoor pollutants can differ in their potential health effects, as well as in their distribution across geographic areas, cultural backgrounds, and socio-economic status.

For example, indoor pollutants include products of combustion, including particulate matter (PM) and oxides of nitrogen, second-hand smoke as well as airborne allergens from pests, pets, and molds as well as toxins from various cleaning products.[16]

In a study conducted in Seattle, USA found that PM2.5 originating from indoor sources was more effective in decreasing lung function than the outdoor-derived PM among school going children.[17]

In another study conducted in 150 preschool children already suffering from asthma, conducted as a part of the Johns Hopkins Center for Childhood Asthma.

The study demonstrates that both indoor coarse PM10 and fine PM2.5 markedly affect respiratory health in children with asthma.


Indoor air pollution is considered more dangerous due to it’s restricted space and time of exposure as compared to outdoor air pollution. Children and women are mostly affected by household air pollutants and men are more susceptible to occupational exposure.


The increase in cases of asthma has become an important health issue worldwide, affecting a large part of the population in many regions.

Exposure to environmental pollutants both outdoor and indoor may partially account for the prevalence of asthma.

Management of asthma requires attention to environmental exposures both indoors and outdoors. Indoor particulate matter and nitrogen dioxide are linked to the majority of asthma cases.

The National Asthma Education and Prevention Program guidelines recommend reducing indoor smoking and improving the air ventilation for reducing indoor air pollution.

Whereas, for outdoor air pollutants that impact asthma, the guidelines recommend that individuals with asthma avoid any physical activity outdoors when these pollutants are elevated.

More research detailing the effectiveness of various methods and precautions to reduce these air pollution exposures and improve asthma outcomes is the need of the hour.

Also, more research is needed to better understand how different pollutant affects differently that can either aggravate the disease or cause it, in order to find an effective and conclusive treatment.

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1 thought on “Indoor & Outdoor Air Pollution and Asthma”

  1. I live in Delhi and we are facing severe pollution now as winter season starts. Nothing has been done to take care of it. I am using mask to avoid when I go out. One of my kids have developed allergy and we got a Honeywell air p last year. I never thought that I will have to pay to get fresh air .. seems it’s just the starting and more is to come inless we do something…. Abt it…


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