Outdoor Air Pollution: Causes, Sources, Effects & Prevention/Solutions

Outdoor Air Pollution: Causes, Sources, Effects & Prevention/Solutions

When it comes to air pollution, there are two main types – indoor air pollution, and outdoor air pollution.

This guide focuses on ambient outdoor air pollution, and we look at causes, sources, examples, effects and potential ways to prevent or solve it.


(*It should be noted that general ambient outdoor air pollution is a lower atmosphere issue which has a separate set of sub issues to deal with than upper atmosphere Greenhouse Gases, Carbon emissions and Climate Change/Global Warming (all of which also affects the outside air environment).

This is a guide specifically about lower atmosphere (non Greenhouse gas) ambient outdoor air pollution.)


Summary – What To Know About Outdoor Air Pollution

First off, outdoor air pollution should be distinguished from greenhouse gas emissions (an upper atmosphere air issue) and global warming – these are separate issues.

Outdoor air pollution is mainly the release of air contaminants into the air that not only lower air quality and contribute to a range of human illnesses (and related deaths), but also contribute to other environmental issues like acid rain for example.

Combustion of fossil fuels in the generation of electricity, industrial activities, and the operation of vehicles/cars are huge emitters of air pollutants. In cities and densely populated areas – vehicles and road transport is the main source.

Air pollution is particularly heavy around cities and heavily populated areas, as it’s mainly an issue caused by humans.

We can measure the levels of outdoor air pollution with outdoor air quality indexes, amongst other measures.

Side effects can be lowering in air quality for humans, which can have health effects, but also environmental side effects like acid rain for example.

The World Health Organisation (WHO) highlights air pollution as the greatest environmental risk to human health – but a changing climate may be the biggest risk in the future.

Cleaner electricity sources like wind and solar, and cleaner vehicle technology (developing electric battery, hybrid, hydrogen and other vehicle types) could go a long way to helping us decrease outdoor air pollution.

You can read about examples of cities that have done something about their air pollution in this guide.


What Is Outdoor (Ambient) Air Pollution?

  • Air pollution in general can be defined as the ’emission of harmful substances to the atmosphere [i.e. the outside environment]’

– OurWorldInData


[Outdoor air pollution] usually has a harmful effect on the living and non-living things that breathe in, absorb or come into contact with that air – such as humans, animals and even the ocean.


Air Pollution Contaminants

When we talk about outdoor air pollution, we are usually talking about the following pollutants:

  • particulate matter (PM10, & PM2.5) (small suspended particles of varying sizes)
  • sulphur dioxide (SO2)
  • nitrogen oxides (NOx)
  • ozone (O3)
  • carbon monoxide (CO)
  • and volatile organic compounds (VOCs)


  • Some of these pollutants are emitted singularly, but some form when two or more pollutants mix together.
  • For example, SO2 and NOx can react in the Earth’s atmosphere to form particulate matter (PM) compounds

– OurWorldInData


  • Note that carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3), and synthetic gases, such as chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs) are more likely to be treated as upper atmosphere/ozone greenhouse gases

– ClimateChangeAustralia.gov.au


Causes, Sources & Examples Of Outdoor Air Pollution

  • The sources and causes of the pollutants listed above can vary
  • They can come from both non-natural, and natural sources.
  • However, most are generally linked to human sources like fuel combustion and industrial (factories, business etc.) activities; pollutants are released as by-products of these processes

– OurWorldInData


  • Examples [of sources might] include petrol and diesel vehicles, burning fuel in houses for cooking and heating (Cookers, heaters, stoves and open fires), emissions from power generation, factories and business, and agriculture.

