The mode of exposure of plants or animals to toxicants, continuous or
intermittent, and the activity level of an exposed animal are also important in
affecting pollutant toxicity. Normally, continuous exposure is more injurious
than intermittent exposure, with other factors remaining the same. For
instance, rats exposed to ozone (O 3 ) continuously for a sufficient period may
develop pulmonary edema. However, when the animals are exposed to the
same dose of O 3 , but administered intermittently, no pulmonary edema may
occur. A similar phenomenon can also occur with plants exposed to various
kinds of air pollutants. One reason for this is that living organisms often can, to
some extent, repair injuries caused by environmental chemicals. The magnitude
of the health effects of O 3 on animals is also highly dependent on the activity
level of the subject. Exercise increases the total volume of inhaled air, and so it
will also increase the total dose of O 3 to the lung. The duration of the exercise is
more important than the dose of the exposure.
3
5.5 ENVIRONMENTAL FACTORS
Environmental factors, such as temperature, pH, humidity, and others, may
affect pollutant toxicity in different ways. Some of these factors are examined in
this section.
5.5.1 TEMPERATURE
Many reports have shown the effects of changes of temperature on living
organisms.
4 Changes in ambient temperature affect the metabolism of
xenobiotics in animals. For example, the rate at which chemical reactions
occur increases with increase in temperature. With fish, an increase in
temperature leads to faster assimilation of waste and therefore faster depletion
of oxygen. Fish and other aquatic life can live only within certain temperature
ranges. For metals, toxicity may increase with either an increase or decrease in
ambient temperature.
5 Temperature also affects the response of vegetation to
air pollution. Generally, plant sensitivity to oxidants increases with increasing
temperature, up to 30
C. Soybeans are more sensitive to O 3 when grown at
28
C, regardless of exposure temperature or O 3 doses.
6
5.5.2 PH
Maintenance of a particular pH in body fluids is critical for the well-being of
animals and humans. The influence of pH on the toxicity of chemical agents
varies according to the organism and the chemical agent. For instance, human
body fluids must be maintained at very near to pH 7.4 for the body’s
metabolism to proceed properly, because most body enzymes function best
when the pH remain around neutral. As noted in Chapter 4, the availability to
plants of metals in the soil varies most markedly with soil pH. Increases in
acidity (decreases in pHs) enhance the mobilization of metals in soil. Acid
Factors Affecting Xenobiotic Action
67
[16:54 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-005.3d]
Ref: 4365 MING-HO YU Chap-005 Page: 67 65-84
intermittent, and the activity level of an exposed animal are also important in
affecting pollutant toxicity. Normally, continuous exposure is more injurious
than intermittent exposure, with other factors remaining the same. For
instance, rats exposed to ozone (O 3 ) continuously for a sufficient period may
develop pulmonary edema. However, when the animals are exposed to the
same dose of O 3 , but administered intermittently, no pulmonary edema may
occur. A similar phenomenon can also occur with plants exposed to various
kinds of air pollutants. One reason for this is that living organisms often can, to
some extent, repair injuries caused by environmental chemicals. The magnitude
of the health effects of O 3 on animals is also highly dependent on the activity
level of the subject. Exercise increases the total volume of inhaled air, and so it
will also increase the total dose of O 3 to the lung. The duration of the exercise is
more important than the dose of the exposure.
3
5.5 ENVIRONMENTAL FACTORS
Environmental factors, such as temperature, pH, humidity, and others, may
affect pollutant toxicity in different ways. Some of these factors are examined in
this section.
5.5.1 TEMPERATURE
Many reports have shown the effects of changes of temperature on living
organisms.
4 Changes in ambient temperature affect the metabolism of
xenobiotics in animals. For example, the rate at which chemical reactions
occur increases with increase in temperature. With fish, an increase in
temperature leads to faster assimilation of waste and therefore faster depletion
of oxygen. Fish and other aquatic life can live only within certain temperature
ranges. For metals, toxicity may increase with either an increase or decrease in
ambient temperature.
5 Temperature also affects the response of vegetation to
air pollution. Generally, plant sensitivity to oxidants increases with increasing
temperature, up to 30
C. Soybeans are more sensitive to O 3 when grown at
28
C, regardless of exposure temperature or O 3 doses.
6
5.5.2 PH
Maintenance of a particular pH in body fluids is critical for the well-being of
animals and humans. The influence of pH on the toxicity of chemical agents
varies according to the organism and the chemical agent. For instance, human
body fluids must be maintained at very near to pH 7.4 for the body’s
metabolism to proceed properly, because most body enzymes function best
when the pH remain around neutral. As noted in Chapter 4, the availability to
plants of metals in the soil varies most markedly with soil pH. Increases in
acidity (decreases in pHs) enhance the mobilization of metals in soil. Acid
Factors Affecting Xenobiotic Action
67
[16:54 26/8/04 P:/CRC PRESS/4365 MING-HO.751 (1670)/4365-005.3d]
Ref: 4365 MING-HO YU Chap-005 Page: 67 65-84