Combustion of coal for heating in winter
accounts for 23% of the annual coal combustion. Furthermore, the F content
of coal consumed in the city is reported to be 163 mg/g, more than double the
mean value of 80 mg/g for coals of other parts of the world.
2
Another
important source of F in Beijing is dust from fresh concrete used for
construction. Factors such as these have contributed to the elevated F
concentrations of wet depositions in the city. For example, the annual
volume-weighted average concentration of soluble F of ambient aerosol is
reportedly 60 mg/m3
, which is 75 times higher than the concentration observed
in the air sample taken in the city ofMorioka, a city without fluoride pollution,
in northern part of Japan.
6
Fluoride has also been traced to runoff from application of insecticides and
herbicides. In addition to direct runoff into surface waters, airborne F may be
deposited into surface water and onto the ground, and eventually taken up by
soils, plants, and animals (Figure 10.2).
10.4 EFFECTS ON PLANTS
HF is the most phytotoxic air pollutant. The high toxicity of F and its
compounds is due to their rapid absorption and the inherent toxicity of the
element. F can cause injury to susceptible plants at concentrations below 1 ppb
(0.8 mg/m3
) for exposure periods of 7 days or less.
7,8,9
Exposure to F can result
in marked increases in foliage F levels. The extent of increases depends upon
factors such as duration of exposure, atmospheric F levels, and species or
variety of plants. F-induced effects in plants may be viewed based on four levels
of biologic organization: cellular, tissue or organ, organism, and ecosystem
(Table 10.2).
1
F accumulates in plant leaves mainly as a result of diffusion from the
atmosphere through the stomata or through absorption from soil by root. In
contrast to other major air pollutants, such as sulfur dioxide (SO2), nitrogen
dioxide (NO2), and ozone (O3), discussed in Chapter 8, F accumulates in the
154 Environmental Toxicology
# 2005 by CRC Press LLCfoliage of plants. The plants then serve as a vehicle for transfer of F to
herbivores, with the potential for inducing dental and skeletal fluorosis.
F induces both structural and functional changes in plant cells. Changes
occur in cellular and subcellular membranes, causing subsequent injuries.
Although plants differ widely in their susceptibility to F injury, accumulation
of high levels of F in leaves normally leads to chlorosis and necrosis. Chlorosis
is associated with lowered chlorophyll content in the leaf and thus leads to
lowered photosynthesis. Similarly, the destruction of part of the leaf, resulting
from necrosis, will cause a comparable reduction in photosynthesis. Both
chlorosis and necrosis lead to reduced growth and yield. Tree death can result
when the injuries are severe (Figure 10.3a). Contrary to NO2 or SO2, F induces
damage in leaf tips and margins of many plant species (Figure 10.3b and Figure
10.3c).
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