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Chlorinated phenoxy acids and their dioxins
Environment International
Vol. 2, pp. 53-54. Pergamon Press Ltd. 1979. Printed in Great Britain
Book Reviews
A very different set of effects has been associated with the dioxin o f 2,4,5-T. This c o m p o u n d , tetrachlorodiphenyl-para-dioxin, is extremely toxic. It is described by A . H . Westing as "perhaps the most poisonous substance ever to have been synthesized." Like the phenoxy acids themselves, the dioxins associated with them do not occur naturally. Nor is there evidence that dioxins are produced from the parent c o m p o u n d under ambient natural conditions. Dioxin, notably the dangerous T C D D , is an unintended impurity formed in the manufacturing process. T C D D has been found in 2,4,5-T in varying a m o u n t s from 0.1 to 2.0 ppm. The chronic toxicities associated with 2,4,5-T apparently are due to the presence of T C D D in the sprayed materials. Manufacturing processes for 2,4,5-T are now available which minimize the formation of T C D D from trichlorophenol. It is postulated that if the level o f T C D D in phenoxy acid herbicides could be kept at, or below, 0.1 ppm, consideration of health risks associated with these substances should be based on the toxicity o f the parent c o m p o u n d itself, not the dioxin. The m e c h a n i s m for toxicity o f T C D D is not known, but it is a powerful poison. Toxic effects have resulted from doses in the/a g kg 1 range. Single dose LDs0 values of less than 50 # g kg -1 (oral) have been established for m a n y laboratory animals. Mortality is frequently protracted (2 to 6 weeks after single dose). The toxicity of T C D D is not unique. It resembles the action o f certain chlorinated biphenyls, chlorinated dibenzo furans and certain brominated homologs. H u m a n s exposed to accidental discharges of T C D D exhibit a classic lesion of persistent chloracne. Other effects, observed mainly in laboratory animals, include progressive wasting, weight loss and hypoplasia o f lymphoid tissues. The liver is the target organ. Bioaccumulation of T C D D has been demonstrated in several aquatic organisms exposed to extremely high dose rates. Bioaccumulation ratios (BR) as high as 13000 over the concentration in the water have been attributed to snails, and 15000 for mosquito fish. Catfish, daphnids and algae also show significant accumulative properties. Algae, snails and daphnids showed no toxic effects themselves at the concentrations used in this research. However, mortality a m o n g several fish species was caused by 54 ppt T C D D for 24-96 hr. Fatalities were delayed up to 60 days. These studies lead to the conclusion that level of exposure is more important than its duration. The experimental conditions o f these experiments included those where T C D D levels in the soil were at least 1 million times higher than is likely to be found in sediments after normal use of contaminate 2,4,5-T. Many conclusions are drawn from the studies related in this book. Notable a m o n g these is the lack o f definitive information regarding mutagenicity of these chlorinated phenoxy acetic acids and their dioxins, and possible genetic effects in plants and h u m a n s . Recommendations include recognition by users o f these substances o f the ecologic impacts to nontarget flora and fauna and the need to specially guard against accidental discharges. Having successfully dealt with one source of controversy surrounding the use o f chlorinated phenoxy acetic acid herbicides, the book concludes by raising a new issue with potential for continuing the furor, but along a different line. That paper speculates about the use of tetrachlorodibenzo-para-dioxin (TCDD) as an agent of chemical warfare.
