120
Floyd M. Ashton
mg/kg, of moderately toxic herbicides are as follows: paraquat (150), bromoxynil
(250), diallate (395), diquat (400-440), 2,4,5-T (300), and 2,4-D (300-1000).
These should be handled with unusual care. The three very toxic herbicides are
dinoseb (5-60), endothall (38-51), and acrolein (46). These herbicides should
be handled very, very carefully. The LDso values given are from the Herbicide
Handbook of the Weed Science Society of America (Anonymous, 1974).
During the past few years the safety of the use of 2,4,5-T and silvex has
been under review by the EPA. This appears to be primarily related to the
presence of a contaminant formed during the high-temperature synthesis of the
intermediate 2,4,5-trichlorophenol from 1,2,4,5-tetrachlorobenzene (Crosby et
al., 1971; Woolson et al., 1972). The contaminant 2,3,7,8-tetrachlorodibenzop-dioxin (TCDD) is a very toxic, embryotoxic, and teratogenic compound (Higgenbotham et al., 1968; Sparschu et al., 1971). However, oral doses of 2,4,5T containing 0.5 ppm of TCDD given to rats (24 mg 2,4,5-Tlkg, 6-15 days of
pregnancy) and rabbits (40 mg 2,4,5-T/Kg, 6-18 days of pregnancy) produced
no evidence of teratogenicity or embryotoxicity (Emerson et al., 1971). In another study, doses of 2,4,5-T containing 0.1 ppm of TCDD given to mice (20
mg 2,4,5-T/kg/day) was established as the no-effect level (Roll, 1971). However,
Roll also reported that doses of 35-130 mg 2,4,5-T/kg administered from 6 to
15 days of pregnancy produced an increase in cleft palates and embryotoxic
effects. Current manufacturing methods can result in TCDD level less than 0.1
ppm in 2,4,5-T or silvex (Warren, 1979). Until recently TCDD had not been
detected in nature (Crosby, 1976); however, within the past year, using refined
methods, TCDD has been detected in trace amounts in nature (Warren, 1979).
It has also been shown that TCDD and other dioxins are produced in trace
amounts in the burning of wood, petroleum, etc. (L. E. Warren, personal communication, 1979). While TCDD is quite stable in soil or water, in the presence
of organic hydrogendonors (i.e., solvents, alcohol) it is subject to rapid photodecomposition by dechlorination (Crosby et al., 1971; Plimmer et al., 1973).
The low levels of TCDD currently present in carefully manufactured 2,4,5-T
and silvex, the extremely low levels found in the environment, and the production
of TCDD by burning of natural substances would suggest that the use of these
materials is safe. However, in spite of these facts there is a continuing organized
effort to prohibit their use in the United States; therefore, in the long run, their
continued use remains obscure. TCDD is not produced during the manufacture
of the phenoxy-type herbicides not using 2,4,5-trichlorophenol as an intermediate, i.e., 2,4-D.
5.2. PERSISTENCE
The persistence of a herbicide at the site of application depends upon a
number of factors in addition to biodegradation, as illustrated in Fig. 5.1. Some
Floyd M. Ashton
mg/kg, of moderately toxic herbicides are as follows: paraquat (150), bromoxynil
(250), diallate (395), diquat (400-440), 2,4,5-T (300), and 2,4-D (300-1000).
These should be handled with unusual care. The three very toxic herbicides are
dinoseb (5-60), endothall (38-51), and acrolein (46). These herbicides should
be handled very, very carefully. The LDso values given are from the Herbicide
Handbook of the Weed Science Society of America (Anonymous, 1974).
During the past few years the safety of the use of 2,4,5-T and silvex has
been under review by the EPA. This appears to be primarily related to the
presence of a contaminant formed during the high-temperature synthesis of the
intermediate 2,4,5-trichlorophenol from 1,2,4,5-tetrachlorobenzene (Crosby et
al., 1971; Woolson et al., 1972). The contaminant 2,3,7,8-tetrachlorodibenzop-dioxin (TCDD) is a very toxic, embryotoxic, and teratogenic compound (Higgenbotham et al., 1968; Sparschu et al., 1971). However, oral doses of 2,4,5T containing 0.5 ppm of TCDD given to rats (24 mg 2,4,5-Tlkg, 6-15 days of
pregnancy) and rabbits (40 mg 2,4,5-T/Kg, 6-18 days of pregnancy) produced
no evidence of teratogenicity or embryotoxicity (Emerson et al., 1971). In another study, doses of 2,4,5-T containing 0.1 ppm of TCDD given to mice (20
mg 2,4,5-T/kg/day) was established as the no-effect level (Roll, 1971). However,
Roll also reported that doses of 35-130 mg 2,4,5-T/kg administered from 6 to
15 days of pregnancy produced an increase in cleft palates and embryotoxic
effects. Current manufacturing methods can result in TCDD level less than 0.1
ppm in 2,4,5-T or silvex (Warren, 1979). Until recently TCDD had not been
detected in nature (Crosby, 1976); however, within the past year, using refined
methods, TCDD has been detected in trace amounts in nature (Warren, 1979).
It has also been shown that TCDD and other dioxins are produced in trace
amounts in the burning of wood, petroleum, etc. (L. E. Warren, personal communication, 1979). While TCDD is quite stable in soil or water, in the presence
of organic hydrogendonors (i.e., solvents, alcohol) it is subject to rapid photodecomposition by dechlorination (Crosby et al., 1971; Plimmer et al., 1973).
The low levels of TCDD currently present in carefully manufactured 2,4,5-T
and silvex, the extremely low levels found in the environment, and the production
of TCDD by burning of natural substances would suggest that the use of these
materials is safe. However, in spite of these facts there is a continuing organized
effort to prohibit their use in the United States; therefore, in the long run, their
continued use remains obscure. TCDD is not produced during the manufacture
of the phenoxy-type herbicides not using 2,4,5-trichlorophenol as an intermediate, i.e., 2,4-D.
5.2. PERSISTENCE
The persistence of a herbicide at the site of application depends upon a
number of factors in addition to biodegradation, as illustrated in Fig. 5.1. Some