Nitrogen oxides (NOx)
Environmental Sources
Nitrogen oxides, or NOx, are the generic term for a group of highly
reactive gases, all of which contain nitrogen and oxygen in varying amounts.
Many of the nitrogen oxides are colourless and odourless. However,
one common pollutant, nitrogen dioxide (NO2) along with particles
in the air can often be seen as a reddish-brown layer over many urban
areas. Nitrogen oxides form when fuel is burned at high temperatures,
as in a combustion process. NOx is one of the main ingredients
involved in the formation of ground-level ozone, which can trigger serious
respiratory problems and reacts to form nitrate particles, acid aerosols,
as well as NO2, which also cause respiratory problems.
Ground-level ozone is formed when NOx and volatile organic compounds
(VOCs) react in the presence of heat and sunlight.
Human Health Effects
NO2 can irritate the lungs and lower resistance to respiratory
infections such as influenza. Short term continued or frequent exposure
to concentrations higher than those normally found in the ambient air
may cause increased incidences of acute respiratory disease in children.
This is so because NO2 attacks the bronchial and lung tissues,
irritate the mucous membrane and can affect those with asthma and bronchitis.
NOx affects breathing and the respiratory system, causing damage
to lung tissue, and may even result in premature death. Small particles
penetrate deeply into sensitive parts of the lungs and can cause or worsen
respiratory disease such as emphysema and bronchitis, and aggravate existing
heart disease.
Exposure limits
- The South African Department of Environmental Affairs
and Tourism’s (DEAT) guideline values for NO2, NOx and NO are as follows:
o Nitrogen dioxide: 500ppb (hourly average), 200ppb (daily
average), 100ppb (monthly average) and 80 ppb (annual average).
o Nitrogen oxides: 1 400ppb (hourly average), 800ppb (daily
average), 400ppb (monthly average) and 300 ppb (annual average).
o Nitrogen Oxide: 900ppb (hourly average), 600ppb (daily
average), 300ppb (monthly average) and 200 ppb (annual average).
- The United States EPA’s National Ambient Air Quality Standards (US
NAAQS) has set an air quality standard of 53 ppb for long-term 1-year
average concentrations of NO2, Short term, 1-hour NO2 concentrations should
not exceed 128 ppb more than once a year.
- The WHO has a guideline limit of 21 ppb for a long term annual average
and 106 ppb for a 1-hour short term average.
- The UK recommendations an hourly average of 150 ppb and an annual
average of 21 ppb.
Sources & Links:
www.ceroi.net/reports/durban/issues/air/guidelin.htm
www.epa.gov/oar/aqtrnd99/chapter2.pdf
www.epa.gov/air/urbanair/nox/
www.modcon.co.il/exposure.htm
www.epa.gov/children/
www.who.int/environmental_information/Air/Guidelines/Chapter3.htm#3.1
www.epa.gov/acidrain/ardhome.html
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