United Nations Environment Programme

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Facts on Pollutants:

Oxides of Nitrogen (NO and NO2 - NOx)

NOx is the common name for nitrogen oxide (NO) and nitrogen dioxide (NO2). Nitrogen oxide (NO) is a colourless gas and is slightly soluble in water. Nitrogen dioxide (NO2) is a reddish-brown gas, is a strong oxidant, and is soluble in water. NO2 reacts with water to form nitric acid (HNO3), a powerful oxidant capable of reacting with almost all metals and many organic compounds. NOx is mostly anthropogenic (man-made). NO2 is derived from NO. Since this transformation occurs rapidly, NO2 is regarded as more important in terms of human impacts. NO2 is also involved in the formation of ground-level ozone (O3) in the lower atmosphere.

Sources of NOx:
NOx results from the combustion processes of motor traffic, power production, and the burning of wood and refuse. Tobacco smoking, use of gas-fired appliances, and oil stoves are also sources. Non-combustion processes include the fertilizer industry, manufacturing of nitric acid (HNO3), welding processes, and the explosives industry. Natural sources of NOx emissions include bacterial action, volcanic action, and lightning. These sources far outweigh those generated by human activity, but because they are distributed over the entire surface of the earth, their background atmospheric concentrations are very small.

Human Health Effects of NOx:
The health effects of NOx are numerous including: respiratory irritation; headaches; pulmonary emphysema (chronically reduced lung functions); impairment of lung defences; oedema of lungs; eye irritations; loss of appetite; and corrosion of teeth. The most vulnerable groups are young children, asthmatics, as well as individuals with chronic bronchitis, emphysema or other chronic respiratory diseases.

Environmental Effects of NOx:
Increased acidic deposition contributes to the formation of acid rain, which may cause extensive damage to materials, vegetation, terrestrial and aquatic ecosystems, and eventually to human populations. Furthermore, both NO and NO2 are implicated in stratospheric ozone layer depletion.

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