C. Ramel. Chlorinated Phenoxy Acids and Their Dioxins. Ecological Bulletins, No. 27, Swedish Natural Science Research Council, 1978. Few topics have aroused more controversy than the use o f chlorinated phenoxy acids as herbicides. The best known representatives of this class of c o m p o u n d s are 2,4-D; 2,4,5-T; Silvex and M C P A . The dioxin T C D D (2,3,7,8-tetrachlorodibenzo-para-dioxin) found in 2,4,5-T is the only dioxin in this class which has environmental significance, but that one is enormously important. This highly readable book deals successfully with these potentially emotional issues by presenting a series of 24 papers by more than 30 respected authors representing a wide range o f expertise. The result is an authoritative, up-to-the-minute description o f the present knowledge of these useful but hazardous compounds. The papers were first presented in Stockholm in February, 1977 at a conference arranged by the Royal Swedish Academy o f Sciences, and have been updated to include m a n y references to research done in 1978. I strongly doubt that one ~could raise a question about chlorinated phenoxy acids which is not discussed in this volume. Since their development in the mid-1940's, these c o m p o u n d s have been used as herbicides all over the world. The actual tonnage is not known, but enormous quantities have been and are still being used to control unwanted vegetation in forests, pastures, farms, plantations, and even along roadsides in urban areas. Evidence presented under the section on Ecology and Economy, though incomplete, shows that these c o m p o u n d s control weeds at lower cost than other methods. Because o f favorable economics, the use o f chlorinated phenoxy acid herbicides probably will continue on a large scale. In the discussion on Plant Physiology, it is apparent that the herbicidal m e c h a n i s m involves a derangement o f the plant growth regulators (auxins). Most probably the toxic effect is due to interference with the action of the natural auxin, indolylacetic acid. T h o u g h growth regulation by auxins is c o m m o n a m o n g m a n y plants (seed plants, ferns) animal cells are not known to contain auxins nor do they possess similar mechanisms. Controversy over the use o f these chemicals has been due principally to their supposed toxicity to h u m a n s and animals. Cases o f acute eye irritation and nausea a m o n g persons close to spray operations are noted. By 1970 phenocy acids were suspected in reported cases o f malformed infants in Sweden. There was also an increased incidence of birth defects in Viet N a m where U.S. troops were spraying " A g e n t O r a n g e " , a mixture of 2,4,5-T and 2,4-D. Such effects were not expected because, when exposed to levels associated with normal use, laboratory animals show phenoxy acids to be completely metabolized and quickly excreted (except in dogs where excretion in urine is less efficient). Prolonged exposure to low levels does not result in bioaccumulation in laboratory animals. The half-life a f most of these c o m p o u n d s under normal use conditions is 3 to 4 weeks, and up to l0 weeks for 2,4,5-T. There is no evidence of carcinogenicity associated with chlorinated phenoxy acids. There is limited evidence that 2,4-D is a weak mutagen in plant cells, but the impact on crops cannot be determined from the limited studies available. Variable, conflicting and inconclusive results have been reported from research into mutagenic effects in various animals treated with 2,4-D; 2,4,5-T and M C P A . Teratogenic effects have been attributed to high doses of 2,4,5-T in mice, but other c o m p o u n d s and lower doses have not had this result.
Office o f Research and Development, U.S. Environmental Protection Agency, Washington D.C., U.S.A. 53
Richard E. Marland
Vol. 2, pp. 53-54. Pergamon Press Ltd. 1979. Printed in Great Britain
Book Reviews
A very different set of effects has been associated with the dioxin o f 2,4,5-T. This c o m p o u n d , tetrachlorodiphenyl-para-dioxin, is extremely toxic. It is described by A . H . Westing as "perhaps the most poisonous substance ever to have been synthesized." Like the phenoxy acids themselves, the dioxins associated with them do not occur naturally. Nor is there evidence that dioxins are produced from the parent c o m p o u n d under ambient natural conditions. Dioxin, notably the dangerous T C D D , is an unintended impurity formed in the manufacturing process. T C D D has been found in 2,4,5-T in varying a m o u n t s from 0.1 to 2.0 ppm. The chronic toxicities associated with 2,4,5-T apparently are due to the presence of T C D D in the sprayed materials. Manufacturing processes for 2,4,5-T are now available which minimize the formation of T C D D from trichlorophenol. It is postulated that if the level o f T C D D in phenoxy acid herbicides could be kept at, or below, 0.1 ppm, consideration of health risks associated with these substances should be based on the toxicity o f the parent c o m p o u n d itself, not the dioxin. The m e c h a n i s m for toxicity o f T C D D is not known, but it is a powerful poison. Toxic effects have resulted from doses in the/a g kg 1 range. Single dose LDs0 values of less than 50 # g kg -1 (oral) have been established for m a n y laboratory animals. Mortality is frequently protracted (2 to 6 weeks after single dose). The toxicity of T C D D is not unique. It resembles the action o f certain chlorinated biphenyls, chlorinated dibenzo furans and certain brominated homologs. H u m a n s exposed to accidental discharges of T C D D exhibit a classic lesion of persistent chloracne. Other effects, observed mainly in laboratory animals, include progressive wasting, weight loss and hypoplasia o f lymphoid tissues. The liver is the target organ. Bioaccumulation of T C D D has been demonstrated in several aquatic organisms exposed to extremely high dose rates. Bioaccumulation ratios (BR) as high as 13000 over the concentration in the water have been attributed to snails, and 15000 for mosquito fish. Catfish, daphnids and algae also show significant accumulative properties. Algae, snails and daphnids showed no toxic effects themselves at the concentrations used in this research. However, mortality a m o n g several fish species was caused by 54 ppt T C D D for 24-96 hr. Fatalities were delayed up to 60 days. These studies lead to the conclusion that level of exposure is more important than its duration. The experimental conditions o f these experiments included those where T C D D levels in the soil were at least 1 million times higher than is likely to be found in sediments after normal use of contaminate 2,4,5-T. Many conclusions are drawn from the studies related in this book. Notable a m o n g these is the lack o f definitive information regarding mutagenicity of these chlorinated phenoxy acetic acids and their dioxins, and possible genetic effects in plants and h u m a n s . Recommendations include recognition by users o f these substances o f the ecologic impacts to nontarget flora and fauna and the need to specially guard against accidental discharges. Having successfully dealt with one source of controversy surrounding the use o f chlorinated phenoxy acetic acid herbicides, the book concludes by raising a new issue with potential for continuing the furor, but along a different line. That paper speculates about the use of tetrachlorodibenzo-para-dioxin (TCDD) as an agent of chemical warfare.
