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A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Michigan State University, Oregon State University, and University of California at Davis. Major support and funding was provided by the USDA/Extension Service/National Agricultural Pesticide Impact Assessment Program.

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DCPA

Publication Date: 9/93

TRADE OR OTHER NAMES

Trade names for products containing the compound include Dacthal, DAC 893, and Dacthalor. It may be formulated with several other herbicides such as methazole and propachlor. Its only restrictions apply in Washington State which requires supplemental labeling for Dacthal W-75 (9).

INTRODUCTION

DCPA also known as chlorthal dimethyl is a phthalate preemergent herbicide used on annual grasses and certain annual broadleaf weed species in a wide range of vegetable crops. About one-half of the use of this compound in the United States is for homes and gardens.

While there is no toxicological concern for DCPA per se, the technical product may contain very small amounts of dioxin (2, 3, 7, 8, TCDD) and HCB as impurities which are of considerable toxicological concern (8). There are also many gaps in the available information on the herbicide and EPA has stated that no new uses will be approved until some of the gaps are filled in.

DCPA is classified as a general use pesticide.

TOXICOLOGICAL EFFECTS

ACUTE TOXICITY

DCPA is a slightly toxic compound that carries the signal word CAUTION on the label. The compound has a very low toxicity to mammals. Therefore, LD50 values are difficult to obtain. The LD50 figures for DCPA in rats range from greater than 3000 mg/kg to 12,500 mg/kg. DCPA in rabbits and beagle dogs has an LD50 of greater than 10,000 mg/kg. The dermal LD50 for rabbits is greater than 10,000 mg/kg. DCPA is not a skin sensitizer and it is only a mild eye irritant.

CHRONIC TOXICITY

The lowest dose that caused observable effects in rats was 758 mg/kg/day over a month of exposure. Dogs given 800 mg/kg/day a month showed some adverse effects in the liver. In longer term studies with rats (90 days), similar doses (about 750 mg/kg) caused no adverse effects in the animals at the highest dose tested (10). In a two year study with rats, a dose of around 50 mg/kg/day was responsible for chronic toxic effects. Changes were noted in the adrenal weights of the females and in the kidney weights of the males (10).

Reproductive Effects

Rats fed moderate doses of DCPA (500 mg/kg/day) showed no changes in fertility, gestation, live births or lactation (2). The study was conducted over one full generation. It is unlikely that the compound would present a reproductive threat to humans.

Teratogenic Effects

Pregnant rabbits fed moderate doses (up to 300 mg/kg) of DCPA on days 8 to 16 of gestation had no maternal effects and there were no skeletal or organ abnormalities in the offspring (10). It is unlikely that the compound presents a significant risk of birth defects in humans. However, more research is needed in this area.

Mutagenic Effects

No mutagenicity was seen in any one of a number of tests (mutation frequency and activity, cytogenetic tests, DNA repair and dominant lethal tests) (10). Strong evidence indicates that the compound is not mutagenic.

Carcinogenic Effects

Based on gross and microscopic examination, no carcinogenic effects were noted in rats in a two-year study where diets contained up to 500 mg/kg/day of DCPA. However, there is not enough information to draw any firm conclusion about the carcinogenic risk to humans. The EPA has stated that the compound currently is not classifiable (10).

Organ Toxicity

Humans given 25 or 50 mg oral doses showed no evidence of abnormalities in blood, liver, kidney and urine analysis. A 3 mg dose in a rabbit eye produced mild irritation which was gone in 24 hours.

Fate in Humans and Animals

Much of the compound that is ingested is not absorbed. Cows excreted nearly all of a small dose of DCPA within five days and dogs absorbed only small amounts (3%) of the compound. The remaining amount was eliminated within four days. The half-life of the parent compound is about five minutes.

Dairy cows had some DCPA or metabolites in their milk. Diets up to 200 ppm for 24 days gave 0.26 ppm in milk, while 30 to 90 ppm for 9 or 23 days gave residues of 0.036 ppm and 0.066 ppm in milk. Residues in other tissues were generally less than 1 ppm (6).

ECOLOGICAL EFFECTS

DCPA appears to be moderately toxic to some young wildfowl and practically non-toxic to the young of other species and to adult birds. The LD50 in young bobwhite quail is 5500 mg/kg. Young mallards and young quail were more sensitive to the herbicide than adult birds. Diets containing about 250 mg/kg caused heavy mortality in the first five days to younger ducks. Older birds had a higher survival rate at this level over a 100-day period (3). The total consumption of DCPA for the study was nearly 40,000 mg/kg.

