Air Pollution Causes

While air pollution causes may be both natural and human-made,  it is undoubtedly the human activity that is the largest source of atmospheric pollution.

So what are the main causes of air pollution?

The largest amounts of air pollution come from human activity though there are some natural sources as well.

Fossil fuels are no doubt the most important source of harmful atmospheric emissions.

We cannot (just yet!) imagine our lives without fossil fuels.

pollution causes, bus, car
Cars & Buses: Major Causes of Air Pollution © Doron Laor

They are used in virtually every area of human life starting from gasoline for our cars to a multitude of applications in industrial production, agriculture, etc.

But fossil fuels are not the only culprits. There are other pollution sources, ex. processes used to produce non-ferrous metals, which also cause air pollution.

In one word, anthropogenic (human-induced) causes are far more important than natural ones in raising the current levels of atmospheric pollution dangerously high.

We discuss major air pollution causes below.

Anthropogenic Air Pollution Causes

Air Pollution Causes: Fundamental Causes (Ref. 1)

Some experts now draw our attention to several global trends such as industrialization, population growth & globalization – which form the basis of all pollution including air pollution.

We refer to these trends as fundamental pollution causes – please feel free to learn more about them here.

Air Pollution Causes: Industries

The global industrial development gave rise to a great number of economic sectors, with each generating air pollution to some degree or another.

So these economic sectors act as pollution causes in their own right.

Below we list some important sectors and types of air pollutants produced by each of them:

Table 1: Some Important Air Polluting Sectors (Ref. 2)
Air Pollutants Emitted
Biofuel Combustion
Mostly CO;
also SO2 and NOx
Industry & Refineries
Mostly SO2 and NOx;
also CO
Power Generation
Mostly SO2 and NOx;
also CO
Residential and Commercial Sector
Mostly SO2 and CO;
also NOx
Road, Rail, Air & Other Transport
Mostly NOx and CO;
also SO2

See the breakdown of other air polluting sectors here.

Air Pollution Causes: Air Pollutants

Air pollutants are basically the waste products generated by the above mentioned economic sectors.

They come in the form of gases and finely divided solid and liquid particles suspended in the air (aerosols).

Air pollutants can also be of primary or secondary nature.

air pollution causes, green field
Air Pollution Over Green Field © Ken Douglas

Primary pollutants are the ones that are emitted directly into the atmosphere by the sources ( ex., power plants).

Secondary pollutants are the ones that are formed as a result of reactions between primary pollutants and other elements in the atmosphere.

Air pollutants are direct pollution causes, in other words they are the actual pollution agents which directly affect the health of living beings as well as the wider environment.

We discuss the most important air pollutants below.

Sulfur Dioxide (SO2)

Sulfur dioxide (SO2) is a colorless gas with a pungent, suffocating odor. SO2 is corrosive to organic materials and it irritates the eyes, nose and lungs; therefore it is quite a dangerous air pollutant. (Ref. 3)

Also Check Out →  Pollution And Tropical Rainforests

Sulfur is contained within all fossil fuels, and is released in the form of sulfur dioxide during fossil fuel combustion. Fossil fuel combustion accounts for almost all anthropogenic sulfur emissions. (Ref. 4)

Table 2: Top 5 Sectors for Global SO2 Emissions in Gg, Year 2000 based on EDGAR Data (Ref. 5)
SO2 Emissions, Gg
Power Generation
Industry (excl. refineries)
Non-ferrous metals
Transformation Sector (incl. refineries)
Residential & Commercial Sector

Power generation (public electricity & heat production) is by far the largest industry for global SO2 emissions.

pollution causes, coal burning
Coal Burning © Marcus Vega

Power plants require large amounts of energy for their operations.

This use of energy by power plants produces emissions of a whole cocktail of air pollutants including greenhouse gases (causing global warming) and non-greenhouse gases.

It is worth singling out coal as the dirtiest fossil fuel of all; it is still widely used as a source of energy by power plants.

Coal combustion is a very serious source of sulfur dioxide emissions.

But in addition to that, it produces a number of greenhouse gases and other waste products such as arsenic, lead, mercury etc.

Industry (manufacturing industries & construction) is the second biggest source of sulfur dioxide emissions globally.

It involves the use of energy for the manufacture of industrial products (ex., iron, steel etc) as well as consumer goods, which releases SO2 into the atmosphere.

Non-ferrous metals’ production is the third largest source of SO2 emissions. Some examples of non-ferrous metals are aluminium, copper, lead, zinc, gold.

The manufacture of non-ferrous metals generates SO2 emissions via:

  • the use of fossil fuels, and
  • the specificity of the industrial process involved

The industrial process we are talking about involves the heating of sulfide ores in the air and results in the separation of non-ferrous metal and the sulfur from the ore.

The separated sulfur combines with oxygen in the air, turns into sulfur dioxide and thus becomes a source of SO2 emissions.
(Ref. 6)

Nitrogen Oxides (NOx)

Nitrogen oxides (NOx) are produced by combustion of all fossil fuels including coal- and gas-fired power stations and motor vehicles. (Ref. 7)

There are two main nitrogen oxides: nitric oxide (NO) and nitrogen dioxide (NO2).

