Organophosphates

Organophosphates are a group of synthetic organic chemicals with some common elements in their chemical structure and the same toxic mechanism of acute toxicity. The general structure of organophosphates is characterized by a central phosphorus atom that is attached to an oxygen or a sulfur atom by a double bond and to three side chains by single bonds.

Organophosphates are mainly used as nerve gases and agricultural pesticides. In fact, organo-phosphate insecticides are the most extensively used pesticides for insect control in the world today. Organophosphates are also widely present in many commonly used products in modern life, such as solvents, plasticizers, flame retardants, gasoline additives, and therapeutic drugs, in both human and veterinary medicine.

All organophosphates have the same mode of toxic action for insects and mammals and can affect the nervous system by inhibiting acetylcholinest-erase, an enzyme that degrades the neurotransmit-ter acetylcholine. Inhibition of acetylcholinesterase can result in excessive accumulation of acetylcho-line at the synapses and neuromuscular junctions causing overstimulation of nerve cells. The nerve transmission will continue indefinitely and produce numerous symptoms, including slurred speech, loss of reflexes, weakness, fatigue, vomiting, headache, tremors, and eventually paralysis of body extremities and respiratory muscles.

Although some organophosphate compounds were described and synthesized in the late 1800s, organophosphate insecticides were first synthesized by Gerhard Schrader in Germany in the late 1930s and were brought to the market in 1940s. The toxic properties of organophosphates have also made them suitable agents to be developed as chemical warfare weapons. Several organophosphate compounds were developed as nerve agents and manufactured in significant quantities in Germany during World War II. However, organophosphate-based chemical weapons have rarely been used during warfare. One possible organophosphate-poisoning event in relation to war is Gulf War syndrome. After the end of the 1991 Gulf War, the U.S. and U.K. governments have conducted many studies on Gulf War veterans’ illnesses. On November 17, 2008, a U.S. congressionally mandated research advisory committee has concluded that Gulf War veterans’ illnesses are likely linked to exposure to neurotoxins, including a drug administered to protect troops against nerve gases and pesticides that were used against sand flies and other pests during deployment. In fact, nerve gas attack on the Tokyo subway can be considered the most well-known organophosphate-based chemical weapon event. On the morning of March 20, 1995, the Aum Shin-rikyo cult carried out a sarin attack on the Tokyo subway. This attack killed 12 people and injured more than 5,000.

Despite their high toxicity to human and wildlife health, organophosphates can break down more rapidly in the environment and persist from only a few hours to several months. Organophos-phates were promoted as an attractive alternative to replace persistent organochlorine insecticides during the 1960s and 1970s. The use of organo-phosphate insecticides increased substantially after many organochlorine insecticides, such as DDT (dichlorodiphenyltrichloroethane), aldrin, and dieldrin, were banned in 1970s. Based on the Food Quality Protection Act of 1996, the U.S. Environmental Protection Agency has suggested that organophosphates pose the greatest risks to human health and the environment among currently used pesticides.

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