Introduction

Poor air quality is known to be detrimental to our health, especially to our respiratory system, as they are the most prone to external stimuli from the air surrounding us. The culprit for poor air quality is often pollutants from various sources, ranging from factory output to household contaminants. Air pollution might be a lot more common than you expect. According to the World Health Organisation, 9 in 10 people reside in areas with abundant pollutants in the air.¹ With prolonged exposure or sometimes acute responses, these pollutants can result in wheezing and other bodily reactions. In this article, we will delve into how exposure to pollutants and poor air quality can contribute to the worsening of our respiratory health.

Air pollution and its common source

Types of air pollutants

• Particulate matter (PM10, PM2.5)

Particulate matter is particles that are small enough to be inhaled, consequently triggering health issues. The number followed by PM stands for the size of the particulate matter. For instance, PM10 stands for particulate matter with a diameter of 10 micrometres or less, including PM2.5. PM2.5 is the most abundant pollutant in particulate matter. The main source of PM2.5 comes from the combustion of fossil fuels, gasoline, and oil.²

• Ground-level ozone (O₃)

O₃ is a colourless gas that is produced from a chemical reaction between nitrogen oxides and volatile organic compounds with sunlight exposure. Human activities such as burning gasoline, fossil fuels, and power plant operations speed up the release of O₃ formation by releasing nitrogen oxides and volatile organic compounds.³

• Nitrogen dioxide (NO₂)

NO pollution is present both indoors and outdoors. Indoor NO₂ mainly comes from tobacco smoke and poorly maintained burning appliances such as stoves, ovens, and radiators. For outdoor NO₂, it is primarily from road traffic.⁴

• Sulfur dioxide (SO₂)

Like most other pollutants, sulfur dioxide is produced by the process of burning fossil fuels and other industrial processes. Besides, sulfur dioxide is also released naturally from volcanic eruptions⁵.

• Volatile organic compounds (VOCs)

VOCs are produced in both human activities and in nature. They are organic chemical compounds that can be vaporised easily. Events like forest fires, fertiliser use, and the burning of fossil fuels can all give rise to VOCs. 

Sources of air pollutants

• Traffic emissions
• Industrial activities and power plants
• Household pollution (smoke, cleaning agents)
• Natural sources (wildfires, volcanic eruptions)

Mechanisms of air pollutants affecting the respiratory system

There are ample studies with evidence to prove the causal relationship between air pollutants and deteriorating respiratory health. The human body might react in different ways when exposed to air pollutants, as different reactions depend on the type and duration of exposure. In the following section, we will explore the key mechanisms by which air pollutants affect our respiratory system.

• Impact on Respiratory Pathway

The pollutants will enter the respiratory pathway through ventilation, some larger particles might deposit on the nasal passages and throat, while finer particles like PM2.5 can penetrate deeper into the respiratory tract such as the bronchi and air sacs of the lungs, while the deposition can cause localised irritation, in long-term, it might cause chronic inflammation of the airways, increasing susceptibility to respiratory infections and diseases such as bronchitis or asthma.

Air pollutants can compromise the natural defence mechanisms of the respiratory tract, namely the mucociliary clearance system, which is the first line of defence against inhaled agents. A well-functioning mucociliary clearance system is responsible for removing pathogens and debris from the respiratory tract; therefore, once the system is weakened, it will lead to the respiratory system being more susceptible to infections like pneumonia, bronchitis, and other viral or bacterial diseases. A study conducted in Germany shows a positive correlation between exposure to NO₂ and PMs and increased incidents of croup (acute laryngotracheobronchitis), mostly due to influenza viral infections.⁸

• Oxidative stress and inflammation

When pollutants, especially ozone and particulate matter (PM2.5) enter the respiratory tract epithelial cells, they can trigger the generation of reactive oxygen species (ROS) and nitrogen species in our lungs, resulting in the initiation of the downstream redox reaction pathway that will eventually damage our body’s lipids, proteins, and DNA, which associate with the potential development of cancer.⁷

Sustained oxidative stress can promote lung tissue damage, causing problems like reduced lung capacity and tissue elasticity, leading to the development of chronic respiratory diseases like emphysema or chronic obstructive pulmonary disease (COPD), which significantly impacts a person’s lung function and ability to breathe effectively.⁷

