Encyclopedia of Environmental Science and Engineering, Volume I and II

AIR POLLUTANT EFFECTS 35

fetus. The effects of combining exposure to carbon monox-

ide with sudden significant changes in altitude or the intake

of drugs or alcohol upon the performance of body functions

should be considered and avoided.

NITROGEN OXIDES (NOx)

“Nitrogen oxides” refers to the mixtures of nitric oxide and

nitrogen dioxide that are formed when combustion causes

the nitrogen and oxygen in the atmosphere to combine to

form nitric oxide, some of which then oxidizes further to

nitrogen dioxide; combustion gases contain about 5 to 10%

nitrogen dioxide mixed with nitric oxide. The mechanism

for the process is believed to be

O 2  2O

N 2  O  NO  N

N  O 2  NO  O

N  OH  NO  H

The overall reaction for the formation of nitrogen dioxide is

2NO  O 2  2NO 2

Nitric oxide is oxidized rapidly by ozone; therefore, ozone

levels tend to be lower in the vicinity of nitric-oxide sources,

such as the tailpipes of vehicles.

Nitrogen dioxide, the most toxic of the nitrogen oxides,

causes damage to lung tissues at concentrations higher than

usually found in ambient atmospheres. Exposures above the

10

10

20

20

30

30

40

40

50

60 80

60

80

100

200

300

400

500

600

800

1000

100 200 300 400 500

duration of exposure (minutes)

CO concentration in air (ppm)

Sedentary

LW

HW

LW = light work

HW = heavy work

FIGURE 2 Criteria for a Recommended Standard Occupational Exposure to Carbon Monoxide. NIOSH, 1972.

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