Air toxics and indoor air quality in Australia

State of knowledge report
Environment Australia, 2001
ISBN 0 6425 4739 4

Part C: Factsheets (continued)

Oxides of nitrogen

Substance name: Oxides of nitrogen
CASR number: NO2: 10102-44-0
NO: 10102-43-9
N2O: 10024-97-2
Molecular–formula: NO2
Synonyms: NO2: nitrogen dioxide
NO: nitric oxide, nitrogen monoxide; mononitrogen monoxide
N2O: nitrous oxide, dinitrogen monoxide; laughing gas; hyponitrous acid anhydride

Physical and chemical properties

NO2 is a dark brown, fuming liquid or gas with a pungent, acrid odour detectable at 0.12 ppm.

NO is a colourless gas with a sharp, sweet odour, brown at high concentrations in air.

N2O is a colourless gas with a slight, sweetish odour.


Melting point: -11°C
Boiling point: -164°C
Vapour density: 1.58


Melting point: -164°C
Boiling point: -151.7°C
Vapour density: 1.04


Boiling point: -88.6°C
Vapour density: 1.53

NO2 is highly soluble in water, to form nitric acid (a strong acid). NO is slightly soluble in water, to form nitrous acid (a weak acid). N2O is nonflammable and has anaesthetic properties.

Common uses

Nitrogen dioxide is produced (by oxidation of nitrogen) for the manufacture of nitric acid (by its dissolution in water). Most nitric acid is used in the manufacture of fertilisers; some is used in the production of explosives, for both military and peaceful (eg in mining operations) purposes.

Nitrous oxide has been used as an anaesthetic (known as 'laughing gas').

Sources of emissions

Point sources

The oxides of nitrogen (nitric oxide and nitrogen dioxide) are formed by the direct combination of oxygen and nitrogen during a variety of thermal processes. These include operation of internal combustion engines, thermal power generating plants, and in the air surrounding the arc or flame as a result of heat from electric arc or gas torches (oxidising flames). In outdoor or open shop welding, hazardous abnormal concentrations are unlikely, except perhaps for short periods. In confined spaces, hazardous concentrations of nitrogen oxides may rapidly build up in welding operations. High concentrations of nitrogen oxides have been found during gas tungsten-arc cutting of stainless steel.

Diffuse sources, and point sources included in aggregated emissions data A variety of agricultural (both cultivation and animal feeding) and forestry (both 'burn-offs' and clear-felling) activities increase the rate of natural processes that produce oxides of nitrogen.

Natural sources

The biological cycling of nitrogen includes processes that produce NO and N2O as intermediates.

Thermal processes in the atmosphere (during lightning) produce oxides of nitrogen from nitrogen gas.

Mobile sources

Vehicle exhausts.

Consumer products that may contain oxides of nitrogen

Nitrate-containing fertilisers.

Health effects

How might I be exposed to oxides of nitrogen?

Most people are exposed outdoors to low levels of oxides of nitrogen from a variety of combustion processes. These include biological processes (formation during natural nitrogen cycling), tobacco smoke, car and thermal power generating plant exhausts, and some welding operations.

In some indoor situations, NO2 may reach concentrations significantly higher than those measured in the ambient air (eg from nonvented gas heaters). People living near industries that produce oxides of nitrogen, or near freeways, may also be exposed to relatively higher levels in the surrounding air.

By what pathways might oxides of nitrogen enter my body?

Oxides of nitrogen can enter the body by breathing in the gases, by absorption through the skin, or by dissolution of nitrogen dioxide in moist surfaces such as the eyes and mouth.

Health guidelines

National Occupational Health and Safety Commission (NOHSC):

There are no short-term exposure limits for nitric oxide and nitrous oxide.

The NOHSC lists nitrogen dioxide as a substance under review for respiratory effects.

What effect might oxides of nitrogen have on my health?

Exposure to nitrogen dioxide and nitric oxide may not always produce immediate effects but may result in the accumulation of fluid in the lung tissues (pulmonary oedema), which may prove fatal some hours after the exposure stops. Nitrogen dioxide is classified by the NOHSC as harmful by inhalation at concentrations greater than 0.1% and very toxic by inhalation at concentrations greater than 10%. It is also irritating to the skin, eyes and respiratory system at concentrations greater than 0.5%.

Environmental effects

Environmental fate

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

In the atmosphere, oxides of nitrogen are rapidly oxidised to nitrogen dioxide (half-life about 50 days), which dissolves in water to produce dilute nitric acid and precipitates in rain. An increased rate of formation of oxides of nitrogen therefore contributes to 'acid rain'.

In the stratosphere, oxides of nitrogen play a crucial role in maintaining the level of ozone. Ozone is formed through the photochemical reaction of nitrogen dioxide and oxygen. However, too little nitrogen dioxide results in too little ozone being formed, On the other hand, too much nitric oxide reduces the level of ozone because of an increase in the reaction of ozone to convert nitric oxide to nitrogen dioxide.

In the lower atmosphere, oxides of nitrogen play a major role in the formation of photochemical smog in a complex set of reactions that lead to the formation of a variety of nitrated organic compounds (from volatile organic matter) and excessive levels of ozone.

Environmental transport

The oxides of nitrogen travel as gases through soil and the atmosphere, and in solution in water in soils, rivers and lakes, and rain and snow.

Environmental guidelines

National Ambient Air Quality Standards and Goals:

What effect might oxides of nitrogen have on the environment?

Excessive levels of the oxides of nitrogen, particularly nitrogen dioxide, can cause death in plants and roots and damage the leaves of many agricultural crops. Excessive levels increase the acidity of rain (ie lower the pH) and thus lower the pH of surface and groundwaters as well as soils. In turn, this lowered pH can have harmful effects, including death, on a variety of biota.


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