• Immune system response

Some chronic respiratory diseases, like asthma, are caused by an abnormally upregulated immune system response due to stimuli from air pollutants. Some pollutants, like diesel exhaust particles, have been shown to act as allergens, triggering hypersensitivity reactions in the airways. A hyperactive immune system might trigger airway constriction, promoting symptoms like wheezing, shortness of breath, and increased frequency and severity of asthma attacks.⁷

Wheezing and air quality

Wheezing is a clinical symptom of a specific sound that is produced while having difficulty breathing, most commonly seen in respiratory conditions like asthma, bronchitis, and COPD. The wheezing sound is described as high-pitched, adventitious, and more noticeable when exhaling, and is produced anywhere in the respiratory pathway from the larynx to the bronchioles.⁶ In light of this, wheezing is a major sign of airway obstruction, and air pollution has been demonstrated to worsen the symptoms of wheezing. 

Relationship between poor air quality and wheezing

There is substantial evidence to show that higher pollutant levels can exacerbate wheezing symptoms, mainly by increasing oxidative stress and allergic inflammation.⁹ Air contaminants from outdoors and indoors can all increase the frequency of wheezing, particularly in children, the elderly, and people who have pre-existing breathing issues such as asthma and COPD.

Besides airway inflammation and allergies, some pollutants like O₃ and NO₂ can trigger bronchoconstriction, a condition of narrowing of the airways due to the contraction of the smooth muscles surrounding the bronchi. The narrowing of the airway can trigger the production of a wheezing sound while breathing. Our lungs might also secrete excess mucus in response to external pollutants. In a healthy respiratory system, the mucus is meant to trap and obstruct contaminants from passing through, but if the mucus production is hyperactive, the thickened mucus might trigger breathing difficulties by narrowing the airway, resulting in wheezing.

Besides adults, perinatal exposure to air pollution can also cause the early onset of respiratory diseases and contribute to infants demonstrating symptoms like wheezing. Maternal exposure to second-hand smoke and environmental tobacco smoke can cause the infant to have a 20% higher likelihood of developing asthma wheeze.⁸ Other pollutants, like traffic-related air pollutants, NO₂, and PMs, have demonstrated similar results. 

Measures to mitigate the effects of poor air quality on respiratory health

Even though air pollutants are ubiquitous and unavoidable in our daily lives, there are still some ways that we can minimise our exposure to take care of our respiratory health.

• Avoid outdoor activities during rush hour

As outdoor pollutants from traffic are a major issue, try to avoid activities outdoors when air pollution is the most serious during certain times of the day, usually during rush hours, when traffic emissions are the highest. Limit your activities near roads during rush hours, and experiment with going to parks and green spaces for physical activity to reduce exposure to traffic-related pollutants.

• Wear face masks in high-pollution areas

It is understandable if your work requires you to be constantly exposed to air pollutants; consider wearing protective face masks if this is the case. Opt for face masks that are designed to block fine particulate matter, such as N95 and N99, which can filter at least 95% and 99% of harmful airborne particles, respectively.¹¹ They are also effective in filtering aerosols and other pollutants, limiting the inhalation of these harmful substances into our respiratory system.

• Implement an indoor air purifier

Other than outdoor air pollution, indoor pollution is also a concern. This is especially significant in residential areas that are located near industrial areas, busy roads, or in cities with poor overall air quality. An air purifier with a High-Efficiency Particulate Air (HEPA) filter can effectively trap and remove PMS, allergens, and other pollutants from the air¹⁰, minimising the health issues caused by indoor pollutants. If possible, try to look for some advanced air purifiers that come with additional features like activated carbon filters, which can absorb a greater variety of pollutants, such as volatile organic compounds (VOCs) from cleaning agents or building materials.

Summary

Air pollution poses a significant risk to respiratory health, contributing to symptoms like wheezing, bronchitis, asthma, and deteriorating respiratory functions. Pollutants such as particulate matter (PM2.5, PM10), nitrogen dioxide (NO₂), and ozone (O₃) irritate the respiratory tract, causing inflammation, oxidative stress, and bronchoconstriction. Individuals with asthma or COPD are particularly vulnerable, as exposure to poor air quality worsens their symptoms and can trigger more frequent and severe exacerbations. Therefore, it is of utmost importance that we try our best to mitigate the harmful effects of various air pollutants. While personal measures are essential, the air pollution problem should also be addressed on a larger scale through government and global regulatory efforts, eventually improving public health and reducing the prevalence of respiratory issues.