C. Ramel. Chlorinated Phenoxy Acids and Their Dioxins. Ecological Bulletins, No. 27, Swedish Natural Science Research Council, 1978. Few topics have aroused more controversy than the use o f chlorinated phenoxy acids as herbicides. The best known representatives of this class of c o m p o u n d s are 2,4-D; 2,4,5-T; Silvex and M C P A . The dioxin T C D D (2,3,7,8-tetrachlorodibenzo-para-dioxin) found in 2,4,5-T is the only dioxin in this class which has environmental significance, but that one is enormously important. This highly readable book deals successfully with these potentially emotional issues by presenting a series of 24 papers by more than 30 respected authors representing a wide range o f expertise. The result is an authoritative, up-to-the-minute description o f the present knowledge of these useful but hazardous compounds. The papers were first presented in Stockholm in February, 1977 at a conference arranged by the Royal Swedish Academy o f Sciences, and have been updated to include m a n y references to research done in 1978. I strongly doubt that one ~could raise a question about chlorinated phenoxy acids which is not discussed in this volume. Since their development in the mid-1940's, these c o m p o u n d s have been used as herbicides all over the world. The actual tonnage is not known, but enormous quantities have been and are still being used to control unwanted vegetation in forests, pastures, farms, plantations, and even along roadsides in urban areas. Evidence presented under the section on Ecology and Economy, though incomplete, shows that these c o m p o u n d s control weeds at lower cost than other methods. Because o f favorable economics, the use o f chlorinated phenoxy acid herbicides probably will continue on a large scale. In the discussion on Plant Physiology, it is apparent that the herbicidal m e c h a n i s m involves a derangement o f the plant growth regulators (auxins). Most probably the toxic effect is due to interference with the action of the natural auxin, indolylacetic acid. T h o u g h growth regulation by auxins is c o m m o n a m o n g m a n y plants (seed plants, ferns) animal cells are not known to contain auxins nor do they possess similar mechanisms. Controversy over the use o f these chemicals has been due principally to their supposed toxicity to h u m a n s and animals. Cases o f acute eye irritation and nausea a m o n g persons close to spray operations are noted. By 1970 phenocy acids were suspected in reported cases o f malformed infants in Sweden. There was also an increased incidence of birth defects in Viet N a m where U.S. troops were spraying " A g e n t O r a n g e " , a mixture of 2,4,5-T and 2,4-D. Such effects were not expected because, when exposed to levels associated with normal use, laboratory animals show phenoxy acids to be completely metabolized and quickly excreted (except in dogs where excretion in urine is less efficient). Prolonged exposure to low levels does not result in bioaccumulation in laboratory animals. The half-life a f most of these c o m p o u n d s under normal use conditions is 3 to 4 weeks, and up to l0 weeks for 2,4,5-T. There is no evidence of carcinogenicity associated with chlorinated phenoxy acids. There is limited evidence that 2,4-D is a weak mutagen in plant cells, but the impact on crops cannot be determined from the limited studies available. Variable, conflicting and inconclusive results have been reported from research into mutagenic effects in various animals treated with 2,4-D; 2,4,5-T and M C P A . Teratogenic effects have been attributed to high doses of 2,4,5-T in mice, but other c o m p o u n d s and lower doses have not had this result.
Office o f Research and Development, U.S. Environmental Protection Agency, Washington D.C., U.S.A. 53
Richard E. Marland