Air Pollution: Types, Causes & Effects

We explain the most common types of natural and man-made air pollution, as well as the main outdoor and indoor air pollutants. We highlight the close connections between air quality and fossil fuel burning, and look at the most important sources of naturally occurring airborne pollution, including wildfires and volcanic eruptions. We also explain the effects of air pollution on human health and the environment.
Photo: Sardaka//CC BY-SA 4.0

This article explains the main forms of air pollution – both indoor and outdoor – their links to climate change, and their effects on ourselves and the planet. It details the most dangerous pollutants, such as greenhouse gases, dioxins, particulate matter and radioactive materials, among others. It also looks at atmospheric phenomena such as smog, acid rain, the Asian brown cloud and the long range transport of air pollutants (LRTAP).

The quality of the air we breathe has a direct impact on the quality of our life and that of our families. With respiratory diseases such as chronic obstructive pulmonary disease (COPD), and growing numbers of asthma and allergy sufferers, it has never been more important to keep our air clean.

Arguably, the single most important change that the world can make to reduce air pollution is to switch to electric vehicles (EVs). Throughout this article we show how the internal combustion engine is responsible for emitting a huge number of toxic gases and dangerous, microscopic particles. In comparison, electrically powered vehicles produce zero pollutants.

Some environmentalists are hoping that the Coronavirus shutdown may lead to a permanent reduction in air pollution. This now looks doubtful. For more, see: Effect of COVID-19 on Climate Change.

What is Air Pollution?

Air pollution is the emission of pollutants into the air that are potentially harmful to human health or the environment. The causes of air pollution – both outdoor and indoor – involve natural processes as well as human activities, although the most worrisome sources are man-made.

The most common effects of air pollution on human health, include breathing problems and allergies. But toxic air also causes numerous serious illnesses, ranging from diabetes and high blood pressure to pneumonia, cancer, heart disease and stroke.

Studies show that outdoor air pollution accounts for as many as 4.2 million deaths each year. 1

In total, according to the World Health Organization (WHO), air pollution (outdoor and indoor) kills around eight million people worldwide every year. Furthermore, roughly 8 out of 10 people living in towns and cities that monitor air pollution, are exposed to air quality levels that exceed WHO-recommended limits. 2

The environmental damage caused by outdoor air pollution includes loss of biodiversity, contamination of the water supply, ocean deoxygenation, loss of nutrients in plants and soils, and damage to biomes and ecosystems in every corner of the globe. It’s one of the main reasons why scientists are calling for our current Holocene epoch to be renamed the Anthropocene epoch – to reflect the humancentric character of our time.

The most common outdoor pollutants are released by the burning of fossil fuels in power plants, factories and furnaces, as well as vehicle engines. In developing countries, the traditional burning of biomass, such as wood, crop residues and animal dung remains a major source of air pollutants, both indoors and outdoors.

The World Bank and other financial experts calculate that productivity losses and social costs caused by air pollution, cost the world economy $5 trillion annually. 3

Global Air Pollution Map
Global map showing levels of air pollution. Notice the very heavy pollution emanating from areas in Africa and Asia, as well as the oil and gas-ridden Gulf States. Color code yellow-orange-red indicates increasing pollution. Image: World health Organization (WHO)

Indoor Air Pollution

Indoor air pollution (IAP) – also called household air pollution (HAP) – refers to the contamination of indoor air – due to physical, chemical or biological pollutants. According to WHO, it is responsible for causing 3.8 million premature deaths, annually.

In developing countries, the most common form of indoor pollution is smoke from wood burning or other biomass, on open fires lit for cooking and heating. The smoke consists of a mass of tiny particles of black carbon and ash, plus toxic gases such as carbon monoxide, sulphur dioxide and nitrogen dioxide.

This type of indoor pollution is extremely unhealthy for women and children in the household, as they spend the most time around the family hearth. It also impacts more on the poorest people in developing countries, since typically they have no money for the healthier liquid fuels. In this sense, indoor air pollution illustrates the inequitable effect of global warming. For more, see: Ethics of Climate Change.

It’s also bad for climate change and the long term health of the planet. On average, 3 billion kilos of wood and other solid fuels (including coal) are burned on indoor fires in developing countries every day. This fuel releases a total of 6 billion kilos of carbon dioxide into the air. That’s three times the daily amount of greenhouse gas emissions from all the private cars in America.

In richer countries, where most homes have more modern cooking equipment and use electricity as well as cleaner forms of coal and natural gas, indoor pollution has fallen dramatically.

Is Air Pollution Linked to Global Warming?

Yes. Air pollution is closely linked to global warming in a number of ways. Here are just four.

