Oxides of Nitrogen


Oxides of nitrogen are a mixture of gases that are composed of nitrogen and oxygen. Two of the most toxicologically significant compounds are nitric oxide (NO) and nitrogen dioxide (NO2). Other gases belonging to this group are nitrogen monoxide (or nitrous oxide, N2O), and nitrogen pentoxide (NO5).

Nitrogen dioxide is produced for the manufacture of nitric acid. Most nitric acid is used in the manufacture of fertilisers, while some is used in the production of explosives for both military and mining uses.

Substance details

Substance name: Oxides of nitrogen

CASR number: N/A

Molecular formula: NO, NO2, N2O and N2O5

Synonyms: NO: nitric oxide, nitrogen oxide, nitrogen monoxide, mononitrogen monoxide

NO2: nitrogen dioxide

Physical properties

NO: sharp, sweet-smelling, colourless gas

Melting point: -163.6°C

Boiling point: -151.8°C

Relative Density: 1.04 (air = 1)

NO2: reddish-brown gas with irritating odour.

Melting point: -9.3°C

Boiling point: 21.15°C

Vapour Density: 1.58

Chemical properties

NO only burns when heated with hydrogen, and forms nitric acid (a strong acid) when dissolved in water.

NO2 is sparingly soluble in water to form nitrous acid (a weak acid).

Further information

The National Pollutant Inventory (NPI) holds data for all sources of Oxides of Nitrogen emissions in Australia.


Low levels of oxides of nitrogen can irritate eyes, nose, throat and lungs, possibly leading to coughing, shortness of breath, tiredness and nausea. Exposure can also result in a build up of fluid in the lungs for 1-2 days after exposure. Breathing high levels of oxides of nitrogen can cause rapid burning, spasms and swelling of tissues in the throat and upper respiratory tract, reduced oxygenation of tissues, a build up of fluid in the lungs, and maybe even death.

Skin or eye contact with high concentrations of oxides of nitrogen gases or nitrogen dioxide liquid will likely lead to serious burns.

Entering the body

Oxides of nitrogen may be inhaled or absorbed through the skin.


Most people are exposed to oxides of nitrogen by breathing in polluted air. People who live near combustion sources such as coal burning power plants or areas of high motor vehicle usage, or live in households that burn a lot of wood or use kerosene heaters or gas stoves may be exposed to higher levels of nitrogen oxides. Workers employed in facilities that use welding materials, produce nitric acid or certain explosives, may inhale oxides of nitrogen during their work.

Nitrogen dioxide and nitric oxide are found in tobacco smoke.

Health guidelines

Workplace exposure:
Currently, the eight-hour time weighted average (TWA) exposure limits are 31 milligrams of nitric oxide (NO) per cubic metre of air, 45 milligrams of nitrous oxide (N2O) per cubic metre of air and 5.6 milligrams of nitrogen dioxide (NO2) per cubic metre of air. A 15-minute short term exposure limit (STEL) has been established for NO2 at a level of 9.4 milligrams per cubic metre.

Australian drinking water guidelines:
In 2004, the National Health and Medical Research Council (NHMRC) and the National Resource Management Ministerial Council (NRMMC) established the following guidelines for acceptable water quality:

Based on health considerations, the concentration of nitrates should not exceed 50 milligrams per litre of drinking water for bottle-fed infants up to 3 months of age, up to 100 milligrams per litre of drinking water for adults and children over 3 months of age.

Where a water supply has between 50 and 100 milligrams of nitrate per litre of drinking water, active measures are required to ensure that those caring for infants are aware of the need to use alternative water sources in making up bottle feeds for infants under 3 months of age.

Based on health considerations, the concentration of nitrites should not exceed 3 milligrams per litre of drinking water.


Excessive levels of the oxides of nitrogen, particularly nitrogen dioxide (NO2), can cause death in plants and roots and damage the leaves of many agricultural crops. NO2 is the damaging component of photochemical smog. Excessive levels increase the acidity of rain (lower the pH), and thus lower the pH of surface and ground waters and soil. The lowered pH can have harmful effects, possibly even death, on a variety of biological systems.

Entering the environment

Oxides of nitrogen are part of the biogeochemical cycling of nitrogen, and are found in air, soil and water.

In the atmosphere, the oxides of nitrogen are rapidly equilibrated to nitrogen dioxide (NO2), which eventually forms acid rain. In the stratosphere, oxides of nitrogen play a crucial role in maintaining the levels of ozone. Ozone is formed through the photochemical reaction between nitrogen dioxide and oxygen.

Where it ends up

Oxides of nitrogen are rapidly broken down by reacting with other substances found in the air. Nitrogen dioxide can form nitric acid in sunlight, and is a major constituent of acid rain, tropospheric ozone and smog. Nitrogen oxides react in the soil and the water to nitric acid.

Environmental guidelines

The following ambient air quality standards were established for nitrogen dioxide in 1999:
Averaging period of 1 hour, a maximum of 0.12 parts per million, with a maximum allowable exceedence of 1 day per year.
Averaging period of 1 year, a maximum of 0.03 parts per million.

Industry sources

Industrial sources of oxides of nitrogen are electricity supply, mining, oil and gas extraction, manufacturing industries and petroleum manufacturing.

Diffuse sources, and industry sources included in diffuse emissions data

A variety of agricultural (both cultivation and animal feeding) and forestry (both "burn-offs" and clearfelling) activities increase the rate of natural processes that produce oxides of nitrogen. Fuel burning activities, such as for heating, and cigarette smoking are also diffuse sources of oxides of nitrogen.

Natural sources

The biological cycling of nitrogen includes processes that produce nitric oxide and nitrous oxide as intermediates.

Thermal processes in the atmosphere (during lightning or bushfires/wildfires) produce oxides of nitrogen.

Transport sources

Oxides of nitrogen are present in the exhausts of all vehicles, including motor vehicles, lawn mowers, shipping/boating, aeroplanes and railways.

Consumer products

Nitrate containing fertilisers contain oxides of nitrogen.

Sources used in preparing this information

  • Agency for Toxic Substances and Disease Registry (ASTDR), ToxFAQs: Nitrogen oxides, accessed May 2007.
  • Intergovernmental Panel on Climate Change (2001), Climate Change: Working Group 1: The Scientific Basis (Chapter 4), accessed May 2007.
  • Merck and Co. 2006, Merck Index, 14th Edition, USA.
  • National Health and Medical Research Council (NHMRC) and the National Resource Management Ministerial Council (NRMMC) (2004), Australian Drinking Water Guidelines 6, accessed May 2007.
  • National Pollutant Inventory (1999), Contextual Information.
  • Office of the Australian Safety and Compensation Council, Exposure Standards: nitric oxide, accessed May 2007.
  • Office of the Australian Safety and Compensation Council, Exposure Standards: nitrogen dioxide, accessed May 2007.
  • Office of the Australian Safety and Compensation Council, Exposure Standards: nitrous oxide, accessed May 2007.
  • Technical Advisory Panel 1999, Final Report to the National Environment Protection Council.
  • United Nations, International Chemical Safety Cards: nitric oxide, accessed May 2007.
  • United Nations, International Chemical Safety Cards: nitrogen dioxide, accessed May 2007.
  • United Nations, International Chemical Safety Cards: nitrous oxide, accessed February 2007.
  • United States Environmental Protection Agency. Integrated Risk Information System (IRIS): Nitrogen dioxide, accessed May 2007.