CHAPTER 2-48: CHLOROPHENOXY HERBICIDES (2,4-D)

2,4-Dichlorophenoxyacetic acid (2,4-D) and its chemical derivatives are widely used herbicides. A large number of formulations are available containing different 2,4-D salts (sodium, amine, alkylamine, and alkanolamine) and esters (propanoic acid, butanoic acid, and other alkoxy compounds). The most frequently used agricultural product, based upon 2013 California pesticide use data, is the dimethylamine salt of 2,4-D. Current California registration data (November 2015) show 205 formulations for the dimethylamine salt, with concentrations ranging from 0.12% (for the most dilute home use product) to 46.8–96.9% (for agricultural formulations). Although some concentrated formulations of 2,4-D esters are wettable powders, others contain petroleum solvents (identified on the "first aid" statement on the pesticide label); even though these solvents are considered "inert" ingredients because they are not pesticides, they may have their own innate toxicity (see "Toluene and Xylene" and "Hydrocarbons").

Agent Orange was a mixture of the chlorophenoxy herbicides 2,4-D (dichlorophenoxyacetic acid) and 2,4,5-T (trichlorophenoxyacetic acid) that also contained small amounts of the highly toxic contaminant TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin, derived from the process of manufacturing 2,4,5-T. Manufacture of 2,4-D by chlorination of phenol does not produce TCDD. Populations involved in the manufacture or handling of 2,4,5-T may show elevated levels of TCDD on serum testing and overall increased rates of cancer compared with the general population.

In plants, the compounds act as growth hormone stimulators. The mechanism of toxicity is unclear but may involve mitochondrial injury. In animals, cell membrane damage, uncoupling of oxidative phosphorylation, and disruption of acetyl coenzyme A metabolism are found, widespread muscle damage occurs, and the cause of death is usually ventricular fibrillation. Toxicity is markedly increased at doses that exceed the capacity of the renal anion transport mechanism (approximately 50 mg/kg). Massive rhabdomyolysis has been described in human patients, most often in cases involving ingestion of formulations containing more than 10% active ingredient.

2,4-D doses of 5 mg/kg are reported to have no effect in human volunteer studies. The minimum toxic dose of 2,4-D in humans is 3–4 g or 40–50 mg/kg, and death has occurred after adult ingestion of 6.5 g. Less than 6% of 2,4-D applied to the skin is absorbed systemically, although dermal exposure may produce skin irritation. The degree of dermal absorption may be less with salt formulations than with 2,4-D esters.

1. Acute ingestion. Vomiting, abdominal pain, and diarrhea are common. Tachycardia, muscle weakness, and muscle spasms occur shortly after ingestion and may progress to profound muscle weakness and coma. Massive rhabdomyolysis, metabolic acidosis, and severe and intractable hypotension have been reported, resulting in death within 24 hours. (A review of 66 published cases reported 33% were fatal.) Neurotoxic effects include ataxia, hypertonia, seizures, and coma. Hepatitis and renal failure may occur.

2. Dermal exposure to 2,4-D may produce skin irritation. Exposures to formulations containing 2,4,5-T may also produce chloracne. Substantial dermal exposure has been reported to cause a mixed sensory-peripheral neuropathy after a latent period.

Depends on a history of exposure and the presence of muscle weakness and elevated serum creatine kinase (CK).

1. Specific levels of 2,4-D can be measured by specialty or agricultural laboratories but may not be available in a timely enough fashion to be of help in establishing the diagnosis. The elimination half-life of 2,4-D is 11.5 hours, and more than 75% is excreted by 96 hours after ingestion. More than 80% is excreted in the urine unchanged.

2. Other useful laboratory studies include electrolytes, glucose, BUN, creatinine, CK, urinalysis (occult heme test result positive in the presence of myoglobin), liver enzymes, 12-lead ECG, and ECG monitoring.

1. Emergency and supportive measures

   1. Maintain an open airway and assist ventilation if necessary.

   2. Treat coma, hypotension, and rhabdomyolysis if they occur.

   3. Monitor the patient closely for at least 6–12 hours after ingestion because of the potential for delayed onset of symptoms.

2. Specific drugs and antidotes. There is no specific antidote.

3. Decontamination

   1. Skin or eye exposure. Remove contaminated clothing and wash affected areas.

   2. Ingestion. Administer activated charcoal orally if conditions are appropriate (see Table 1–38). If a delay of more than 60 minutes is expected before charcoal can be given, consider using ipecac or other emetic if it can be administered within a few minutes of exposure and there are no contraindications. Consider gastric lavage after a large recent ingestion.

4. Enhanced elimination. There is no proven role for these procedures, although alkalinization of the urine may promote excretion of 2,4-D. (As with other weak acids, alkalinization would be expected to promote ionization of the phenoxy acid and decrease reabsorption from the renal tubules.) Hemodialysis has been recommended on the basis of limited clinical data showing clearances similar to those of alkaline diuresis. Plasmapheresis was reported effective in a pediatric case of polyneuropathy associated with 2,4-D ingestion.