(i) The warmer the temperatures the dryer the forests and the greater the risk of widespread wood-burning, with all its harmful air pollution.
(ii) Rising temperatures lead to more photochemical smog being formed in and around urban areas. This is because ultraviolet light from the sun reacts with nitrogen oxides from car exhaust fumes in the air.
(iii) Climate change also boosts levels of allergenic air pollutants such as pollen, by creating a longer pollen season. 4 (iv) Most of the world’s harmful air pollution stems from the burning of fossil fuels, which is also the main driver of climate change.

We Need to Look After Our Planet

All pollution is a sign that we don’t really care about Planet Earth and its biosphere. We consume too much, waste too much and then mismanage our waste disposal.

Unless we change, the situation is going to get a lot worse for at least three reasons. (a) The world population is growing. (b) Consumption of almost everything is growing, especially in developing countries. (c) Consumption of natural gas and petroleum products continues to rise, fuelling the greenhouse effect and placing huge stress on the global and regional climate systems.

Witness, for instance, the vast clouds of air pollutants released by bushfires in Australia, forest fires in California and the Arctic, and the massive Asian brown cloud of toxic pollutants that covers the Indian sub-continent during the dry season.

In the United States – the world’s most technologically advanced nation – air quality is now declining. Data provided by the U.S. Environmental Protection Agency (EPA) in mid-2019 showed a 15 percent increase in the number of days with unhealthy amounts of pollutants in the air in 2017 and 2018, compared to the preceding 3-year average.

What Causes Air Pollution?

Air pollution can be naturally occurring or the result of human action. Here is a short summary of the most common causes.

Man-Made Air Pollution

• Fossil Fuel Combustion
The burning of coal and other fossil fuels in power plants, steel plants, furnaces and waste incinerators, is a major cause of CO2, nitrous oxide (N2O) and sulfur dioxide emissions. Significant amounts of partially combusted particles (particulate matter) are also released.

• Industrial Processes
Certain industrial processes cause significant pollution. For example, cement industry emissions of CO2 are a major contributor to climate change.

• Leakage from Oil & Gas Infrastructure
Oil and gas drilling sites, refineries and pipelines are major emitters of methane gas (CH4), as well as volatile organic compounds (VOCs)

• Farming & Agriculture
Farming produces a range of greenhouse gases and other pollutants. (a) The over-use of nitrogen fertilizers is a significant cause of N20 pollution. (b) Methane is emitted by cows and other ruminants as they digest grass, and also by their waste. (c) Ammonia emitted from fertilizers and animal manure on farms, and then blown over cities, can combine with nitrogen oxides emitted from cars, or sulfur dioxide from coal-burning, to form smog (see below).

• Cars, Trucks and Other Fossil Fuel Engines
Engines emit significant amounts of greenhouse gas emissions including chemicals like carbon monoxide, nitrogen oxides (NOx) and VOCs, as well as particulate matter. Diesel engines also release particulates and soot directly into the air.

• Paint, Hair Spray, Aerosol Sprays, Solvents
Fumes from the above products are an important cause of indoor air pollution from volatile organic compounds.

• Mining, Smelting, Metals Processing
These industrial activities are important sources of heavy metal pollutants such as cadmium, lead and mercury

• Incinerators
The burning of waste materials is a common source of hazardous pollutants, including mercury, dioxins, lead, and others.

• Controlled Burning
The technique of controlled burning is used in both agriculture and forest management in order to conserve and restore forest and grassland ecosystems.

• Nuclear Facilities
Leaks and accidents at nuclear facilities are a well-documented source of radioactive emissions.

Naturally Occurring Air Pollution

• Volcanoes
Volcanic activity causes significant emissions of sulfur and ash particulates. (See below for more details.)

• Forest Fires
These can emit massive amounts of air pollution in the form of CO2 and particulates. For example, during the summer, studies show that wildfire smoke can account for as much as 73 percent of harmful particulates. 5 In Europe, forest and grassland fires consume an average of 600,000 hectares of land per year. (See below for more details.)

• Wind-Blown Dust
Wind-driven mineral dust and sand from deserts, lake beds and other large areas of land with no vegetation.

• Wind-Blown Sea Salt
These salt particles can contribute up to 80 percent of particulates in the air along coastal areas. This is mostly sea salt, whipped into the air by strong winds.

• Plants & Trees
Scientists estimate that emissions from trees and plants accounts for about two-thirds of the VOCs currently in the atmosphere. These natural VOCs are not as toxic as those emitted by paint and pesticides. But, once in the atmosphere, they react with other airborne chemicals to form secondary air pollutants (see below).

• Wetlands
Wetlands (e.g. swamps, marshes, bogs, fens, peat bogs) are the largest natural source of methane, generating about 150 million tonnes of biogenic methane annually. This accounts for up to 33 percent of global methane emissions.

