Types of Air Pollution Explained

Types of Air Pollution Explained

There are various causes of air pollution, and the composition of harmful substances in the air can vary depending on your surroundings. If you want to know what exactly you are exposing yourself to, you need to delve deeper into the types of air pollution, their sources, and their effects on the human body. 

Based on Air Quality Index reports, EPA determined the six most common types of air pollutants as follows:

  • Nitrogen Oxides (NOx
  • Particulate Matter (PM) 
  • Sulfur Dioxide (SO2)
  • Ozone (O3
  • Carbon Monoxide (CO)
  • Lead (Pb)

Nitrogen Oxides

If you ever got caught in the middle of stop-and-go traffic in a busy city, like London, your body is probably familiar with nitrogen oxides and their products. Today, NOx is one of the main reasons for respiratory diseases in people near busy transportation networks and industrial areas. According to Ian Colbeck, a professor at the University of Sussex, NOx pollution caused the death of 23,500 people in the UK every year.

So, where does this deadly gas come from, and why is it dangerous for our health?


Nitric oxide (NO) and nitrogen dioxide (NO2)  form through chemical reactions inside your car engine. Especially in metropoles, vehicle exhausts constitute a significant portion of the emission. Recent data revealed that 50% of the total emission in London was caused by road transport, especially around busy roads and main highways.

NOx As a Poison

Chronic NOx exposure ruptures and inflames the lining of the lung, which weakens your respiratory immune system.  That’s why NOx is strongly associated with asthma and bronchitis.  In addition, reactions of NOx with Ozone, VOC, and ammonia can cause DNA mutations and cancer. The number of cases lays out the scope of the threat much more clearly.

A study conducted in Europe with 4000 children of ages 0-12 found that the children living in areas with above-average NOx levels were two times more likely to develop non-allergic asthma [1]. Another study in Brazil between 2011-2012 revealed that exposure to NOx for 30 days increased respiratory-related risk of mortality by 5% [2].

Particulate Matter (PM)

Health organizations declared PMs as a primary health threat for their ability to cross our defense mechanisms and accumulate in our lungs and blood. This makes them one of the top reasons for heart and lung disease, resulting in increased hospital admissions and premature deaths. But that is only the tip of the iceberg; with current findings, we are discovering other adverse effects of PM, especially related to cancer, diabetes, and many other diseases.

Today, PM is perhaps the most prevalent air pollutant, especially in metropoles. For instance, a 2019 report revealed that there was no area in London, where the PM2.5 levels were within the WHO tolerable range!

Some of them are so small that you can only see them via an electron microscope. Perhaps it is because of their invisibility that we are constantly inhaling PMs without even noticing and without taking precautions. The Daily Air Quality Index clearly proves this statement, as people in the UK were found to spend 200-300 hours per year in moderate-and-higher PM2.5 pollution levels.

Types and Sources

PM is the common name for small solid and liquid particles suspended in the air. Particulates are found in three groups according to their size. (The subscript refers to their size in microns.) 

Coarse particles (PM10): Bacteria, smoke, dust, spores, pollen grains, and certain viruses

Fine particles (PM2.5):  These particles result from the fuel-burning in vehicle engines, indoor appliances, and power plants.

Ultrafine particles (PM0.1): The main resources include automobile exhaust, indoor charcoal grill, tobacco, vacuum cleaners, and printer toner.

PM as a Poison

While all PMs can do us harm, the smaller they are, the more hazardous they become. Smaller particles can penetrate deeper into our defense mechanism and mix into our bloodstream. What’s worse is, smaller PMs can stay afloat for much longer and travel far away from their original site, affecting people in a larger area, compared to large PMs. For example, meteorological data from China showed that PM2.5 could travel up to 2000km in two days [3].

PM2.5 and PM0.1 can infiltrate far deeper into our lungs and mix into our bloodstream. Once in our blood, they can cause inflammation, cardiovascular diseases, and cancer [4]. 

Studies linked chronic exposure to PM2.5 to hospital admissions and age-specific mortality due to cardiovascular diseases. In the UK, approximately 29,000 people per year prematurely died of cardiac arrest because of PM2.5 exposure [5]. The data was backed by WHO, stating that long term exposure could increase the risk of cardiopulmonary death by up to 13%. 

PM2.5 also affects lung function by causing inflammation, exacerbating asthma, chronic obstructive pulmonary disease (COPD), and bronchitis. In children, it was even found to halt lung growth. Finally, a strong correlation was discovered between PM2.5 and lung cancer, due to mutations in the DNA facilitated by PMs [6]. In a recent WHO report, PM accounted for 5% of lung cancer deaths globally. 

