Nitrogen gas, normally relatively inert (unreactive), comprises about 80% of
the air. At high temperatures and under certain other conditions it can combine
with oxygen in the air, forming several different gaseous compounds collectively
called oxides of nitrogen (NOx). Nitric oxide (NO) and nitrogen
dioxide (NO2 - the criteria pollutant) are the two most important.
Major sources of nitrogen oxides include
Fuel combustion in power plants and automobiles.
Processes used in chemical plants.
Certain members of this group of pollutants, especially nitrogen dioxide (NO2),
are known to be highly toxic to various animals as well as to humans. High
levels may be fatal, while lower levels affect the delicate structure of lung
tissue. In experimental animals this leads to a lung disease that resembles
emphysema in humans. As with ozone, long-term exposure to nitrogen oxides makes
animals more susceptible to respiratory infections. Nitrogen dioxide exposure
lowers the resistance of animals to such diseases as pneumonia and influenza.
Humans exposed to high concentrations suffer lung irritation and potentially
lung damage. Increased respiratory disease has been associated with lower level
The human health effects of exposure to nitrogen oxides, such as nitrogen
dioxide, are similar to those of ozone. These effects may include:
Short-term exposure at concentrations greater than 3 parts per million (ppm)
can measurably decrease lung function.
Concentrations less than 3 ppm can irritate lungs.
Concentrations as low as 0.1 ppm cause lung irritation and measurable
decreases in lung function in asthmatics.
Long-term lower level exposures can destroy lung tissue, leading to
Children may also be especially sensitive to the effects of nitrogen oxides.
Oxides of nitrogen also can:
Seriously injure vegetation at certain concentrations. Effects include:
Bleaching or killing plant tissue.
Causing leaves to fall.
Reducing growth rate.
Deteriorate fabrics and fade dyes.
Corrode metals (due to nitrate salts formed from nitrogen oxides).
Oxides of nitrogen, in the presence of sunlight, can also react with
hydrocarbons, forming photochemical oxidants, as discussed in the section on
ozone. Also, NOx is a precursor to acidic precipitation, which may
affect both terrestrial and aquatic ecosystems.