Primary & Secondary Air Pollutants

Air pollutants are divided into two basic types: primary and secondary. Primary ones are those that are released directly into the air (e.g. from a volcano, a car’s tailpipe, a factory chimney, a forest fire). Wind-driven sea salt, or mineral dust are also primary pollutants.

Secondary ones are not emitted directly into the air. Instead, they form in the atmosphere when primary pollutants interact with each other, or with sunlight. The best examples are ground-level ozone and acid rain. Some materials may be both primary and secondary.

What Are the Main Types & Effects of Man-Made Air Pollutants?

Air pollution caused by human activities includes the following pollutants:

Carbon Dioxide (CO2)

Carbon dioxide is the most dangerous of Earth’s long-lived greenhouse gases and by far the biggest contributor to global warming. It captures less heat per molecule than other greenhouse gases like methane or nitrous oxide, but there’s a lot more of it, and it remains active in the atmosphere for much longer.

Present-day atmospheric levels of CO2 are around 414 parts per million (ppm) of earth’s atmosphere. This is an increase of about 47 percent over the 280 ppm levels of pre-industrial times. Roughly 38.0 billion tonnes of CO2 were pumped into the atmosphere in 2019, as a result of fossil fuel combustion. This figure represents a 66 percent increase over emissions in 1990. 6

Methane (CH4)

Methane comes from livestock agriculture and waste, wetlands, fossil fuel industries, biomass burning and permafrost thawing. It has a shorter lifespan than CO2, but it’s more powerful. Its 20-year global warming potential is 84, which means that over a 20-year period it traps 84 times more heat escaping from the planet’s surface, than carbon dioxide.

There is little difference between methane and natural gas, so it is extremely flammable and may, in the right quantity, form an explosive mixture with air.

Since pre-industrial times, atmospheric levels of methane have increased two and a half times, from 722 ppb (parts per billion) to 1863 ppb (August 2019) – the highest level of atmospheric methane for 800,000 years. 7

On the Indo-Gangetic plain and eastern China, a combination of farming and industrial power plants have combined to produce significant emissions of methane, as have the swampy Sudd wetlands of southern Sudan. For more, see: Why are Methane Levels Rising?

In 2014, in the Four Corners region of the southwestern United States, SCIAMACHY – one of ten instruments on board the ESA’s Environmental Satellite, ENVISAT – observed a methane cloud about the size of the state of Delaware. Detected quite by chance over the San Juan Basin, it was the largest methane gas cloud ever detected by satellite in North America.

Sulfur Dioxide (SO2)

Sulfur dioxide is produced from the combustion of coal and petroleum and the smelting of mineral ores (aluminum, copper, iron, lead and zinc) that contain sulfur. Sulfur dioxide dissolves easily in rainwater to form sulfuric acid, making it a key component of acid rain. When combined with VOCs and ground level ozone, or alternatively with ammonia emissions from farms, it forms urban smog. For more, see: The Sulfur Cycle: A Simple Guide.

Nitrogen oxides (NOx)

Nitrogen oxides are a cluster of seven gaseous compounds composed of nitrogen and oxygen, often referred to as NOx gases. They include nitrogen dioxide (NO2), mostly emitted during high temperature combustion of fossil fuels, and nitrous oxide which is released during agricultural fertilizer-related activities, and during the burning of fossil fuels and solid waste.

Nitrogen and sulfur oxides also react with water vapor in the air to form nitric and sulfuric acids, while nitrous oxide (N2O) combines with oxygen in the air to form nitrogen dioxide (N02), a major component of smog. Nitrous oxide is a very powerful greenhouse gas which traps about 265 times more heat than CO2 over a 100-year period. For more, see: The Nitrogen Cycle: How Does It Work?

Carbon Monoxide (CO)

Carbon monoxide is produced during the burning of fossil fuels in a low-oxygen environment. Exhaust fumes from vehicle engines are the biggest source of carbon monoxide in the atmosphere. Other sources include certain industrial processes, and the burning of wood. It also occurs naturally in the atmosphere in minute quantities. Carbon monoxide is toxic and sometimes fatal. It combines with other primary pollutants (e.g. VOCs and nitrous oxide) and sunlight to form photochemical smog.

Volatile Organic Compounds (VOCs)

VOCs are well-known outdoor and indoor air pollutants. A number of them are produced as a result of incomplete combustion of fossil fuels, as well as from solvents and paints. They include a wide variety of chemicals, some of which have short- and long-term health-effects.

VOCs are commonly encountered outdoors in the form of urban smog. Indoors they are usually experienced in the form of fumes from household products such as paint, varnish, waxes, wood preservatives, air fresheners, hair spray, aerosol sprays and solvents. Concentrations of many VOCs are consistently up to ten times higher indoors than outdoors. One reason why VOCs have been linked to sick building syndrome.

