DCPA (Dacthal) By Caroline Cox mal irritation (in rabbits).3 Based on are being used even though the study is other EPA data, the acute oral LD50 ap- inadequate. A chronic feeding study of Dimethyl tetrachloroterephthalate, pears to be greater than 10,000 milli- dogs found no adverse effects,3 but a commonly known as dacthal, DCPA, grams per kilogram (mg/kg) of body subchronic (28 day) study found weight or chlorthal-dimethyl, is an herbicide weight in both rats and dogs.8 loss, decrease in liver weight and liver active ingredient used to control crab- degeneration at the only dose tested, 800 grasses, other annual grasses, and cer- Chronic Toxicity mg/kg/day.10 tain broad-leaved weeds in turf, home In a 1963 study, rats fed DCPA for Tests required for U.S. registration of flower gardens, nursery stock, and a two years had increased kidney weights dacthal found no teratogenic (causing number of fruit and vegetable crops.1 (in males), increased adrenal gland birth defects) or mutagenic (causing ge- First registered for use in the United weights (in females), excessive growth netic damage) effects,2 although a Rus- States in 1958, it is manufactured by of the thyroid, abnormalities of the liver, sian study of mouse bone marrow cells the Fermenta Plant Protection Com- and accumulation of iron in the spleen found that dacthal caused abnormalities pany and sold under the trade name (in females). The No Observable Effect in cell division.11 Valid tests of DCPA’s Dacthal.2 Estimated annual usage in Level (NOEL) for these symptoms was oncogenicity (ability to cause tumors) 1988 was between 3.2 and 4.7 million 50 mg/kg/day.2 EPA believes that the and reproductive toxicity had not been pounds. Only 37 percent of this is used test was not adequate to detect all of completed by 1988,2 although the two- on crops; the majority is used on turf the chronic effects of DCPA because a year rat study (above) found a variety sod farms, golf courses, home lawns, persistent lung infection was present in of tumors.9 and home gardens.3 Estimated use on both treated and untreated animals, re- golf courses in 1982 was 400,000 Contaminants and “Inert” (Secret) 4 Ingredients pounds. Figure 1 DCPA is a chlorinated benzoic acid2 DCPA and Its Contaminants DCPA is contaminated by two toxic (see Figure 1) in the same chemical impurities, 2,3,7,8-tetrachlorodibenzo-p- family as the herbicides dicamba and CO CH dioxin (2,3,7,8-TCDD) and chloramben.5 2 3 hexachlorobenzene (HCB).3 (See Figure Cl Cl 1.) Levels of 2,3,7,8-TCDD in DCPA are Mode of Action Cl Cl as high as 100 parts per trillion, while DCPA kills germinating seeds. Micro- HCB has been found at levels up to three CO CH scopic studies indicate that it does this 2 3 parts per thousand.3 Since both of these by disrupting microtubule formation in DCPA compounds are toxic, their presence in exposed cells and causing abnormal cell DCPA is of concern. division. Microtubules are slender cylin- O 2,3,7,8-TCDD is the most toxic mem- Cl Cl drical structures made of polymerized ber of the dioxin family of chlorinated proteins. They move chromosomes to organic (carbon-containing) compounds. Cl Cl newly forming daughter cells and con- O In animals, dioxins cause a variety of ad- trol the orientation of cell walls in plant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) verse health effects, including cancer, cells. Abnormal microtubules disrupt birth defects, atrophy of the thymus, cell wall formation, resulting in the for- liver damage, reduced functioning of the mation of large multinucleate cells when Cl immune system, reproductive failure, cell division is inhibited. They also dis- Cl Cl skin disfiguration, and weight loss12 (JPR rupt the process by which chromo- 9(4):32-36). somes replicate and divide.6,7 Cl Cl Many of these effects are found fol- Cl lowing exposure to minute amounts of Acute Toxicity Hexachlorobenzene (HCB) 2,3,7,8-TCDD. For example, both carci- Of the six acute toxicity tests required nogenic and reproductive effects have by the U.S. Environmental Protection been found at levels in animals of one Agency (EPA) for the registration of a sulting in a high mortality rate. In addi- part per trillion.13,14,15 Mortality, reduced pesticide, only one (a skin sensitization tion, the sample size of the test was growth, and lethargic behavior in rain- test in guinea pigs) had been completed smaller than current EPA standards.9 bow trout was found in water contain- at the time DCPA’s registration standard EPA is requiring that the test be re- ing 38 parts per quadrillion of 2,3,7,8- was issued in 1988. The five missing tests peated. NOELs are used by EPA and TCDD.16 include oral toxicity or LD50* (in rats), other regulatory agencies to establish 2,3,7,8-TCDD persists in soil; esti- dermal toxicity (in rabbits), inhalation permissible exposure levels to pesti- mates of its half-life in soil range from 10 toxicity (in rats), eye irritation, and der- cides, and the results of this rat study to 30 years.17 It also concentrates in ex- posed animals, particularly in fish. In the rainbow trout study mentioned above, * The LD50 is the dose of a chemical that kills Caroline Cox is JPR’s editor. 