– British Lung Foundation


Specific examples of air contaminants include:

  • sulphur dioxide (SO2) – About 99% of the sulfur dioxide in air comes from human sources. The main source of sulfur dioxide in the air is industrial activity that processes materials that contain sulfur, eg the generation of electricity from coal, oil or gas that contains sulfur. [and, industrial activities and motor vehicles] – Environment.gov.au.
  • Sulphur dioxide can also be produced by volcanoes – Wikipedia


  • nitrogen oxides (NOx) – comes from the burning of fossil fuels like coal, oil and gas. Most of the nitrogen dioxide in cities comes from motor vehicle exhaust (about 80%) – Environment.gov.au. Can also come from electrical storms via electrical discharge, and plants, soil and water – although only a very small amount comes from these natural sources. Other sources of nitrogen dioxide are petrol and metal refining, electricity generation from coal-fired power stations, other manufacturing industries and food processing. Unflued gas heaters and cookers are the major sources of nitrogen dioxide in Australian homes – Environment.gov.au


  • ozone (O3) – Tropospheric, or ground level ozone, is not emitted directly into the air, but is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC). It is formed when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants, and other sources chemically react in the presence of sunlight – Epa.gov


  • particulate matter (PM10, & PM2.5) (small suspended particles of varying sizes) – Particulate matter, also known as particle pollution or PM, is a term that describes extremely small solid particles and liquid droplets suspended in air. Particulate matter can be made up of a variety of components including nitrates, sulphates, organic chemicals, metals, soil or dust particles, and allergens (such as fragments of pollen or mould spores). Particle pollution mainly comes from motor vehicles, wood burning heaters and industry – Health.NSW.Gov.Au. 
  • Most particles form in the atmosphere as a result of complex reactions of chemicals such as sulfur dioxide and nitrogen oxides, which are pollutants emitted from power plants, industries and automobiles. Some are emitted directly from a source, such as construction sites, unpaved roads, fields, smokestacks or fires. – EPA.gov
  • The friction of brakes and tyres on the road also creates particulate matter. – British Lung Foundation


  • carbon monoxide (CO) – It is a product of combustion of fuel such as natural gas, coal or wood. Vehicular exhaust contributes to the majority of carbon monoxide let into our atmosphere. In 2013, more than half of the carbon monoxide emitted into our atmosphere was from vehicle traffic and burning one gallon of gas will often emit over 20 pounds of carbon monoxide into the air. – Wikipedia/Union Of Concerned Scientists, is produced in the incomplete combustion of carbon-containing fuels, such as gasoline, natural gas, oil, coal, and wood. The largest anthropogenic source of CO in the United States is vehicle emissions. – NAP.edu


  • volatile organic compounds (VOCs) – VOCs comprise volatile hydrocarbons and other organic molecules released into the atmosphere. They may have biogenic or anthropogenic sources. In the UK it is estimated that less than 5% of the VOCs (2.3 million tonnes per year, expressed in terms of carbon) emitted into the atmosphere are emitted from vegetation. The rest comes from transport, including distribution and extraction losses (50%), solvent use (30%) and other industrial processes (15%). Road transport alone accounts for 30% of VOC emissions. – APIS.ac.uk.
  • Common VOCs include acetone, benzene, ethylene glycol, formaldehyde, methylene chloride, perchloroethylene, toluene and xylene. – SOE.environment.gov.au


  • Overall, in towns and cities, the main source of air pollution is road transport

– British Lung Foundation


  • Other more minor sources may include smoke from bushfires, windblown dust, and biogenic emissions from vegetation (pollen and mould spores)

– NSW Government


How Much Outdoor Air Pollution Is Released Each Year, & What Are The Trends (Increasing or Decreasing)?

Obviously different cities and countries release different amounts of outdoor air pollution and have different policies and measures in place to control outdoor air pollution.

But, here are some amounts and trends from different countries:

  • sulphur dioxide (SO2) – In the US, sulfur dioxide emissions have been decreasing, and are down to 2709 thousand tons in 2016 – Statista.com.
  • Air quality regarding sulfur dioxide in improving in the US – EPA.gov.
  • In Australia, the amount of sulfur dioxide in air is at acceptable low levels in most Australian towns and cities. – Environment.gov.au. In Australia, the highest concentrations of sulfur dioxide in the air are found around petrol refineries, chemical manufacturing industries, mineral ore processing plants and power stations. – Environment.gov.au