In a recent study, researchers found that 57 percent of the volatile organic compounds (VOCs) they measured in the air of a university lecture theatre, was emitted by people and their possessions. The most predominant VOCs identified in the occupied lecture theatre were cyclic volatile methyl siloxanes, found in many personal care products, notably antiperspirants. The amounts found in the lecture theatre were much higher than those typically measured outdoors. 8

The aromatic VOCs benzene, toluene and xylene may be carcinogenic and may cause leukemia with prolonged exposure. Another hazardous VOC is 1,3-butadiene. This is produced during the processing of petroleum and is used in the production of synthetic rubber. Lesser amounts are found in plastics and fuel.


Dioxins are a family of chemicals (CDDs, CDFs, certain PCBs) generated by the combustion of chlorine-containing compounds. Dioxins are created during combustion processes in waste incineration (commercial or municipal), milling and smelting, or from burning fuels (coal, oil, wood). These are the main sources of dioxin-related air pollution.

Dioxins are known as persistent organic pollutants (POPs), meaning they take a long time to break down once they are in the environment. They are highly toxic and can cause cancer, reproductive problems, damage to hormones and to the immune system. The CDD 2,4,5-T was a component of Agent Orange, the defoliant which was used extensively by the U.S. military in the Vietnam War.

Particulate Matter (PM)

The term particulate matter (PM) (or particulates) is used to describe a mixture of solid particles and liquid droplets found in the air. Or, tiny particles of solid or liquid suspended in a gas. Some particles – like coal fly ash, smoke, dust, sea salt, soot, black carbon, or droplets of acid, are large enough to be seen with the naked eye. Others are so tiny they can only be detected using a microscope.

Particulates are classified according to size. PM10 refers to inhalable particles, with diameters of 10 microns or less. (Note: 1 micron = 1 millionth of a meter. A human hair measures about 70 microns in diameter.) PM2.5 refers to particles, with diameters of 2.5 microns or less.

These particles vary significantly in size and shape and may comprise hundreds of microscopic flecks of solid or liquid material.

Where Do Particulates Come From?

Like any pollutant, PM may be emitted directly into the air from a chimney, a vehicle exhaust pipe, or a forest fire. Or it may be carried away by the wind from a breaking wave (sea-salt), or building site (concrete dust), or lake bed (mineral dust), or field (fertilizer). Or it may be shot into the air by a volcanic eruption.

Alternatively, it may be created in the atmosphere as a result of a reaction between chemicals which are emitted directly into the air.

Why is particulate matter so unhealthy? Because any PM less than 10 microns in diameter can penetrate deep into your lungs and may even pass into your bloodstream. In 2016, for example, a study identified a series of microscopic particles lodged inside the brains of 37 people who had lived in heavily polluted cities in Britain and Mexico. The particles were linked to Alzheimer’s disease. 9

One particularly dangerous chemical particulate is lead, a heavy metal. Airborne lead used to come mainly from car tailpipes, but now that petrol is mostly lead-free, the main sources are industrial processes such as smelting, and – indoors – lead-based paint, which is still found in many older houses. Lead is especially hazardous because ingesting lead particles can lead to brain damage.

Other airborne heavy metals, such as mercury and zinc, also come from smelting and, to a lesser extent, old waste incinerators (the newer facilities are almost emission-free). Heavy metal particulates eventually settle out as nonpoint source pollution, mostly in soils in the area surrounding the emitting source.

Radioactive Pollution

Nuclear radiation emitted accidentally by nuclear power plants is too hot a subject to handle for the governments, plant owners and regulatory authorities involved. So, nobody tells the truth because they’re afraid the public will panic.

Keeping this in mind, there have been only three major nuclear alerts: Three Mile island, Chernobyl and Fukushima.

Fukushima Daiichi Nuclear Plant, Japan. (2011)
A release of radiation from the Tsunami-damaged plant contaminated a wide area around the plant and forced the evacuation of half a million residents. On 24 May 2012, more than one year after the disaster, the Tokyo Electric Power Company (TEPCO) stated that approximately 538 petabecquerels (PBq) of iodine-131, caesium-134 and caesium-137 were released from the Fukushima Daiichi Nuclear Plant. (Note: 1 petabecquerel = 1 thousand, million, million becquerels.)

Chernobyl Nuclear Plant, Ukraine. (1986)
Massive amounts of radiation escaped across the western Soviet Union and Europe. As a result of the disaster, some 220,000 people had to be relocated from their homes. Scientists estimate that four hundred times more radiation was emitted from Chernobyl than from the combined bombing of Hiroshima and Nagasaki in 1945.

Three Mile Island Nuclear Plant. USA. (1978)
The plant escaped a core meltdown by the skin of its teeth. About 140,000 people were temporarily evacuated. The exact amount of radioactive air pollution released, is unknown.

