structural termite control agent, and in agriculture. Most residential use of chlorpyrifos has been phased out in the United States; however, agricultural use remains common. EPA estimated that, between 1987 and 1998, about 21 million pounds of chlorpyrifos were used annually in the US. In 2001, chlorpyrifos ranked 15th among pesticides used in the United States, with an estimated 8 to 11 million pounds applied. In 2007, it ranked 14th among pesticide ingredients used in agriculture in the United States. Application Chlorpyrifos is normally supplied as a 23.5% or 50% liquid concentrate. The recommended concentration for direct-spray pin point application is 0.5% and for wide area application a 0.03–0.12% mix is recommended (US). Kinetics Chlorpyrifos enters the insect through several routes. Simon et al. 1998 report that insects encounter the active ingredient in their food plants and eat it. They also find it to enter through the digestive system, skin and membranes of the respiratory system. Mechanism of action Like other organophosphate pesticides chlorpyrifos acts by acetylcholinesterase inhibition. Human toxicity Chlorpyrifos exposure may lead to acute toxicity at higher doses. Persistent health effects follow acute poisoning or from long-term exposure to low doses, and developmental effects appear in fetuses and children even at very small doses. Acute health effects For acute effects, the World Health Organization classifies chlorpyrifos as Class II: moderately hazardous. The oral LD50 in experimental animals is 32 to 1000 mg/kg. The dermal LD50 in rats is greater than 2000 mg/kg and 1000 to 2000 mg/kg in rabbits. The 4-hour inhalation of LC50 for chlorpyrifos in rats is greater than 200 mg/m3. Symptoms of acute exposure Acute poisoning results mainly from interference with the acetylcholine neurotransmission pathway, leading to a range of neuromuscular symptoms. Relatively mild poisoning can result in eye-watering, increased saliva and sweating, nausea, and headache. Intermediate exposure may lead to muscle spasms or weakness, vomiting, or diarrhea, and impaired vision. Symptoms of severe poisoning include seizures, unconsciousness, paralysis, and suffocation from lung failure. Children are more likely to experience muscle weakness rather than twitching; excessive saliva rather than sweat or tears; seizures; and sleepiness or coma. Frequency of acute exposure Acute poisoning is probably most common in agricultural areas in Asia, where many small farmers are affected. Poisoning may be due to occupational or accidental exposure or intentional self-harm. Precise numbers of chlorpyrifos poisonings globally are not available. Pesticides are used in an estimated