Pollution types


Pollutants occur as gases (e.g. carbon monoxide, nitrogen dioxide, sulfur dioxide, ozone, VOCs such as benzene and formaldehyde) and PM. In air pollution, PM refers to solid and liquid particles suspended in air, and the PM and air mixture is referred to as aerosol. PM with recognised human health impacts is PM10 and PM2.5. Pollutants can be primary or secondary (Table ATM3). Many gaseous and particulate pollutants are emitted from combustion in vehicle engines, industrial processes and domestic wood heaters.

Primary air pollutants are emitted directly into the air from a source. They can have direct impacts or be precursors for secondary air pollutants (formed through reactions in the atmosphere), as discussed below.

PM10 and PM2.5 are complex mixtures of particles with different sizes and chemical components; their formation is influenced by multiple sources and processes. Primary particles are emitted directly to the atmosphere and are generally large, so contribute most to PM10. Natural sources of PM are processes that occur naturally in the Earth system (e.g. bubbles bursting on the sea surface to produce sea salt particles, erosion that generates dust, naturally lit bushfires that generate smoke). Anthropogenic sources include:

  • dust associated with agricultural, mining and urban developments
  • traffic-related suspension of road particles
  • smoke from bushfires, prescribed burning and household wood heaters
  • emissions from vehicle exhaust, industrial processing and commercial activities
  • spray drift from aerial application of agricultural and horticultural chemicals.

Secondary pollutants (such as ozone) are not directly emitted by a source, but result from the chemical reactions of primary pollutants (such as oxides of nitrogen and VOCs), often in the presence of sunlight (photochemical reactions)—they therefore predominate during the warmer months. Secondary particles are formed by chemical reactions in the atmosphere that convert gases to particles; these are also often referred to as secondary aerosols. These conversions lead to the production of large numbers of very small particles (nucleation) and the growth in size of existing particles (condensation).

It is worth noting that every location has a certain level of naturally occurring air pollution, which defines the background air quality. The sources can be distant sources, because pollutant gases and fine particles can travel hundreds or thousands of kilometres. They can also be local, such as the emissions of VOCs from eucalypts, which are a precursor to the formation of secondary particles that scatter light and cause the Blue Mountains to be ‘blue’. Control strategies can do very little to abate background sources of natural air pollution.

Pollen and fungal spores are other airborne pollutants that can have adverse health impacts. When inhaled, proteins and glycoproteins associated with pollens can interact with the immune systems of sensitive individuals to produce an allergic response in the form of hayfever or allergic asthma (e.g. Simpson et al. 2005).

Table ATM3 Major sources of air pollutants and particulate matter (PM)

Type of pollutant


Major sources

Primary pollutants

Carbon monoxide

  • Combustion, including biomass (vegetation) burning in domestic wood heaters, prescribed burns and bushfires, motor vehicles and metal manufacturing


  • Road dust, metal manufacturing and metal ore mining

Nitrogen dioxide (NO2) and nitric oxide (NO), generalised as NOx

  • Combination of nitrogen and oxygen during high-temperature combustion of fossil fuels
  • Motor vehicle exhaust (responsible for about 80% of urban NO2)
  • Electricity generation in fossil-fuelled power stations, petrol and metal refining, food processing and other manufacturing industries

Sulfur dioxide

  • Electricity generation in coal-fired power stations; metal smelting of sulfurous ores, including lead, copper, zinc, aluminium and iron


  • In non-urban areas: biomass (vegetation) burning in domestic wood heaters; prescribed burns and bushfires; windblown dust from agriculture, mining, other land uses and the natural environment; road dust
  • In urban areas: motor vehicles, domestic wood heaters (in winter), construction activities and secondary particles


  • Contains both primary and secondary pollutants
  • Combustion sources, secondary nitrates and sulfates, secondary organic aerosol and natural-origin dust

Secondary pollutants


  • Atmospheric photochemical reactions of primary pollutants, NOx and hydrocarbons (volatile organic carbons) from motor vehicles and industry
  • Naturally occurring background ozone

PM2.5, PM10 = particulate matter with a diameter smaller than 2.5 or 10 microns, respectively

Keywood MD, Emmerson KM, Hibberd MF (2016). Ambient air quality: Pollution types. In: Australia state of the environment 2016, Australian Government Department of the Environment and Energy, Canberra, https://soe.environment.gov.au/theme/ambient-air-quality/topic/2016/pollution-types, DOI 10.4226/94/58b65c70bc372