For more, see: Is Nuclear Energy a Replacement for Fossil Fuels?

Air Pollution in Europe Levels: Chart
Share of the EU urban population exposed to air pollutant levels in excess of EU and WHO thresholds (2015-2017). The difference in exposure is because the WHO has stricter air quality standards than the EU. Image: European Environment Agency. 10


Urban smog is a secondary pollutant which comes in two basic forms: classical or photochemical smog. Classical smog usually occurs in urban areas with a humid climate. It is a combination of smoke particulates from burning coal, plus fog and sulphur dioxide.

In contrast, photochemical smog is typically made up of particulates emitted by car exhausts, plus ground level ozone, which are acted upon by ultraviolet light from the sun. (The ozone is another secondary pollutant resulting from a mixture of NOx and VOCs.)

Photochemical smog also results from a mixture of ammonia, plus NOx from cars, plus sunlight. For example, researchers analyzing urban haze in Salt Lake City, in 2017, discovered it mostly consisted of tiny particles, less than 2.5 microns in diameter (PM2.5). Some particles were smoke, soot or dust, but roughly three-quarters consisted of ammonium nitrate. This air pollutant results from a reaction between nitrogen oxides (from vehicle and industrial emissions) and ammonia blown into the air from farms that overuse ammonia-based fertilizers or generate large amounts of animal manure. 11

China too has experienced severe haze problems caused by particulates of an ammonium compound – in her case it is ammonium sulfate. This particulate matter is created when sulfur dioxide emissions from coal-burning power plants combine with ammonia emissions from heavy-handed use of ammonia fertilizers. 12

Ground Level Ozone (O3)

Ground level ozone is formed from NOx and VOCs. In high concentrations, caused largely by the combustion of fossil fuels, it is a pollutant and a key component of photochemical smog.

Acid Rain

Acid rain has a pH less than 5. (For comparison, the pH of battery acid is 1; lemon juice, 2; vinegar, 3.) However, it’s worth noting that all rain is rather acidic; the pH of “natural” rain, for example, is about 5.6.

Rainwater is naturally acidic because there are gases in the air, notably C02, which mix with water vapor to create acids. Carbon dioxide reacts with water vapor to make carbonic acid (HC03). Groundwater is naturally slightly acidic for the same reason. Rainwater will also become more acidic (pH 3-5) if it mixes with smoke from a bushfire or ash from a volcano. Acid rain is a type of acid deposition, which can be wet or dry, liquid or solid.

Acids are formed in the atmosphere because acid precursors in the air react with water. The most common of these chemicals are carbon dioxide, sulfur oxides (SOx) especially sulfur dioxide (SO2), and nitrogen oxides (NOx). These chemicals occur naturally, but their concentrations have increased significantly because of anthropogenic emissions.

The amount of man-made sulfur in the air, for instance, is now thought to exceed the total amount of sulfur arising from natural sources, including volcanic eruptions.

Man-made CO2 and SO2 both come from the burning of fossil fuel, mostly coal. The smelting of sulfur-bearing ores is another major source of sulfur emissions. Nitrogen oxides are also emitted from power stations and from vehicle emissions; natural sources of NOx include forest fires, soil gases, and animal waste.

Once in the atmosphere, these aerosols are carried by the wind and oxidized. If they fall to earth quickly, they become acidic when they react with moisture on the ground.

The rest of the airborne particles react with water vapor in the atmosphere. Thus, carbon dioxide becomes carbonic acid (HC03), sulfur dioxide becomes sulfuric acid (H2S04), and nitrogen oxides become nitric acid (HN03). Depending on weather conditions and circulation patterns, these acidic pollutants can remain in the atmosphere for up to two weeks.

To recap. The constituents of acid rain are airborne aerosols resulting mainly from fossil fuel combustion. (See also: Environmental Effects of Fossil Fuels.) They form acids through oxidation and chemical interaction with water vapor in the atmosphere. The resulting acids eventually fall to the ground as either dry or wet deposition.

The areas hardest hit by acid precipitation tend to be downwind of heavily industrialized areas that use large amounts of fossil fuels. Such areas include the northeastern United States, southeastern Canada, most of eastern Europe from Poland northward into Scandinavia, and the Indian subcontinent.

In 2006, according to the Xinhua News Agency, one-third of China including Beijing was affected by acid rain. It is still an endemic problem in the country, which is suffering the effects of rapid growth in industrialization. China produces the world’s highest emissions of ammonia, but climate models are showing that ammonia emission control might aggravate acid rain pollution. 13

Acid rain harms both natural and human systems. Acids tend to speed the degradation of paint, plastics and rubber, as well as masonry, cement, steel, and building stone. Acid precipitation has negative impacts on trees, rendering them more susceptible to the effects of cold and disease. At the same time, soils and surface waters can become acidified. In lakes, the acidification of water can lead to reproductive failure and death in fish.

