such as chronic bronchitis. Many researchers consider that chronic bronchitis
in exposed persons may result from continued irritant exposures. In
asthmatics, inhalation of acidic aerosols may lead to bronchospasm. Certain
morphological changes are associated with the observed clinical symptoms in
human chronic bronchitis. The changes include an increase in the number and
size of epithelial mucus secretory cells, or both, in both proximal bronchi and
in peripheral airways. The changes are accompanied by an increase in the
volume of mucus secretion.
19 These changes are followed by an increase in
epithelial thickness and a decrease in airway diameter, similar to those
observed in laboratory animals.
Synergism may be observed in elevated airway resistance induced by SO 2 in
combination with certain other air pollutants. For example, the response to
inhaled SO 2 can be exacerbated by prior exposure to O 3 . Also, the presence of
H 2 SO 4 on ultrafine ZnO particles (simulating coal combustion effluent) in a
mixture with SO 2 has been shown to increase lung reactivity responses by tenfold over those produced by pure droplets of H 2 SO 4 of comparable size.
20
Published reports support the hypothesis that acidic pollutants contribute
to carcinogenesis in humans. Researchers have also examined possible
biological mechanisms for such a contribution, including pH modulation of
toxicity of xenobiotics and pH-dependent alteration of cells involving mitotic
and enzyme regulation. Based on review of the mortality data from London for
the period 1958 to 1972, the EPA
21 concluded that marked increases in
mortality occurred, mainly among the elderly and chronically ill, and that the
increases were associated with black smoke and SO 2 concentrations above
1000 mg/m
3 . The conclusion was especially favored when such an elevation of
pollutants occurred for several consecutive days.
8.3 NITROGEN DIOXIDE
8.3.1 FORMS AND FORMATION OF NITROGEN OXIDES
Six forms of nitrogen (N) oxides occur in the atmosphere: nitrous oxide (N 2 O),
nitric oxide (NO), nitrogen dioxide (NO 2 ), nitrogen trioxide (N 2 O 3 ), nitrogen
tetroxide (N 2 O 4 ), and nitrogen pentoxide (N 2 O 5 ). Of these, NO 2 is the most
important air pollutant because of its relatively high toxicity and its ubiquity in
ambient air, while N 2 O, N 2 O 3 , and N 2 O 4 have low relative toxicity and air
pollution significance. Basic chemical reactions involved in NO 2 formation are
as below:
12108C
N 2 þ O 2 ! 2NO
ð8:4Þ
2NO þ O 2 ! 2NO 2
ð8:5Þ
Air Pollution – Inorganic Gases
117
[16:53 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-008.3d]
Ref: 4365 MING-HO YU Chap-008 Page: 117 111-134
in exposed persons may result from continued irritant exposures. In
asthmatics, inhalation of acidic aerosols may lead to bronchospasm. Certain
morphological changes are associated with the observed clinical symptoms in
human chronic bronchitis. The changes include an increase in the number and
size of epithelial mucus secretory cells, or both, in both proximal bronchi and
in peripheral airways. The changes are accompanied by an increase in the
volume of mucus secretion.
19 These changes are followed by an increase in
epithelial thickness and a decrease in airway diameter, similar to those
observed in laboratory animals.
Synergism may be observed in elevated airway resistance induced by SO 2 in
combination with certain other air pollutants. For example, the response to
inhaled SO 2 can be exacerbated by prior exposure to O 3 . Also, the presence of
H 2 SO 4 on ultrafine ZnO particles (simulating coal combustion effluent) in a
mixture with SO 2 has been shown to increase lung reactivity responses by tenfold over those produced by pure droplets of H 2 SO 4 of comparable size.
20
Published reports support the hypothesis that acidic pollutants contribute
to carcinogenesis in humans. Researchers have also examined possible
biological mechanisms for such a contribution, including pH modulation of
toxicity of xenobiotics and pH-dependent alteration of cells involving mitotic
and enzyme regulation. Based on review of the mortality data from London for
the period 1958 to 1972, the EPA
21 concluded that marked increases in
mortality occurred, mainly among the elderly and chronically ill, and that the
increases were associated with black smoke and SO 2 concentrations above
1000 mg/m
3 . The conclusion was especially favored when such an elevation of
pollutants occurred for several consecutive days.
8.3 NITROGEN DIOXIDE
8.3.1 FORMS AND FORMATION OF NITROGEN OXIDES
Six forms of nitrogen (N) oxides occur in the atmosphere: nitrous oxide (N 2 O),
nitric oxide (NO), nitrogen dioxide (NO 2 ), nitrogen trioxide (N 2 O 3 ), nitrogen
tetroxide (N 2 O 4 ), and nitrogen pentoxide (N 2 O 5 ). Of these, NO 2 is the most
important air pollutant because of its relatively high toxicity and its ubiquity in
ambient air, while N 2 O, N 2 O 3 , and N 2 O 4 have low relative toxicity and air
pollution significance. Basic chemical reactions involved in NO 2 formation are
as below:
12108C
N 2 þ O 2 ! 2NO
ð8:4Þ
2NO þ O 2 ! 2NO 2
ð8:5Þ
Air Pollution – Inorganic Gases
117
[16:53 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-008.3d]
Ref: 4365 MING-HO YU Chap-008 Page: 117 111-134