Air pollutants we monitor

Our air quality monitoring networks provide measurements for six criterion pollutants. We also conduct short-term campaign monitoring activities.

'Criteria air pollutants' describes air pollutants that have been regulated and are used as indicators of air quality. These pollutants are regulated based on criteria that relate to health and/or environmental effects. 

Find out more about the six criteria pollutants we measure below.

Chemical symbol: O3

How it is formed

When near the ground, ozone is a colourless, gaseous secondary pollutant formed by chemical reactions between reactive organic gases and oxides of nitrogen on sunny days. 

Some interesting facts


  • is a secondary pollutant near the ground level and is a major component in photochemical smog.
  • is also formed  in the upper atmosphere or ‘stratosphere’ (the ozone layer) but isn’t a pollutant there because it is produced naturally and is important in absorbing harmful UV radiation, preventing it from reaching the Earth’s surface.
  • forms more readily in summer and reaches its highest concentrations in the afternoon or early evening.

How it can affect us

Ozone can irritate human eyes and airways and damage plants. Breathing ozone can affect lung function and worsen asthma. You may notice difficulty in breathing, coughing, and throat irritation if you are exercising outdoors when ozone levels are high.

Chemical symbol: NO2

How it is formed 

Nitrogen dioxide can be directly formed by oxidisation of nitrogen during fuel combustion processes, or when nitric oxide combines with oxygen (‘oxidises’), especially in warm, sunny conditions.

Some interesting facts

Nitrogen dioxide

  • is one of the main oxides of nitrogen in the atmosphere along with nitric oxide (NO) and nitrous oxide (N2O). Nitrous oxide occurs in much smaller quantities than the other two but is a powerful greenhouse gas and so contributes to global warming.
  • is produced by fuel combustion, especially in motor vehicle. Mostly this is nitric oxide with less than 10% nitrogen dioxide.
  • may, along with other oxides of nitrogen, remain in the atmosphere for several days, during which time chemical processes may generate nitric acid, and nitrates and nitrites as particles.
  • plays a major role in the chemical reactions which generate photochemical smog, along with other oxides of nitrogen.
  • is at highest concentrations near busy roads.

How it can affect us

Nitrogen dioxide irritates the lungs, making people with asthma more susceptible to lung infections and to asthma-triggers like pollen and exercise.

Chemical symbol: CO

How it is formed

Carbon monoxide is an odourless, colourless gas produced by incomplete oxidisation of carbon, such as incomplete combustion of fuel.

Carbon monoxide

  • is produced naturally by bushfires and by oxidation in the oceans and air of methane produced from organic decomposition
  • is produced by humans from combustion processes. In cities, the motor vehicle is by far the largest human source

Some interesting facts

  • usually remains in the atmosphere for a month or two.
  • is removed by oxidation to form carbon dioxide, absorption by some plants and micro-organisms, and rain

How it can affect us

  • reduces oxygen transport in the body when inhaled, binding to the oxygen-carrying site on the blood's haemoglobin. At high concentrations it is very toxic, causing headaches, dizziness, reduced ability to think, and nausea.
  • mostly affects people suffering from heart disease. They may experience chest pain if exposed to carbon monoxide, particularly while exercising.

Chemical symbol: SO2

How it is formed

Sulfur dioxide can be formed from both natural and human activities. Natural processes include decomposition and combustion of organic matter; sea spray and volcanic eruptions. The main human activities are smelting sulfur-containing mineral ores and fossil fuel combustion

Some interesting facts

Sulfur dioxide dissolves in water to form sulfuric acid, a corrosive substance that damages materials and plant and animal tissue. Acid rain, a serious environmental issue worldwide, can harm historic buildings, forests and lakes.

How it can affect us

Sulfur dioxide irritates airways of the lungs - people with asthma who are physically active outdoors are most vulnerable. Effects may include wheezing, chest tightness, and shortness of breath.

Symbol: NEPH

  • this provides an indication of the reduction in visibility caused by fine particles in the air  
  • is measured using a technique is called nephelometry (NEPH) by shining light through an air sample and determining how much is scattered. The greater the concentration of particles, the greater the light scattering and the higher the reading, indicating lower visibility in the atmosphere. 

Symbols: PM2.5 and PM10

What are they

  • particles (or 'particulates') are solid or liquid particles that may be suspended in the air

Some interesting facts

  • are measured as PM10 (particles less than 10 micrometers diameter) and PM2.5 (particles less than 2.5 micrometers diameter)
  • include dust, smoke, plant spores, bacteria and salt.
  • may be a primary pollutant, such as smoke particles, or a secondary pollutant formed from the chemical reaction of gaseous pollutants.
  • may result from human activities like mining; burning fossil fuels; transportation; agricultural and hazard reduction burning; the use of incinerators; and the use of solid fuel for cooking and heating.
  • can be classified by size. Large particles usually settle out of the air quickly but smaller ones can remain suspended for days or months. Rain is important for removing particles from the air.

How they can affect us

  • may reduce visibility and harm health.
  • have potential impact on human health depending on size. Larger particles, like total suspended particles (TSP), are usually trapped in the nose and throat and swallowed. Smaller particles may reach the lungs and irritate them. Fine particles can be carried deep into the lungs and irritate the airways.
  • may cause people with heart disease to experience symptoms like chest pain, and shortness of breath. Particle pollution can aggravate existing respiratory diseases such as asthma and chronic bronchitis.