Sulfur Dioxide

It’s not only a traffic jam but also trains, ships, and petroleum refineries that contribute to the spread of this deadly gas. According to Greenpeace statistics, several countries boast intolerable levels of SO2 emission, including the US, India, Russia, China, Saudi Arabia, Iran, South Africa, Bulgaria, Serbia, and Romania. 

Not only can SO2 damage the protective layer of our lungs, but it can increase the PM2.5 level in the air.

Formation and Sources

Sulfur-containing coal and diesel can give rise to sulfur dioxide (SO2) when burned. Recent reports indicate that approximately 6.5 million tons of SO2 are emitted every year in the US through petroleum refining, metal extraction, and diesel engines of trains and ships. Globally, SO2 emissions increased from 25 million tons in 1995 to 31 million tons in 2020 [7]. 

Sulfur Dioxide as a Poison

Damaging the mucous membranes of the throat, nose, and lungs, SO2 leaves you vulnerable to respiratory diseases like asthma. Besides, SO2 can react with other airborne chemicals to form sulfate, a type of PM2.5. Penetrating deep into your lungs and circulation, it can increase the risk of cardiovascular diseases and lung cancer [8].

In fact, a clinical study from Brazil revealed that above-average exposure to SO2 for seven days increased the risk of mortality from circulatory diseases by 3.6% [9].

Ozone (O3)

While ozone is essential for the sustainability of life on our planet, it can become an enemy if formed in the wrong atmospheric layer. In the US alone, 132 million people currently live in areas that don’t meet the standard ground-level ozone criteria [10].

The ground-level ozone gradually accumulates near the surface and gives rise to the infamous smog mostly attributed to Chinese air pollution. It gets even more severe in the summer months because sunlight accelerates the formation of ozone.

Besides giving a gloomy feeling to the urban landscape, ozone can reduce our quality of life significantly by bringing down the protective barrier of our lungs and causing chronic respiratory complications.

Formation and Sources

In the upper layers of the atmosphere, we mostly encounter good ozone, which absorbs harmful ultraviolet rays and increases atmospheric oxygen. So, how can a gas so beneficial for human life be a key factor of atmospheric pollution? The answer is in the sentence itself: human.

Ozone can also form near ground-level when another air pollutant, NOx, interacts with volatile organic compounds (VOC), also created as industrial byproducts. You can find VOC in motor vehicles, paint thinners, and dry cleaning agents. (Now you know where that distinct sharp smell comes from). 

In other words, any source that emits NOx, such as vehicle exhaust, contributes to ground-level ozone formation. 

Ozone as a Poison

Depending on the level of exposure, ground-level ozone can severely affect our respiratory, cardiovascular, and central nervous systems. Even an exposure shorter than 8 hours can disrupt the protective tissue in our lungs to trigger inflammation, asthma, and bronchitis [11]. A study found that prolonged inhalation of ozone had strong correlations with increased mortality rates, especially in vulnerable groups with compromised immune systems [12].

Carbon Monoxide

With no taste, odor, or colour, carbon monoxide (CO) is quite frankly the silent killer in air pollutants. When it comes to the release of CO, we all play a significant part whenever we drive to work, turn on our boilers, or our ovens and stoves. This pollutant that surrounds us from all sides - indoors and outdoors - can have severe health effects, including permanent brain damage.


Both carbon dioxide and carbon monoxide form through the burning of fuel, depending on the oxygen supply. If the oxygen in the environment is limited, more carbon monoxide (CO) will be generated instead of CO2


Overall, automobile use constitutes a vast portion of CO emission. Reports of EPA from the last two decades indicate that 84% of carbon monoxide emission was due to human activity, with automobile use accounting for 75% of human-related CO release [13]. According to the London Air Quality Network Guide, automobiles make up  90% of CO emissions in London.

That said, there are various other sources of CO, most of which are inside your homes. Any fuel-burning appliance or engine at home is prone to producing CO when ventilated improperly. These include boilers, ovens/grills, water heaters, fireplaces, cooking ranges, and stoves. In the US, 17% of 1800 randomly-selected houses have above-average levels of CO.

CO As a Poison

Once CO reaches our bloodstream, it binds to our red blood cells and prevents them from carrying oxygen to our organs. A prolonged lack of oxygen transfer ultimately damages our cells and causes dysfunctionalities in our body. Besides the acute and severe exposure that causes CO poisoning, chronic exposure to lower levels should not be underestimated.

Long term exposure to CO can cause severe brain damage that can affect a person’s functionality. Some of these long term effects include memory problems, concentration loss, hearing and vision loss, and Parkinsonism (not to be confused with Parkinson’s Disease), which is a general term for neurological diseases involving movement and speech impairment. The prolonged lack of oxygen also triggers coronary heart disease and even heart attack[14].


