precipitation, therefore, may greatly increase the availability of toxic metals,
such as aluminum, to plants.
5.5.3 HUMIDITY
The sensitivity of plants to air pollutants increases with increase in relative
humidity. For instance, high relative humidity has been found to contribute to
acute damage to forest vegetation caused by sulfur dioxide (SO 2 ).
7 Injurious
effects of O 3 and nitrogen dioxide (NO 2 ) on vegetation have also been found to
be greater when the relative humidity is high. A similar effect was found with
fluoride toxicity, gladiolus plants exhibited a higher sensitivity to fluoride when
relative humidity increased from 50 to 80%.
8
5.6 INTERACTION
The actions of individual toxicants are affected by many factors, such as
portals of entry, mode, metabolism, and others described previously. However,
organisms are generally exposed to a complex mixture of different pollutants.
Simultaneous exposure to more than one toxicant can have a dramatic impact
on the outcome of exposure. Toxicants may interact to produce additive,
potentiation, synergistic, or antagonistic effects. The factors affecting the
outcome of exposure are complex and include, among others, the characteristics of the chemicals and the physiological condition of the organism.
5.6.1 SYNERGISM, ADDITIVE AND POTENTIATION
An additive interaction occurs where the combined effects of two compounds
are simply additive. Synergism, however, describes a combined toxicity that is
greater than the simple additive effect of two compounds. Potentiation
describes synergism where one compound is generally assumed to have little
or no intrinsic toxicity when administered alone (with synergism, both
compounds have appreciable toxicity when administered separately).
Smoking and exposure to asbestos, for example, may have a synergistic
effect, resulting in increased lung cancer. The presence of particulate matter
such as sodium chloride (NaCl) and SO 2 , or SO 2 and sulfuric acid mist
simultaneously, would have potentiation or synergistic effects on animals.
Many insecticides exhibit synergism or potentiation. A recent study with
female rats showed that when the animals were exposed to fluoride and
benzene hexachloride (BHC) simultaneously, a synergistic effect occurred,
causing decreasing red blood cell counts and relative weight of the ovary.
9
Exposure of plants to both O 3 and SO 2 simultaneously is more injurious
than exposure to either of these gases alone. Laboratory studies showed that
a single exposure to O 3 at 0.03 ppm and to SO 2 at 0.24 ppm for 2 hours or 4
hours did not injure tobacco leaves. However, when the leaves were exposed
to a mixture of O 3 at 0.031 ppm and SO 2 at 0.24 ppm for 2 hours, a
68
Environmental Toxicology
[16:54 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-005.3d]
Ref: 4365 MING-HO YU Chap-005 Page: 68 65-84
such as aluminum, to plants.
5.5.3 HUMIDITY
The sensitivity of plants to air pollutants increases with increase in relative
humidity. For instance, high relative humidity has been found to contribute to
acute damage to forest vegetation caused by sulfur dioxide (SO 2 ).
7 Injurious
effects of O 3 and nitrogen dioxide (NO 2 ) on vegetation have also been found to
be greater when the relative humidity is high. A similar effect was found with
fluoride toxicity, gladiolus plants exhibited a higher sensitivity to fluoride when
relative humidity increased from 50 to 80%.
8
5.6 INTERACTION
The actions of individual toxicants are affected by many factors, such as
portals of entry, mode, metabolism, and others described previously. However,
organisms are generally exposed to a complex mixture of different pollutants.
Simultaneous exposure to more than one toxicant can have a dramatic impact
on the outcome of exposure. Toxicants may interact to produce additive,
potentiation, synergistic, or antagonistic effects. The factors affecting the
outcome of exposure are complex and include, among others, the characteristics of the chemicals and the physiological condition of the organism.
5.6.1 SYNERGISM, ADDITIVE AND POTENTIATION
An additive interaction occurs where the combined effects of two compounds
are simply additive. Synergism, however, describes a combined toxicity that is
greater than the simple additive effect of two compounds. Potentiation
describes synergism where one compound is generally assumed to have little
or no intrinsic toxicity when administered alone (with synergism, both
compounds have appreciable toxicity when administered separately).
Smoking and exposure to asbestos, for example, may have a synergistic
effect, resulting in increased lung cancer. The presence of particulate matter
such as sodium chloride (NaCl) and SO 2 , or SO 2 and sulfuric acid mist
simultaneously, would have potentiation or synergistic effects on animals.
Many insecticides exhibit synergism or potentiation. A recent study with
female rats showed that when the animals were exposed to fluoride and
benzene hexachloride (BHC) simultaneously, a synergistic effect occurred,
causing decreasing red blood cell counts and relative weight of the ovary.
9
Exposure of plants to both O 3 and SO 2 simultaneously is more injurious
than exposure to either of these gases alone. Laboratory studies showed that
a single exposure to O 3 at 0.03 ppm and to SO 2 at 0.24 ppm for 2 hours or 4
hours did not injure tobacco leaves. However, when the leaves were exposed
to a mixture of O 3 at 0.031 ppm and SO 2 at 0.24 ppm for 2 hours, a
68
Environmental Toxicology
[16:54 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-005.3d]
Ref: 4365 MING-HO YU Chap-005 Page: 68 65-84