  • nitrogen oxides (NOx) – There’s been a 60% decrease in the US national average of nitrogen dioxide from 1980 to 2017 – EPA.gov.
  • In the US, there was 12,412 thousand tons of nitrogen oxide emissions in 2014 – Statista.com.
  • In Australia, since the early 1990s, even the highest levels of nitrogen dioxide reached in most Australian towns and cities are thought to be acceptable for humans. – Environment.gov.au


  • ozone (O3) – there’s been a 32% decrease in ground level ozone national average in the US from 1980 to 2017 – EPA.gov


  • particulate matter (PM10, & PM2.5) (small suspended particles of varying sizes) – there’s been a 41% decrease in the particulate matter 2.5 national average in the US from 1980 to 2017. There’s also been a 34% decrease in the particulate matter 10 national average in the US from 1980 to 2017  – EPA.gov. 
  • Particle pollution is a major air quality issue in Australia. – Environment.gov.au


  • carbon monoxide (CO) – there’s been a 84% decrease in the carbon monoxide national average in the US from 1980 to 2017 – EPA.gov. 
  • In most Australian towns and cities, the levels of carbon monoxide in air are below levels that are hazardous for human health. Only larger cities, like some capital cities, have the potential to have harmful levels of carbon monoxide. – Environment.gov.au


  • and volatile organic compounds (VOCs) – there are different VOC’s such as Butadiene, Dichlorobenzene,  Benzene, Chloroform, Methylene Chloride, m,p-Xylene, o-Xylene, and Toluene.
  • DEC.ny.gov has done some studies on the levels of these VOCs across the NY state. VOCs particularly affect indoor air quality—concentrations of many VOCs are consistently higher indoors (up to 10 times higher) than outdoors. – SOE.environment.gov.au


OurWorldInData also shows levels of the different levels of different air pollutants over the years. You can see that there was a huge increase up until 1970/1980 for most regions, followed by a steady decline – https://ourworldindata.org/air-pollution

Although some pollutants have decreased since 1980, there has been small increases or flatlines in progress in the years before 2017. Particulate matter 10 levels are one example of this – with minimal progress being made since 2004.


Effects Of Outdoor Air Pollution

  • Outdoor air pollution can have an impact on human health, damage to ecosystems, food crops and the built environment 
  • The World Health Organisation (WHO) highlights air pollution as the greatest environmental risk to human health (note that this is based on current risk, and that longer-term environmental threats such as climate change may exceed this in the future). 
  • The World Health Organization estimates that 3 million people die from ambient outdoor pollution every year

– OurWorldInData/WHO


  • Although that number can vary by up to a million depending on the source and year you read it from.
  • It is important to emphasize the difficulties in directly attributing deaths to air pollution. A ‘death’ from air pollution is defined as someone who dies prematurely (could be in the range of months or years) than would be expected in the absence of air pollution.
  • In many cases, air pollution exacerbates pre-existing cardiorespiratory illnesses—individuals suffering from asthma, for example, are particularly vulnerable.

– OurWorldInData/StateOfGlobalAir.org


The three key sources of air pollution deaths are from the indoor burning of solid fuels (indoor air/household pollution), exposure to ambient outdoor ozone (O3), and ambient outdoor particulate matter (PM) pollution.

In 2015, deaths from these 3 pollutants were as follows (as total %’s):

  • Ozone – 3.45%
  • Particulate Matter – 57.54%
  • Indoor Air Pollution/Solid Fuels – 38.72%

– OurWorldInData


Aside from death, ambient outdoor air pollution can cause other health related problems such as:

  • sulphur dioxide (SO2) – affects people when it is breathed in. People most at risk are those with asthma or breathing conditions. It irritates the nose, throat, and airways to cause coughing, wheezing, shortness of breath, or a tight feeling around the chest. – Environment.gov.au


  • nitrogen oxides (NOx) – causes increased likelihood of respiratory problems. Nitrogen dioxide inflames the lining of the lungs, and it can reduce immunity to lung infections. This can cause problems such as wheezing, coughing, colds, flu and bronchitis. People with asthma, and in particular children and older people are most at risk. – Environment.gov.au