Before the discovery of its life-threatening effects in the 70s, lead was one of the pioneers of industrial processes, from gasoline and ceramics to the manufacturing of fertilizers. 

Although lead is used much less frequently today in industrial applications, you can still find significant amounts of it in the soil and the air. According to Penn State University, 5-10 million children in the US are still exposed to lead suspended in the dust. Clearly, the environmental effects of lead are much more permanent than anticipated.

Unlike some of the air pollutants on our list, lead is much more difficult to get rid of. In other words, its exposure can have irreversible effects ranging from brain damage to cancer.

Formation and Sources

Although the use of lead in various industries has been reduced since 1970, the recovery of our planet from industrial lead is not fast enough. In particular, urban soil remains a hurdle because it contains large amounts of lead that it can release into the air. With other sources like leaded aviation gasoline of aircraft and lead-battery manufacturers, the airborne lead still poses serious health risks.

Lead as a Poison

Because lead is difficult to excrete once taken in, even short amounts of lead exposure can cause various complications. In children, lead poisoning was discovered in triggering behavioural problems, brain damage, as well as liver and kidney damage [15]. Occupational lead exposure in adults can disrupt reproductive function, digestion, nervous system, and even cause cancer [16].




If you are curious about the complications caused by air pollution on various parts of the body, here is an article you might find interesting:

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[1] Gruzieva, Olena et al. “Exposure to air pollution from traffic and childhood asthma until 12 years of age.” Epidemiology (Cambridge, Mass.) vol. 24,1 (2013): 54-61. doi:10.1097/EDE.0b013e318276c1ea

[2] César, A C G et al. “Association between NOx exposure and deaths caused by respiratory diseases in a medium-sized Brazilian city.” Brazilian journal of medical and biological research = Revista brasileira de pesquisas medicas e biologicas vol. 48,12 (2015): 1130-5. doi:10.1590/1414-431X20154396

[3] Wang, J., Zhang, M., Bai, X. et al. Large-scale transport of PM2.5 in the lower troposphere during winter cold surges in China. Sci Rep 7, 13238 (2017). https://doi.org/10.1038/s41598-017-13217-2

[4] Ghosh, Sudakshina, and Serpil C. Erzurum. "Nitric oxide metabolism in asthma pathophysiology." Biochimica et Biophysica Acta (BBA)-General Subjects 1810.11 (2011): 1008-1016.

[5] Hamanaka, Robert B., and Gökhan M. Mutlu. "Particulate matter air pollution: effects on the cardiovascular system." Frontiers in Endocrinology 9 (2018): 680.

[6] Feng, Shaolong, et al. "The health effects of ambient PM2. 5 and potential mechanisms." Ecotoxicology and environmental safety 128 (2016): 67-74.

[7] Zhong, Qirui, et al. "Global sulfur dioxide emissions and the driving forces." Environmental Science & Technology (2020).

[8] Meng, Ziqiang. "Oxidative damage of sulfur dioxide on various organs of mice: sulfur dioxide is a systemic oxidative damage agent." Inhalation toxicology 15.2 (2003): 181-195.

[9] Amancio, Camila Trolez, and Luiz Fernando Costa Nascimento. "Association of sulfur dioxide exposure with circulatory system deaths in a medium-sized city in Brazil." Brazilian journal of medical and biological research 45.11 (2012): 1080-1085.

[10] Laumbach, Robert J. “Outdoor air pollutants and patient health.” American family physician vol. 81,2 (2010): 175-80.

[11] Jule, Yvon, et al. "Ozone induced acute and chronic alterations in the lung in mice: a combined digital imaging and functional analysis." (2018).

[12] Turner, Michelle C., et al. "Long-term ozone exposure and mortality in a large prospective study." American journal of respiratory and critical care medicine 193.10 (2016): 1134-1142.

[13] Wu, Lingyun, and Rui Wang. "Carbon monoxide: endogenous production, physiological functions, and pharmacological applications." Pharmacological reviews 57.4 (2005): 585-630.

[14] Wilbur, Sharon, et al. "Toxicological profile for carbon monoxide." (2012).

[15] Pan, Shangxia, et al. "Effects of lead, cadmium, arsenic, and mercury co-exposure on children's intelligence quotient in an industrialized area of southern China." Environmental pollution 235 (2018): 47-54.

[16] Dumat, Camille, et al. "Lead pollution and human exposure: forewarned is forearmed, and the question now becomes how to respond to the threat!." Lead in Plants and the Environment. Springer, Cham, 2020. 33-65.


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