DCPA is slightly toxic to practically non-toxic to fish depending on the species. For example it is non-toxic to blue gill sunfish and slightly toxic to rainbow trout. The compound is practically non-toxic to estuarine and marine organisms (invertebrates and some fish). The available data suggest that DCPA poses no hazard to endangered aquatic species (10).

When exposed to rates of 100 g/bee, DCPA was not toxic. At 229.63 g/bee there was only three percent mortality. Thus, DCPA is only slightly toxic to bees.

ENVIRONMENTAL FATE

The half-life of DCPA is from 14 to one hundred days in most soils, However, moisture is essential for degradation. In one study there was no apparent buildup of pesticide residues in soil even after repeated application. The DCPA concentration declined slowly to 75 or 80% in 28 days. Later sampling showed a continued decline of DCPA and its breakdown products. No actual half-time was found for one of its breakdown products (diacid), but it is thought to be about a year.

There is virtually no degradation of DCPA in water ranging from moderately acidic to moderately alkaline (pH 5.0 to pH 9.0). The half- life due to sunlight action in water is greater than one week.

Plants may produce the same two breakdown products that are seen in soils with the proportion varying in different species. DCPA affects the seed and pre emergence stage, but has little effect on crops or weeds after they have emerged. Limited information suggests that plants may remove the chlorine molecules of DCPA (6). In one study pine trees took up nearly 1% of the soil applied chemical. The majority of the compound taken up by the trees remained in the root system where it was rapidly diluted.

The metabolite, tetrachloro-terephthalic acid (TTA or diacid), is much more water soluble than the parent compound and is subject to leaching in some soils. This metabolite has been detected below the health advisory level in groundwater in the onion growing areas of Eastern Oregon (5). It has been detected in several other states in the US as well (10).

Exposure Guidelines:

NOEL (rat): 50 mg/kg/day
DWEL: 2.0 mg/l
HA: 4.0 mg/l lifetime
ADI: 0.5 mg/kg/day (ppm) (EPA)
RfD: 0.5 mg/kg/day (EPA)
LEL: 500 mg/kg/day (rat)

Physical Properties

CAS #: 1861-32-1
Chemical name: dimethyl 2,3,5,6-tetrachlorobenzene-1,4dicarboxylate
Chemical class/use: phthalate herbicide
Solubility in water: 0.5 mg/l
Solubility in other solvents: benzene 25 g/100 g; toluene 17 g/100 g; acetone 10 g/100g; carbon tetrachloride 7 g/100 g
Melting Point: 155-156 degrees C
Vapor Pressure: 12.5 x 10 to the minus 6 power mm Hg
Partition Coefficient: 4.95 (octanol/water)

BASIC MANUFACTURER

ISK Biotech Corp.
5966 Heisley Rd
PO Box 8000
Mentor, OH 44060-8000
Telephone: 216/357-4100

Review by Basic Manufacturer:

Comments solicited: October, 1992
Comments received:

REFERENCES

  1. SDS Biotech Corporation (1986). DCPA (Dathal) Supporting Data, Tetrachloroterephthalic Acid, Volume I, SDS-954, Painesville, OH.
  2. U.S. Environmental Protection Agency (1987). Health Advisory, Office of Drinking Water.
  3. Beste, C.E., Chairman (1983). Herbicide Handbook of the Weed Science Society of America. Weed Science Society of America, Champaign, IL.
  4. Midwest Research Institute (1975). Initial Scientific and Minieconomic Review No. 14, DACTHAL, Kansas City, MO.
  5. Malheur County Sanitarian (1985) News Release, October 7, Vale, OR.
  6. Food and Drug Administration (1986). The FDA Surveillance Index. Bureau of Foods, Dept of Commerce, National Technical Information Service, Springfield, VA.
  7. Forest Service (1987). Pesticide Background Statements, Vol III Nursery Pesticides, U. S. Dept. of Ag., Agriculture Handbook N. 7.
  8. Walker, Mary M. and Lawrence H. Keith. (1992). EPA's Pesticide Fact Sheet Data Base. Lewis Publishers. Chelsea, MI.
  9. The Farm Chemicals Handbook. (1992). Meister Publishing. Willoughby, OH.
  10. Health Advisories for 50 Pesticides. 1988. United States Environmental Protection Agency. Dacthal.