50 percent of a population of test animals. bioconcentration factors in fish were cal- JOURNAL OF PESTICIDE REFORM / FALL 1991 17 ingredients.1 Figure 2 Estimated Number of Cancers Human Exposure Caused by Exposure to DCPA's Contaminants3 DCPA residues have been found in the Total Diet Study conducted by the Pest control operators U.S. Food and Drug Administration. The treating lawns with DCPA study samples a “market basket” of 234 food items collected four times a year from different regions of the U.S. and Farmworkers treating crops tests for pesticide residues. The samples are analyzed for over 200 chemicals. In 1987, DCPA was found 20 times in the Children playing on 936 items sampled; it ranked twentieth treated lawns in frequency out of the 53 pesticides found in the survey. HCB was found 91 times and ranked sixth in frequency.22 Homeowners treating lawns EPA has established a Provisional Acceptable Daily Intake for DCPA of .5 mg/kg/day based on the inadequate two Consumers of treated TCDD year rat study discussed above.2 produce HCB Environmental Fate EPA’s “negligible risk” standard In 1988, nine of the ten environmen- tal fate studies required by EPA for DCPA’s registration were incomplete.3 0.01 0.1 1 10 100 1000 Degradation of DCPA in soil is depen- Number of cancers per million people dent on both temperature and soil mois- ture. Half-lives in a recent study varied culated to be over 28,000. Similarly large estimated between three and six years.18 from 18 days at 30° C to 92 days at 10° bioconcentration factors have been cal- HCB bioconcentrates by a factor of 11 in C. Low soil moisture (10 percent) in- culated for carp and fathead minnows. chickens, over 6,000 in catfish, and creased DCPA’s half-life.23 Volatilization In cattle, 2,3,7,8-TCDD concentrations in 140,000 in seagull eggs.20,21 DCPA contains (evaporation) also increased with in- beef and milk fat are approximately five almost ten times as much HCB as the creasing temperature.24 times those found in the cattle feed.17 other three pesticides contaminated with Drift of DCPA onto parsley, daikon, HCB has been associated with arthri- HCB (chlorothalonil, picloram, and pen- dill, and kohlrabi (crops on which DCPA tis, osteoporosis of bones in the hands, tachlorophenol). A total of about 12,500 use is not registered) has been docu- liver damage (including the disease por- kilograms of HCB per year are released mented in California. A field study of phyria cutanea tarda), skin lesions, into the air in the United States as these DCPA drift to quantify this problem muscle weakness and short stature in pesticides are used.18 showed that DCPA volatilization contin- humans.18 HCB is concentrated in breast EPA estimated the carcinogenic risks ued up to 60 days after application. milk and high concentrations are passed associated with the 2,3,7,8-TCDD and About 10 percent of the DCPA was lost from mother to child during nursing. In HCB contaminants in DCPA. The results to the atmosphere in the first 21 days Turkey (where three to four thousand are as high as six hundred cancers per after application. Half-life in the soil in people consumed HCB contaminated million pest control operators exposed this study was 50 days.25 bread in 1955) all infants born to moth- to HCB while treating lawns with DCPA DCPA and its metabolites (breakdown ers with porphyria died. Mortality of and two hundred cancers per million products which EPA considers toxico- nursing babies was also high.18 In ani- children playing on DCPA-treated lawns.3 logically equivalent to DCPA3) have been mals, exposure to HCB causes similar (See Figure 2. For a discussion of the repeatedly found in groundwater and effects. In addition, HCB in animals problems with quantitative risk assess- surface water. In EPA’s National Pesti- causes kidney damage, immune system ment, see JPR 8(1):7-12 and JPR 10(1):2- cide Survey, over 6 percent (about 6,000) suppression, neurologic effects, cleft pal- 7).) of the community water system wells ate, a reduction in the survival of nurs- The public has no information about contained DCPA metabolites as did over ing pups, and cancer.18 the identity of the “inert” ingredients 2 percent (about 264,000) of rural do- HCB residues have been found in the used in DCPA formulations or their tox- mestic wells.