The degree of acidification experienced by lakes and streams varies according to their underlying rocks, sediments, and soils. Some surface waters are particularly vulnerable to acidification; others less so. Soils rocks and other materials that have the chemical composition to neutralize acids are known as buffers. Carbonate-rich materials, such as chalk, limestone and similar soils, for instance, are efficient buffers that help a water body to resist acidification. In contrast, lakes that are underlain by granites, which do not act as buffers, are very sensitive to acidification.

Some toxic metals — especially aluminum, lead, and mercury – are more soluble in more acidic water, which means that the concentration of these toxic metals usually rises in line with a lake’s acidity. This may be a reason for the death of fish in acidified lakes.

In some cases, an acidified lake can be improved by adding buffers such as lime. It is a temporary and expensive answer that must be repeated at regular intervals.

Acid rain has a less harmful effect on oceans as a whole, except in shallower coastal waters. Ocean acidification makes it more difficult for coastal species – such as crustaceans, lobsters, shrimp, snails and crabs – to create the exoskeletons they need to survive.

Air Pollution in Cities

In addition to photochemical smog resulting from photochemical reactions in the troposphere on sunny days, city dwellers also suffer the artificial warming effects of large expanses of asphalt, concrete, and other artificial surfaces. These materials absorb and re-radiate the sun’s energy upwards. Warming is further increased through the release of heat from vehicles, factories, and heated buildings.

In addition, airborne pollutants – typically found in higher concentrations in city environments – directly absorb and re-radiate the sun’s heat. As a result, air temperatures in urban areas tend to be significantly higher than those in the surrounding countryside – a phenomenon known as the urban heat island effect.

Tall buildings promote air turbulence as well as the convection of heat above the city, while pollutants provide nucleation sites for the formation of water droplets in clouds. This causes increased cloudiness and more frequent rainfall. In addition, the typical pattern of air circulation in an urban heat island, tends to trap particulate matter above the city, contributing to a bad air quality syndrome known as a dust dome.

Weather and topography sometimes amplify local air pollution. Normally, air temperature in the lower part of the atmosphere decreases the higher you go. However, sometimes, when a warm air mass moves into the area, it can trap a pocket of cool air underneath it — a phenomenon known as a thermal inversion.

Thus, when warm, contaminant-ridden gases are emitted from factories or vehicles, instead of rising higher and higher into the sky as normal, the polluted air rises until it hits the overlying warm air mass, then spreads out laterally polluting the nearby city boroughs. Most chronic episodes of smog and acute air pollution are caused by the presence of thermal inversions. Local topographic features such as valleys and basins can intensify the effects of a thermal inversion. Because Mexico City and the Los Angeles, for example, are built on geographic basins that trap cool air and concentrate pollutants, they suffer from chronic air pollution.

Asian Brown Cloud, Satellite View
The so-called Asian brown cloud, shown here over northern India and Bangladesh. This haze is caused by vast amounts of aerosols (like soot, black carbon, dust) produced by the incomplete combustion of coal and biomass across the region. Photo: Jesse Allen/ Earth Observatory/ Modis / NASA.

Long Range Transport of Air Pollutants (LRTAP)

The long-distance transportation of atmospheric pollutants is becoming a major problem as industrialization grows and climate change intensifies. Known as LRTAP, it is defined by the European Environment Agency as the “atmospheric transport of air pollutants within a moving air mass for a distance greater than 100 kilometres”.

These pollutants include particulates of lead, mercury, sulfur, NOx, hydrocarbons and other compounds emitted from industry and vehicles, as well as carbon dioxide, black carbon, soot and dust released by forest fires.

Windblown contaminants of this sort can be transported for thousands of kilometers across continents, oceans, hemispheres, or even around the globe. Their behavior is influenced by numerous factors, including their own chemical characteristics and local topography, as well as prevailing weather patterns and conditions.

The pollutant’s own characteristics will determine how long it remains in suspension in the atmosphere. Wind systems and rainfall are also important. When the air is still, pollutants are not transported far.

Likewise, when rainfall occurs it washes out the particulate matter from the atmosphere. The lack of rainfall is a key contributor to the atmospheric phenomenon known as the Asian Brown Cloud, which occurs over eastern China and southern Asia during the dry season, between December and March. Composed of black carbon, soot, dust and industrial pollution, the formation of the brown cloud is helped by the absence of rainfall that would otherwise wash it out of the sky.

The height of the emissions source (chimneys etc) is also relevant. Pollutants emitted from very tall smoke stacks can remain in the atmosphere a long time and may be transported hundreds of kilometers before falling to earth.

