Know your pesticide: 2,4-D
by Dzulkifli Abdul Razak
The Sun, May 31, 1997
2,4-D IS 2,4-DICHLOROPHENOXYACETIC acid and is used in herbicides. 2,4-D, a chlorinated phenoxy compound, functions as a systemic herbicide and is used to control many types of weeds. This compound is used in cultivated agriculture and in pasture and rangeland applications, forest management, home and garden situations and for the control of aquatic vegetation.
At very low application rates, 2,4-D is an efficient plant growth regulator when mixed with other substances. There are many forms or derivatives (esters, amines, salts) of 2,4-D and these vary in solubility and volatility. It is a white powder with a slight phenolic odour.
2,4-D enters the body through inhalation and the skin during occupational exposure. Its mechanism of action is related to uncoupled oxidative phosphorylation and decreased oxygen consumption in tissues, as well as to disturbances in carbohydrate and other metabolic processes.
Though the LD50 of 2,4-D is only moderately toxic, the product carries the DANGER signal word on the label, indicating that it is highly toxic. This is because 2,4-D has produced serious eye and skin irritation in agricultural workers.
2,4-D is used in many commercial products. Over 150 products containing 2,4-D exist in Malaysia, either on its own or in combination with other chemicals.
In humans, prolonged breathing of 2,4-D causes coughing, burning, dizziness and temporary loss of muscle coordination. Acute oral poisoning by 2,4-D results in headache, dizziness, drowsiness, unsteadiness, dilated pupils, diplopia, vomiting, diarrhoea, mucous burns, muscular weakness, impaired coordination, speech difficulty, some muscle fibrillation, hyporeflexia, clonic spasm, convulsions, fever, cyanosis, heart disturbance, loss of consciousness, coma and urinary incontinence.
Symptoms vary with the different commercial products because of the specific amounts and types of additives such as surfactants and solvents. Only poor occupational practice makes possible massive dermal and inhalation overexposure with signs and symptoms of acute intoxication.
Respiratory tract irritation and dyspnoea with functional pulmonary disorders are common following occupational exposure. Bronchitis, peribronchitis, and pneumosclerosis have been connected with long-term occupational exposure.
- Teratogenic effects
2,4-D has a very limited ability to cause birth defects. However, rats fed 150 mg/kg on days 6-15 of pregnancy had increase in skeletal abnormalities such as delayed bone development and wavy ribs, which are reflection of general toxicity. The same conclusions may be drawn for 2,4-D's potential to cause teratogenic effects in humans.
- Carcinogenic effects
Low doses fed to rats for two years caused an increase in malignant tumours. There was some question about whether the tumours were associated with specific organs or were non-specific. Female mic given a single injection of 2,4-D developed cancer (reticulum-cell sarcomas).
In humans, a variety of studies give conflicting results.
Several studies in Sweden and the United States suggest an association of 2,4-D exposure with cancer. An increase occurrence of non-Hodgkin's lymphoma was found among Kansas and Nebraska farm population associated with the spraying of 2,4-D. Other studies done in New Zealand, Washington, New York, Australia and on Vietnam War veterans from the United States were all negative. There remains considerable controversy about the methods used in the various studies and thus with the results of the various studies.
- Organ toxicity
Most symptoms disappear within a few days but there is a report of liver dysfunction from long term exposure. The absorption of the herbicide is almost complete in mammals after ingestion and nearly all of the dose is excreted in the urine. The compound is readily absorbed through the skin and lungs also. Only traces of the compound have been found in the milk of lactating animals for six days following exposure.
The half-life of 2,4-D is between 10 and 20 hours in living organisms. There is little evidence to suggest that the compound accumulates to any significant level in mammals or in other organisms. Peak concentration of 2,4-D were found in the blood, liver, kidney, lungs and spleen with lower levels in muscle and brain between six and eight hours after small doses (1 mg/kg) were given to rats. After 24 hours, there were no detectable tissue residues.
2,4-D is slightly toxic to birds. Some formulations of 2,4-D are highly toxic to fish while others are less so, For example the LD50 ranges between 1.0 mg/L to 200 mg/L in cut-throat trout, depending on the formulation used. Channel catfish have less than 10% mortality at 100 mg/L in 48 hours. Limited studies indicate a half-life of less than two days in fish and osyters when exposure is discontinued.
Brood production is severely impaired when honeybees were fed moderate doses, but at lower levels of exposure, they live significantly longer than the controls. The honeybee LD50 is 11.5 micrograms/bee.
Half-life figures for the herbicide in soil are seven days (15-25 degree Celcius with 65% moisture) and 10 days in non-sterile soil and 1.5 to 16 days in other soils. Soil microbes are primarily responsible for its disappearance in soil.
In aquatic environments, microorganisms readily degrade 2,4-D and breakdown by sunlight is not a major reason for loss. Rates of breakdown increase with increased nutrients, sediment load and dissolved organic carbon.
Under oxygenated conditions, the half-life can be short, in the order of one week to several weeks.
2,4-D interferes with normal plant growth processes. Uptake of the compound is through leaves, stems and roots. However, it is generally non-persistent. In one study, when 2,4-D was applied to grass, there were 80 ppm at day zero, 45 ppm at 14 days and 6 ppm at 56 days. Breakdown in plants us by a variety of biological and chemical pathways.
In cases of dermal exposure, the management of such exposure involves removing all contaminated clothing and flooding the skin surface with water. Following this, the exposed skin is then washed with soap and water. A close examination of the skin may be required of pain or irritation exist after decontamination. All clothing athat is contaminted should be laundered before it is worn again.
In eye exposure, the exposed eyes should be irrigated with copious amounts of water or saline for at least 15 minutes. Pour the water from a cup or glass held 10 cm from the eye. If irritation, pain, swelling, lacrimation or photophobia persist, the patient should be seen in a healthcare facility.
In case of inhalation exposure, move the patient to a place with fresh air. Monitor for respiratory distress. If cough or difficulty in breathing develops, evaluate for respiratory tract irritation, bronchitis or penumonitis.
In case of ingestion, there is no specific antidote for 2,4-D poisoning. Treat ingestions of greater than 40 mg/kg with gastric emptying if within four hours of ingestion. Emesis may be indicated in recent substantial ingestion unless the patient is or could rapidly become comatose or convulsing. It is most effective if initiated within 30 minutes.
The writer is a Science Officer at the National Poison Centre, Universiti Sains Malaysia, 11800 Minden, Penang.