  • ozone (O3) – ground level ozone can cause the muscles in the airways to constrict, trapping air in the alveoli. This leads to wheezing and shortness of breath. People with asthma and children, older adults, and people who are active outdoors, especially outdoor workers are most at risk. There are other health issues ground ozone can cause as well – EPA.gov


  • particulate matter (PM10, & PM2.5) (small suspended particles of varying sizes) – Studies have linked exposure to particle pollution to a number of health problems including respiratory illnesses (such as asthma and bronchitis) and cardiovascular disease. In addition, the chemical components of some particles, particularly combustion products, have been shown to cause cancer. These effects are often more pronounced for vulnerable groups, such as the very young and the elderly. Particle pollution is the major cause of reduced visibility. – Environment.gov.au


  • carbon monoxide (CO) – Increased levels of carbon monoxide reduce the amount of oxygen carried by haemoglobin around the body in red blood cells. The result is that vital organs, such as the brain, nervous tissues and the heart, do not receive enough oxygen to work properly. For healthy people, the most likely impact of a small increase in the level of carbon monoxide is that they will have trouble concentrating. Some people might become a bit clumsy as their coordination is affected, and they could get tired more easily. People with heart problems are likely to suffer from more frequent and longer angina attacks, and they would be at greater risk of heart attack. Children and unborn babies are particularly at risk because they are smaller and their bodies are still growing and developing. – Environment.gov.au


  • volatile organic compounds (VOCs) – Different VOCs have different health effects, and range from those that are highly toxic to those with no known health effect. Breathing low levels of VOCs for long periods of time may increase some people’s risk of health problems. Several studies suggest that exposure to VOCs may make symptoms worse in people who have asthma or are particularly sensitive to chemicals. – SOE.environment.gov.au


Countries Where Outdoor Air Pollution Can Be An Issue

Some interesting trends in air pollution related deaths according to OurWorldInData are:

  • Death rates from air pollution—across countries of all income levels—have shown a general decline over the last few decades. [usually] by more than 50 percent.
  • Globally, it’s estimated that outdoor air pollution resulted in 4.2 million deaths in 2016; this represents an increase from 3.4 million in 1990. Overall, we see that the majority of pollution-related deaths are in Asia – South, Southeast and East Asia alone accounted for nearly 3 million in 2016.

You can read more about air pollution related deaths by type, country and more here – https://ourworldindata.org/air-pollution 


We’ve also put together a guide that details the countries where outdoor and indoor air pollution might be the worst.


Measuring Outdoor Air Pollution – Air Quality Index

  • One way to measure and keep track of outdoor air pollution in a particular area or city is with an Air Quality Index.
  • An Air Quality Index can give you a range of information, but should usually tell you the main pollutants in an area and give you a general health rating for the air in that area.
  • There are large online sites that keep track of the Air Quality Index (such as WAQI, Airnow and AQICN), and individual governments also have their own tracking programs.
  • The NSW Government in Australia for example has their own air monitoring networks where they measure particles (PM10, PM2.5), sulfur dioxide (SO2), carbon monoxide (CO), ozone (O3), nitrogen dioxide (NO2) and visibility. Wind speed and direction, air temperature and humidity are also recorded


  • In metropolitan areas (greater Sydney, Newcastle, and Wollongong regions), the main air pollutants of concern are ozone (O3) and particles (particulate matter or PM). For regional areas in NSW, particle pollution is the main concern.

– NSW Government


Countries & Cities With The Most & Least Polluted Outdoor Air In The World

Obviously pollution can vary from city to city within a country, and even from year to year.