LRTAP in East Asia

As more and more companies shift their manufacturing operations to China, LRTAP has become a major concern for the rest of East Asia. Studies show numerous examples of aerosols emitted in China that cause episodes of severe air pollution in South Korea. In certain weather conditions, air pollutants released in eastern China and southwestern Manchuria are gradually spread into neighboring countries by weak westerly winds. 14

Japan is also badly affected by natural and anthropogenic pollution coming from the East Asian mainland. For example, dust particles from arid areas in China and Mongolia carried eastwards by prevailing westerlies, are often observed in Japan during the spring. In addition, in recent decades, a marked increase in the consumption of fossil fuels (notably coal) has led to a marked increase in severe air pollution from the East Asian mainland, including mutagens, such as polycyclic aromatic hydrocarbons (PAHs). 15

LRTAP in Europe and the USA

One of the most notorious examples of LRTAP occurred following the nuclear disaster in Chernobyl, Ukraine, in April 1986. In the wake of the disaster, radioactive particles encircled the globe within two weeks, much of the contamination eventually settling out as dry deposition (settling to the ground as dry, solid particles).

Another problem linked to LRTAP is arctic haze. This red-brown mist, which typically occurs in the lower atmosphere during the arctic winter, is made up of fine particles of dust, soot, sulfates, hydrocarbons, metals, and pesticides. It is caused by the transport of air pollutants from sources in Europe and Asia, and it has significant negative impacts on sensitive arctic ecosystems and on global warming. Researchers believe that this long-range transport is the main reason for the presence of toxic contaminants such as mercury in remote Arctic lakes and wildlife populations.

Aside from Chernobyl and Arctic haze, the United States and Europe have both benefited hugely from the Long-Range Transboundary Air-Pollution treaty which they signed in 1979. It is probably the single most effective piece of international anti-pollution legislation yet passed.

Natural Airborne Pollution

The two most common sources of natural air pollution are: wildfires and volcanic activity. Forest fires release a wide range of pollutants, from particulate matter and acrolein (a respiratory irritant) to cancer-causing toxins such as formaldehyde and benzene. 16 Volcanoes emit sulfur dioxide, carbon dioxide, hydrogen fluoride, chlorine, and ash particulates.


As forests and bushlands become tinder-dry due to record-breaking heatwaves, wildfires are easily ignited. These types of blazes have been happening with increasing frequency throughout the world.

During 2019-20, for instance, Arctic fires have raged across Siberia, Australian bushfires have burned up large parts of Victoria and New South Wales, while huge tracts of forest in California and Oregon (including unique redwood forests) have gone up in smoke. As a result, vast amounts of soot and black carbon pollutants have been released into the atmosphere, much of which was transported by the wind for thousands of miles before dropping to earth.

The 2019-20 Australian bushfires, for example, emitted a gigantic plume of pollution that drifted 11,000 kilometers (6,800 miles) across the Pacific to South America. And according to the World Meteorological Organization, the 2019 fires in Siberia emitted smoke clouds that covered more than 5 million square kilometers – greater than the size of the European Union (EU). These clouds contain huge amounts of tiny microscopic particles (including PM10 and PM2.5) that can penetrate the lungs.

In addition to the respiratory and other health dangers posed by these tiny particles, the carbon dioxide emitted helps to ramp up the greenhouse effect which drives global warming.

Volcanic Activity

Gases emitted by volcanoes that pose the greatest danger are sulfur dioxide, carbon dioxide, and hydrogen fluoride. Other toxic material includes ammonia and vanadium, as well as ash particulates.

The sulfur gases mix with the water vapor released by the eruption to form sulfurous and sulfuric acid. This forms vog (volcanic smog) a toxic mixture of acid particulates and water vapor that causes breathing problems.

If volcanic plumes of acid pollutants reach the stratosphere, they can form a persistent haze of liquid droplets, reflecting away incoming solar radiation and cooling the earth for a year or two.

The volcanic ash emitted is gritty, abrasive, and sometimes corrosive. It’s not especially toxic, but it affects infants, the old, and anyone with breathing difficulties. Ash can also damage drinking water and wastewater treatment facilities by clogging equipment.

According to the U.S. Geological Survey (USGS), the world’s volcanoes, terrestrial and undersea, emit around 200 million tons of carbon dioxide (CO2) annually. This amounts to less than half of one percent of anthropogenic emissions, but they still help to raise global temperatures and boost climate change.

Air Quality Index & Air Quality Standards

Air quality is now regulated in many (but far from all) countries. Air quality standards vary significantly between the World Health Organization, the United States, Europe and the rest of the world. In terms of legislation, the U.S. Clean Air Act is a useful global model.

