Methyl parathion is a toxic organophosphate insecticide, also known as parathion-methyl and metafos. Its chemical formula is C8H10NO5PS. This substance was widely used in agriculture due to its effectiveness as a pesticide.
Understanding Methyl Parathion
Methyl parathion is an organophosphate insecticide containing phosphorus. It controls pests like boll weevils, mites, and mosquito larvae by contact, stomach, and respiratory action. Historically, its primary use was in agriculture, especially on cotton and corn crops in the United States.
Pure methyl parathion is a colorless crystalline solid, but the technical product is often a light to dark tan substance with about 80% purity. It has a pungent, garlic-like odor when impure. While sparingly soluble in water, it dissolves well in many organic solvents. This pesticide is not widely dispersed or persistent in the environment under normal use, with a reported field half-life of 1 to 30 days. Temperature and sunlight can influence its degradation rate.
Pathways of Exposure
Humans can be exposed to methyl parathion through inhalation, dermal absorption, and ingestion. Inhalation occurs by breathing contaminated air, especially near agricultural spraying areas or from volatilization off plants and soil after application. Dermal exposure is a major route for workers involved in manufacturing, application, cleanup, and for field workers.
Skin contact can happen when touching contaminated plants or surfaces, particularly in cases of illegal indoor spraying where the chemical does not break down quickly. Ingestion may occur through consuming contaminated food or water, or accidental swallowing. Residues have been detected in air, water, fish, soil, and agricultural crops, making food consumption a potential exposure pathway.
Health Impacts
Methyl parathion exerts its toxic effects by inhibiting acetylcholinesterase activity in the nervous system and at the motor end-plate. Its active metabolite, methyl paraoxon, phosphorylates the enzyme’s active site, preventing acetylcholine breakdown. This accumulation overstimulates cholinergic end organs, leading to various symptoms.
Acute exposure symptoms can manifest within minutes to 12 hours. Symptoms include headaches, dizziness, nausea, vomiting, abdominal cramps, and diarrhea. More severe signs involve blurred vision, pinpoint pupils, excessive sweating and salivation, muscle twitching, tremors of the tongue or eyelids, and difficulty breathing. Very high doses can lead to incoordination, slurred speech, loss of reflexes, paralysis of respiratory muscles, irregular heartbeat, unconsciousness, convulsions, and death from respiratory failure or cardiac arrest. Children exposed have shown lethargy, increased salivation, increased respiratory secretions, and pinpoint pupils, with some cases resulting in death.
While long-term human health effects from methyl parathion exposure are not fully established, chronic occupational exposure to organophosphates, including methyl parathion, has been linked to neuropsychiatric disorders. Animal studies suggest chronic oral exposure could lead to neurological effects like distal axonopathy or decreased nerve conduction velocity. Chronic exposure in animals has also been associated with hematological changes and ocular alterations.
Regulatory Status and Preventing Exposure
Methyl parathion is classified as a Category Ia (extremely toxic) pesticide by the World Health Organization (WHO) and Toxicity Category I by the U.S. Environmental Protection Agency (EPA). Its use has been heavily regulated and largely phased out for many agricultural applications due to health and safety concerns. The U.S. EPA canceled all methyl parathion registrations for insecticide and acaricide use, with the last legal use date for end-use products being December 31, 2013.
Internationally, methyl parathion is recommended for inclusion in the Prior Informed Consent (PIC) procedure under the Rotterdam Convention. This convention facilitates information exchange on hazardous chemicals and supports national decisions on their import, rather than banning them.
Preventing Exposure
To minimize personal exposure, individuals should avoid contaminated areas or products. Workers handling methyl parathion are advised to wear protective clothing, including respirators, and to wash all equipment and clothing thoroughly after use. Contaminated clothing should be laundered separately from family clothing. A 48-hour re-entry period is recommended for agricultural workers after field treatment.