  • For example, the most polluted city in a 2016 report, Zabol in Iran, has had its pollution level cut fourfold in the latest version of the database, and now appears to be cleaner than Australia’s capital Canberra
  • Based on the amount of particulate matter under 2.5 micrograms found in every cubic metre of air, Indian regions and cities are the most polluted in the world in 2018, followed by China. Some places in Saudi Arabia are also highly polluted

– WeForum.org


  • Egypt, Mauritania, Libya, Niger, Cameroon and Pakistan also show high mean annual averages of migrograms per cubic meter of PM 2.5 air pollution in 2015 
  • Some of the least polluted countries in the world in terms of mean annual averages of migrograms per cubic meter of PM 2.5 air pollution in 2015 are Kirbati, Samoa, Brunei, Solomon Islands, Sweden, Finland, Australia, Canada, United States, New Zealand, Norway, Spain, and Iceland. 
  • It’s important to note that there is an additional key factor at play, which has some impact on pollution concentrations over time and space: the weather. Local weather conditions, and seasonal and weather patterns have an important influence on the year-round fluctuations in exposure levels reported in each place

– OurWorldInData


Potential Solutions To, & Prevention Of Outdoor Air Pollution

Solutions to, and prevention of outdoor air pollution involves a wide ranging approach.

It’s definitely not a simple issue with one simple solution, and no solution is perfect.

It really does centre around reducing, or finding alternatives to fuel combustion and other human related air pollution producing activities, and becoming more environmentally friendly with the way we run our households and businesses/industries.


Some things that might be done to reduce the level of outdoor air pollution and lower pollutant emissions are:

  • Switching to electric vehicles
  • Reducing reliance on vehicles in heavily populated cities, and favoring public transport and walking/bikes
  • Switching to cleaner renewable energy over fossil fuels for households and business/industry, and agriculture
  • Switch to diets and agriculture that produces less air contaminants, or becoming more environmentally friendly with production

People can also check air quality websites to see how polluted the air is in the city or area they are living. Moving to places with less air pollution can be an option for better short term and long term health.



1. Hannah Ritchie and Max Roser (2018) – “Air Pollution”. Published online at OurWorldInData.org. Retrieved from: ‘https://ourworldindata.org/air-pollution’ [Online Resource]

2. http://www.health.nsw.gov.au/environment/air/Pages/outdoor-air-pollution.aspx

3. https://www.blf.org.uk/support-for-you/air-pollution/where-does-it-come-from

4. https://www.environment.nsw.gov.au/topics/air/monitoring-air-quality

5. http://www.environment.gov.au/protection/publications/factsheet-sulfur-dioxide-so2

6. http://www.environment.gov.au/protection/publications/factsheet-nitrogen-dioxide-no2

7. https://en.wikipedia.org/wiki/Air_pollution

8. https://archive.epa.gov/ozonedesignations/web/html/faq.html#whatisozone

9. https://www.climatechangeinaustralia.gov.au/en/climate-campus/climate-system/greenhouse-gases/

10. http://www.health.nsw.gov.au/environment/air/Pages/particulate-matter.aspx

11. https://www.epa.gov/pm-pollution/particulate-matter-pm-basics#PM

12. https://www.nap.edu/read/10378/chapter/3

13. http://www.apis.ac.uk/overview/pollutants/overview_VOCs.htm

14. https://www.statista.com/statistics/501303/volume-of-sulfur-dioxide-emissions-us/

15. https://www.epa.gov/air-trends/sulfur-dioxide-trends

16. https://www.epa.gov/air-trends/nitrogen-dioxide-trends

17. https://www.statista.com/statistics/501284/volume-of-nitrogen-oxides-emissions-us/

18. https://www.epa.gov/air-trends/ozone-trends

19. https://www.epa.gov/air-trends/carbon-monoxide-trends

20. https://www.epa.gov/air-trends/particulate-matter-pm25-trends

21. https://www.epa.gov/air-trends/particulate-matter-pm10-trends

22. https://www.dec.ny.gov/chemical/66472.html

23. https://www.epa.gov/ozone-pollution/health-effects-ozone-pollution

24. http://www.environment.gov.au/protection/publications/factsheet-carbon-monoxide-co

25. https://soe.environment.gov.au/theme/ambient-air-quality/topic/2016/volatile-organic-compounds

26. https://www.weforum.org/agenda/2018/05/these-are-the-worlds-most-polluted-cities

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