To begin with, outdoor air pollution in the United States is measured by the Air Quality Index (AQI), which ranks air conditions across the country according to five pollutants: ground-level ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide and particulate matter (particle pollution).

In addition, the Clean Air Act requires the U.S. Environmental Protection Agency (EPA) to set National Ambient Air Quality Standards (NAAQS) for six “criteria” pollutants in outdoor ambient air. These are: lead, carbon monoxide, sulfur dioxide, nitrogen dioxide, particulate matter and ground-level ozone.

The Clean Air Act also regulates threats to air quality from a number of other sources, including: (a) Hazardous or toxic air pollutants that pose life-threatening health risks such as cancer, or environmental threats from the bioaccumulation of heavy metals. (b) Acid rain. (c) Emissions of chemicals that deplete the stratospheric ozone layer, that safeguards us from cancer-causing UV rays.

Individual states are obliged to adopt enforceable plans to meet the federal air quality standards. They must also control emissions that drift into neighboring states.

Other regulatory provisions are aimed at minimizing air pollution emissions from increasing numbers of cars and trucks, and from new factories and industrial plants. New power plants and manufacturing facilities, for example, are required to use the best available technology, while existing plants are governed by less stringent standards.

How Does Air Pollution Affect Health?

The health effects of air pollution are often hard to pin down. Establishing a specific causal link between pollution and a fatal lung disease isn’t always possible. What’s more, deaths caused by air pollution raise unwelcome questions. Environmental and pollution regulations are even described as anti-business rather than pro-health.

But one thing is clear. Air pollution is becoming a major global health problem. According to the World health Organization (WHO), it causes around 8 million premature deaths annually. Of these, roughly 4.2 million deaths are caused by outdoor air pollution, 3.8 million by indoor pollutants. 17 18

A more recent study, however, which uses the new Global Exposure Mortality Model (GEMM) – offering much better assessments of the effects of fine particulate matter (PM2.5) – concludes that ambient (outside) air pollution alone is responsible for 8.8 million deaths annually – more than twice the WHO figure of 4.2 million. 19

The study’s findings are very similar to another recent study (2018) which reported a total of 8.9 million premature deaths from outdoor airborne pollutants. 20

Irrespective of the exact number of fatalities, it’s clear – as any pollution map can confirm – that the major burden of air pollution, along with its health effects, is being carried by Asia and (to a lesser extent) by Africa. This is largely because their development lags behind the West’s, and because the West has shifted most of its manufacturing into developing countries. Nearly all the world’s most polluted cities, for example, are in India and China. 21

The two main airborne pollutants are (a) particulate matter (microscopic particles of dust, soot, or chemical compounds), and (b) Ozone (a mix of NOx gases, methane, carbon dioxide, carbon monoxide, and VOCs). Ozone is mostly seen as a constituent of photochemical smog, which basically consists of ozone + particulates.

Chart: Sources of Fine Particles Air Pollution
Sources of fine particulates in a semi-urban environment. Image: © Puraka Clean Air Research.

Particulate Matter

Particulates and aerosols are associated with increased risk of cerebrovascular disease, cardiovascular disease, chronic kidney disease, chronic obstructive pulmonary disease (COPD), type 2 diabetes, hypertension, lung cancer, pneumonia, dementia.

According to the “State of Global Air (2019)” a key report compiled by the Health Effects Institute (HEI), total air pollution (from particulate matter, ozone, and household air pollution) contributed to more than 5 million deaths globally — nearly 1 in every 10 deaths — in 2017. 22

Ozone/Smog Haze

In the short-term, photochemical smog irritates the eyes, throat and lungs of children, seniors and those who work outside. People with asthma or allergies suffer worst of all, since smog can trigger asthmatic or allergy reactions. Long-term exposure to ozone is linked to serious respiratory diseases as well as increased risk of fatality from chronic obstructive pulmonary disease (COPD).

Constituents of smog and ozone vary around the world. In some cities, for example, ammonia is a key component of smog; in others, sulfur dioxide.

According to the “State of Global Air (2019)” long-term exposure to ozone resulted in almost 500,000 deaths from COPD worldwide in 2017.

Household (Indoor) Pollution

According to the World Health Organization (WHO), household air pollution accounts for 3.8 million deaths annually. 23

In developing countries, the main source of indoor pollution is inefficient cooking equipment, such as open fires or crude stoves, using solid fuels (wood, coal, biomass). This sort of cooking environment – still used by as many as 3 billion people – produces dangerous amounts of fine particulates (PM2.5 or smaller) that are inhaled into the deepest part of the lungs.

In developed countries, the amount of indoor household pollution has been dramatically cut. According to the European Environment Agency, the main household air pollutants in Europe include: tobacco smoke, radon gas emissions, particles from burning fuels, volatile organic compounds, and asbestos. 24


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