While NO is a colorless gas, NO2 is a gas of reddish-brown color with a distinct sharp, biting odor.

Fossil fuel combustion produces both NO2 and NO.

But almost 90% of the total NOx combustion product is released in the form of NO which is then converted to NO2 in the air. (Ref. 8, 9)

Table 3: Top 5 Sectors for Global NOx Emissions in Gg NO2, Year 2000 based on EDGAR Data (Ref. 10)
NOx Emissions, Gg NO2
Power Generation
Deforestation & Savannah Fires
Industry (excl. refineries)

Road transport is the biggest global contributor of nitrogen emissions produced by motor fuel combustion.

US, China, Brazil, India and Russia are the largest producers of global nitrogen emissions. (Ref. 11)

Power generation is indeed another top producer of nitrogen emissions.

For example, in the US power plants are responsible for about a quarter of all nitrogen oxides emitted in the country every year. (Ref. 12)

air pollution causes, deforestation burma
Deforestation in Burma © Isabel Esterman

While rainforest destruction is much better known for being one of the largest causes of carbon dioxide emissions, it (alongside savannah fires) is also a significant source of nitrogen dioxide.

Also Check Out →  Air Pollution Effects

Most of the deforestation is now taking place in tropical countries where rainforests are routinely cleared for a number of reasons, ex. cattle ranching, crop plantations, etc.

Carbon Monoxide (CO)

Carbon monoxide is a highly toxic gas which has no color, odor or taste.

Fossil fuel combustion normally produces carbon dioxide (CO2) but sometimes, when such combustion is incomplete it also becomes a source of carbon monoxide. (Ref. 13)

Table 4: Top 5 Sectors for Global CO Emissions in Gg, Year 2000 based on EDGAR Data (Ref. 14)
CO Emissions, Gg
Deforestation & Other Wildfires
Biofuel Combustion
Residential & Commercial Sector
Agriculture Waste Burning

Deforestation by means of fires (plus other wildfires) has become such a serious global problem that it is now the biggest source of carbon monoxide emissions.

The way biofuel combustion causes CO emissions is similar to that of wildfires’.

Biofuel is used by some power plants and road transport in developed countries, but mostly as a source of energy for residential purposes in developing regions of the world, with Africa and Asia being the biggest biofuel users. (Ref. 15)

Road transport is another major source of carbon monoxide both in developed and developing countries.

It is the largest producer of carbon monoxide emissions in developed countries. (Ref. 16)

Ammonia (NH3)

Ammonia is a pungent, hazardous caustic gas.

Agriculture, specifically livestock farming & animals waste, is the main source of ammonia emissions.

Ozone (O3)

Ozone (O3) is a colorless, poisonous gas with a sharp, cold, irritating odor. (Ref. 17)

It can be found in: (Ref. 18, 19)

air pollution, los angeles smog
Los Angeles Smog © Brooke Ganz
  • the stratosphere (upper layer of the atmosphere) where it occurs naturally, and
  • the troposphere (lowest layer of the atmosphere) where it occurs both naturally and as a product of anthropogenic emissions.

Stratospheric ozone protects the Earth by keeping harmful ultraviolet sunlight from reaching the planet’s surface.

However, human-induced tropospheric ozone is a secondary pollutant produced by the reaction of primary pollutants, nitrogen oxides and hydrocarbons, in the presence of sunlight. (Ref. 20)

Tropospheric ozone is one of the main components of the photochemical smog which is harmful to human and animal health.

Other Air Pollutants

Airborne Particles are tiny fragments of solid or liquid nature suspended in the air (aerosols).

air pollution causes, particles, coal dust
Particulate Pollution: Coal Dust © Paul Vitucci

They may be primary – when emitted directly into the atmosphere by sources (ex., road transport & power plants), or secondary – when particles are formed in the atmosphere through the interaction of primary pollutants.

Inhalation of airborne particles may lead to asthma, lung cancer, and other problems.

Volatile Organic Compounds (VOCs) may include a wide range of organic air pollutants, from pure hydrocarbons to partially oxidized hydrocarbons to organic compounds containing chlorine, sulfur, or nitrogen.

VOCs may affect human health directly (ex., leukemia) or indirectly as contributors to the formation of tropospheric ozone, with all the negative effects of ozone on human health and the environment.

Persistent Organic Pollutants (POPs) are compounds which are resistant to degradation and persistent in the environment and may include dioxins, furans, polychlorinated biphenyls (PCBs) and organochlorine pesticides such as DDT.

Exposure to POPs takes place through diet (ex., animal fat consumption), environmental exposure or accidents and may lead to cancers, neurobehavioral disorders, and other illnesses.

Natural Air Pollution Causes

Natural air pollution causes are mostly forest fires and volcano eruptions but may also include vegetation (ex., forests), oceans and decay processes in soil.