Air pollution can seem quite basic. The sky is brown bad; the sky is blue good. But start trying to understand what clean air truly entails, and it quickly becomes far more technical and complex than meets the eye, quite literally.
Ahead of this years International Day of Clean Air for blue skies, weve built a glossary of the terms most important for understanding air pollution what it is, how it works and how its tackled. We hope it helps readers expand their good-bad barometer to dig deeper into what theyre breathing and why that matters.
Air pollution basics
Air pollution is airborne liquid, solid and gaseous particles that change the natural atmosphere and are harmful to human and planetary health. Notably, around 43 per cent of countries lack a legal definition for air pollution, reflecting the ubiquity of weak air pollution control.
Around half of countries have definitions of air pollution that only address ambient air pollution, whichis another term for outdoor air pollution. These policy definitions exclude indoor air pollution contaminated inside air that includes household air pollution, which is caused by burning solid fuel, like charcoal for cooking.
Of the five major layers of the atmosphere, which is the mixture of gases surrounding Earth, two are most relevant for pollution. They are: the troposphere, which is the first layer where smog and pollution get trapped, and the stratosphere, the second layer, where pollutants can damage the protective, stratospheric ozone layer.
Some harmful substances are emitted directly into the air from human or natural sources, such as factories or wildfires. These are primary pollutants. Others, known as secondary pollutants, are formed in the atmosphere from chemical reactions between pollutant precursors which include chemical substances, some pollutants themselves and sunlight, water vapor and other gases.
Aerosols are any solid particle or liquid droplet suspended in the air (i.e. not gas). Aerosols that are microscopic are called particulate matter (PM).
Common sources of air pollution include stationary sources like factories or household stoves; mobile sources, namely vehicles; biomass burning, the combustion of organic materials; open waste burning, when rubbish is burned in uncontrolled ways; and non-exhaust emissions from vehicles, like the particles released by the wear on tires, brakes and roads.
Common air pollutants
Criteria pollutants, or commonly regulated air pollutants,are those that air-quality laws and standards most routinely control and monitor, because of their deleterious effects on human and planetary health. They include:
- PM2.5Fine particulate matter with a diameter of 2.5 micrometres or less (30 times smaller than the width of a human hair), meaning they can penetrate deep into the lungs and blood.
- PM10:Which has a diameter of 10 micrometres or less, often lodging in the nose, throat and upper airways.
- Ground-level ozone (O): Different from good stratospheric ozone, it sits nearest to the Earth and harms our breathing. The term is often used interchangeably with tropospheric ozone (though, technically, tropospheric ozone extends higher above ground)and isformed when precursors such as nitrogen oxides, carbon monoxide, methane and volatile organic compounds react in sunlight.
- Nitrogen dioxide (NO): A toxic gas that primarily comes from burning fossil fuels.
- Sulphur dioxide (SO): A colourless, poisonous gas released by both natural sources (volcanoes, hot springs), industrial activities and burning fossil fuels.
- Carbon monoxide (CO): Known as the silent killer, a toxic, flammable gas thats difficult to detect. When produced by incomplete combustion of fuels like gas, oil, wood, coal, or kerosene and trapped indoors, it can be fatal.
Heavy metals and other toxic substances are known or suspected to cause cancer, birth defects, or other serious human health and environmental harm and so are also frequently regulated. Some examples are lead, mercury, benzene, arsenic and asbestos. In some countries, these are referred to as hazardous air pollutants or air toxics simply because they are regulated under a different legal framework than criteria pollutants though criteria pollutants are hazardous and toxic too.
Smog is a visible, often hazy mixture of air pollutants. There are a few types of smog, but the most familiar is photochemical smog, smog produced through sunlight-driven chemical reactions of pollutants that creates ground-level ozone and brownish haze.
Super pollutants are compounds that can cause much more global warming per unit emitted than carbon dioxide, especially in the near-term.Many are short-lived climate pollutants (SLCPs), which drive about 45 per cent of global warming but remain in the air for a short period. These include:
- Methane (CH): A colorless, odorless, powerful greenhouse gas.
- Black carbon / soot: Fine particulate matter produced when fuels or organic matter do not burn completely, often taking the form of black powder when deposited.
- Hydrofluorocarbons: Synthetic chemicals largely from refrigeration.
- Tropospheric ozone: See ground-level ozone, above.
There can be overlap between SLCPs and greenhouse gases gases that trap heat and drive climate change but not always. Black carbon, for one, is not a gas; it is a component of particulate matter that can change cloud formation and land on snow and ice and accelerate melting.
There are plenty of other pollutants that have detrimental effects, but here are a few of the common ones. Ammonia (NH) is a colorless, pungent gas often coming from nitrogen fertilizers thats a pollutant precursor to PM2.5. Nitrogen oxides (NOx) are a group of reactive gases borne from fossil fuels burning at high temperatures and contribute strongly to smog and acid rain. Volatile organic compounds (VOCs) are carbon-based chemicals that can easily evaporate from paints and finishes, household cleaners, building materials, fuels, even some cosmetics and trigger ground-level ozone and smog. There are also biogenic VOCs (BVOCs), which are made and released by plants. Stressful effects of climate change like higher temperatures and droughts, can cause them to ramp up their release of BVOCs.
Formation and movement
When a fuel burns with insufficient oxygen think, poorly ventilated heaters and stoves its incomplete combustion results in carbon monoxide, carbon particles (soot or smoke) and a variety of other pollutants, which can be especially dangerous indoors.
Dust and sandstorms often overlooked in pollution dialogue occur when strong winds over dry regions transport dangerous particulate matter, pathogens and pollutants across regions and continents through whats called atmospheric dispersion. When the pollution eventually leaves the air and settles back on land, water or other surfaces, its called atmospheric deposition.
Similarly, acid rain, or acid deposition, is any form of precipitation that contains enough nitric and sulfuric acids and ammonium caused by natural and human-driven pollution to become damagingly acidic, particularly for ecosystems.
When a layer of warm air sits above a layer of cool air, temperature inversion prevents air from moving upward, trapping pollution and smog near the ground.
Measurement and regulation
An airshed is a geographical area affected by the same air mass, which can range from valleys to large swaths of continents. It is arguably the most effective basis for monitoring and regulating air quality, though air quality still tends to be monitored and managed according to administrative boundaries instead.
Air quality is a measurement of pollutants in the air. Many countries define acceptable air quality through legally established ambient air quality standards, which can be mapped on a scale called an Air Quality Index (AQI), used to communicate how relatively clean or polluted the air is. Notably, there is not one single international Index; many countries use their own, and many dont have one at all. As a result, air quality measurements stay the same across locations, but their interpreted risks might differ.
Exposure is the amount of contact a human, plant or animal has with a harmful pollutant, taking into consideration the inhalation process, magnitude, duration and frequency of the contact. It is a key component of air pollution health impact assessments.
An air quality standard is a legally binding limit on a pollutants acceptable concentration in the ambient air. In 2021, UNEP published a guide to help countries embed air pollution policy into national legislation effectively. An air quality guideline is an evidence-based recommendation, and often by default refers to the guidelines published by the World Health Organization (WHO), but its not legally binding.
Source apportionment is one method of source attribution, the scientific practices of identifying where pollutants come from and how much each source contributes to ambient air pollution. This informs whats called an emission limit, a legally binding limit on the amount of a pollutant that a specific source can release.
Air pollution is usually measured in micrograms per cubic metre (g/m), which charts the mass of a pollutant within a volume of air, or parts per million / billion (ppm/ppb), which measures the proportion of pollutant molecules within the air.
