Environmental Engineering Reference
In-Depth Information
vary in composition. Their primary effect would be to irritate the upper
respiratory passages and bronchi. Because of its solubility in tissue fluids,
SO
2
(associated with some combustion appliances) can cause bronchial irri-
tation. Irritants are described in a later section on ETS.
C. Nitrogen oxides
The nitrogen oxides include NO and NO
2
. The former is a relatively nontoxic
gas produced in the high-temperature reaction of nitrogen (N
2
) and O
2
in
combustion. Its importance lies in its relatively rapid oxidation to NO
2
, a
substance with considerably greater toxicity. Nitrogen dioxide is relatively
nonsoluble in tissue fluids. As a consequence, it enters the lungs where it
may expose lower airways and alveolar tissue.
Animal studies at 0.5 ppmv NO
2
have indicated that such exposures can
cause a variety of pathological changes including the destruction of cilia
lining respiratory airways, obstruction of bronchioles, and disruption of
alveoli. Other animal exposure studies indicate that NO
2
exposure may
inhibit respiratory defense mechanisms, allowing bacteria to invade and
multiply in lung tissue.
A causal relationship between low-level NO
2
exposures and respiratory
symptoms or disease is suggested from epidemiological studies that have
attempted to evaluate potential health effects associated with gas cooking
stoves and kerosene heaters. Results of studies with gas cooking stoves have
been mixed. Some studies indicate a significantly increased risk of experi-
encing one or more pulmonary symptoms for children under the age of
seven in homes with a gas stove; exposure to NO
2
has been suggested as
the cause. However, follow-up studies have been unable to demonstrate a
dose-response relationship between respiratory symptoms and NO
2
levels.
In all cases, NO
2
was measured with relatively long averaging times (circa
7 days) and may have not been indicative of potential effects associated with
short-term peak exposures. Many epidemiological studies have been con-
ducted to evaluate the relationship between gas cooking (and, to a limited
extent, associated NO
2
levels) and respiratory illness. They have not shown
a consistent pattern between such exposures and respiratory symptoms or
lung function changes in either children or adults.
However, in a relatively recent Australian study, the presence of a gas
stove and exposure to NO
2
were observed to be significant risk factors for
both asthma and respiratory symptoms in children. The presence of a gas
stove in the household increased the likelihood of asthma by threefold and
respiratory symptoms by twofold. This association between gas stoves and
asthma was significant even after data were adjusted for NO
2
(NO
2
was
significantly associated with asthma when the presence of a gas stove was
not taken into account). These results suggest that an additional risk may be
associated with gas stoves that is separate from 4-day average NO
2
exposures.
A few studies have focused on kerosene heaters. In a U.S. study, children
under the age of seven exposed to average NO
2
levels >16 ppbv were reported
