Factsheet: Air pollutants - gases

Gases can adversely affect our air quality and people’s health.

The National Environmental Standards for Air Quality Regulations 2004 (commonly referred to as the NES-AQ) include four gas contaminants that, when breathed in, can cause negative effects to people’s health:

  • Sulphur dioxide (SO2)
  • Carbon monoxide (CO)
  • Nitrogen dioxide (NO2)
  • Ozone (O3)


These NES-AQ guidelines are for short term concentrations of an hour, or eight hours for CO. There have been few breaches of these NES-AQ at sites in New Zealand. Sites are selected to monitor these gases when there is a likelihood of people being exposed to harmful concentrations.  Monitoring sites are commonly placed in industrial areas for SO2, near roads for CO and NO2 and some distance downwind of cities for O3.

Since 2007 Waka Kotahi NZ Transport Agency (NZTA) has monitored NO2 on state highways in New Zealand, to assess long term trends. They use passive sampling to measure monthly average concentrations, so these cannot be compared to the NES-AQ hour average threshold for NO2.

Other gases, such as benzene, are also monitored by some regional councils and unitary authorities at traffic sites in New Zealand.

Sulphur dioxide (SO2)

A colourless, soluble gas with a characteristic pungent smell which forms sulphuric acid when combined with water. Sulphur dioxide is produced mainly from the combustion of fossil fuels that contain sulphur, such as coal and oil (e.g. coal being burnt in a home fireplace for heating and diesel-powered vehicles). Sulphur dioxide is also produced from some industrial processes (e.g. fertiliser manufacturing, aluminium smelting and steel making). Natural sources of sulphur dioxide include geothermal activity.

Sulphur dioxide can cause respiratory problems such as bronchitis, and can irritate your nose, throat and lungs. It may cause coughing, wheezing, phlegm and asthma attacks. Sulphur dioxide has been linked to cardiovascular disease. Measured concentrations are reported in micrograms per cubic metre (µg/m3) of air.

(Source: https://environment.govt.nz/facts-and-science/air/air-pollutants/sulphur-dioxide-and-effects-on-health/)

Carbon monoxide (CO)

A colourless and odourless gas formed by incomplete combustion of carbon-containing fuels, in particular from motor vehicles (petrol and diesel) and burning wood and coal for home heating or wildfires. Health effects include reducing the blood’s ability to carry oxygen and aggravating heart conditions. Measured concentrations are reported in milligrams per cubic metre (mg/m3) of air.

Nitrogen dioxide (NO2)

Nitrogen dioxide is a reddish-brown, pungent, acidic gas that is corrosive and strongly oxidising. Nitrogen dioxide is not usually released directly into the air. The main source of nitrogen dioxide resulting from human activities is the combustion of fossil fuels (coal, gas and oil) especially fuel used in cars. It is also produced from making nitric acid, welding and using explosives, refining of petrol and metals, commercial manufacturing, and food manufacturing. Natural sources of other nitrogen oxides include volcanoes and bacteria.

The main health effect of nitrogen dioxide is on the respiratory system. Inhalation of nitrogen dioxide by children increases their risk of respiratory infection and may lead to poorer lung function in later life. There is also an association between nitrogen dioxide concentrations in the air and increases in mortality and hospital admissions for respiratory disease. Nitrogen dioxide can decrease the lungs’ defences against bacteria making them more susceptible to infections. It can also aggravate asthma. Measured concentrations are reported in micrograms per cubic metre (µg/m3) of air.

(Source: https://environment.govt.nz/facts-and-science/air/air-pollutants/nitrogen-dioxide-effects-health/)

Ozone (O3)

A colourless and odourless gas that at ground level is formed by chemical reactions involving sunlight, volatile organic compounds, and nitrogen oxides (the main source being vehicle emissions). Ozone also occurs naturally in the upper atmosphere where it can move downwards, contributing to ground level concentrations. Health risks include respiratory and cardiovascular diseases. Measured concentrations are reported in micrograms per cubic metre (µg/m3) of air.


A volatile organic compound. Motor vehicles and home heating are the main sources, and some industrial activities. Benzene is found in petrol products and some solvents. Breathing in benzene over a long period can cause cancer and other diseases. Measured concentrations are reported in micrograms per cubic metre (µg/m3) of air.

Where do I find more information?

Check with your regional council or unitary authority for more information on air quality monitoring in your area.