Methamidophos

Methamidophos, trade name "Monitor," is an organophosphateinsecticide.

Crops grown with the use of methamidophos include potatoes^[1] and some Latin American rice.^[2] Many nations have used methamidophos on crops, including developed nations such as Spain, United States, Japan, and Australia. Due to its toxicity, the use of pesticides that contain methamidophos is currently being phased out in Brazil. In 2009, all uses in the United States were voluntarily canceled.^[3]

LD₅₀ rates of 21 and 16 mg/kg for male and female rats, respectively. 10–30 mg/kg in rabbits, and dermal LD50 of 50 mg/kg in rats. It is rapidly absorbed through the stomach, lungs, and skin in humans, and eliminated primarily through urine.^[4] It is a cholinesterase inhibitor.

Breakdown in soil is 6.1 days in sand, 309 days in water at pH 5.0, 27 days at pH 7.0, and 3 days at pH 9.0. Sunlight accelerates breakdown. It is uptaken through roots and leaves of plants.^[4]

It is classified as a WHOToxicity Class "Class 1b, Highly Hazardous", and its parent chemical, acephate, is "class III, Slightly Hazardous".

Methamidophos is used in great quantities in ricefields in China.^[5] Rice–fish culture is common in the southern parts of China as well as in many other rice-producing countries (e.g., Thailand, Malaysia, and the Philippines). Brown rice (unpolished) in this study contained double the concentration of polished rice. Both plants and animals did not degrade the pesticide well, and fish for human consumption in these cases contains methamidophos in concentrations roughly similar to brown rice.^[5]

Its toxicological effects are primarily associated with inhibition of acetylcholinesterase, an enzyme essential for the regulation of the neurotransmitter acetylcholine. Excessive acetylcholine accumulation can result in cholinergic syndrome, characterized by symptoms such as excessive salivation, sweating, nausea, vomiting, abdominal cramping, respiratory distress, muscle weakness, and, in severe cases, convulsions, respiratory failure, or death.

Consumer exposure may occur through improper handling, accidental ingestion, or contact with contaminated food, water, or surfaces. Due to its high acute toxicity and potential for harmful exposure, methamidophos has been subject to strict regulatory controls and restrictions in many jurisdictions, including limitations or prohibitions on agricultural and consumer use. Residue monitoring and safety regulations have been implemented in various regions to reduce the risk of dietary and environmental exposure.

The compound’s toxicity has led regulatory agencies to classify methamidophos as a highly hazardous pesticide. Appropriate protective measures, including controlled application practices and adherence to established safety standards, are considered necessary wherever its use is permitted.

• GS-7

• Methamidophos in the Pesticide Properties DataBase (PPDB)