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The information in this profile may be out-of-date. It was last revised in 1996. EXTOXNET no longer updates this information, but it may be useful as a reference or resource.

Please visit the National Pesticide Information Center (NPIC) to find updated pesticide fact sheets. If you don't find a fact sheet related to your question, feel free to call 1-800-858-7378. NPIC is open five days a week from 8:00am to 12:00pm Pacific Time.

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E X T O X N E T

Extension Toxicology Network

Pesticide Information Profiles

A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Oregon State University, the University of Idaho, and the University of California at Davis and the Institute for Environmental Toxicology, Michigan State University. Major support and funding was provided by the USDA/Extension Service/National Agricultural Pesticide Impact Assessment Program.

EXTOXNET primary files maintained and archived at Oregon State University

Revised June 1996

Methyl parathion

Trade and Other Names: Alternate common names are parathion-methyl and and metafos. Trade names include Bladan M, Cekumethion, Dalf, Dimethyl Parathion, Devithion, E 601, Folidol-M, Fosferno M50, Gearphos, Kilex Parathion, Metacide, Metaphos, Metron, Nitrox 80, Partron M, Penncap-M, and Tekwaisa.

Regulatory Status: Methyl parathion is a highly toxic insecticide in EPA toxicity class I. Some or all formulations of methyl parathion may be classified as Restricted Use Pesticides (RUPs). RUPs may be purchased and used only by certified applicators. Labels for products containing methyl parathion must bear the Signal Word DANGER.

Chemical Class: organophosphate

Introduction: Methyl parathion is an organophosphate insecticide and acaricide used to control boll weevils and many biting or sucking insect pests of agricultural crops, primarily on cotton. It kills insects by contact, stomach and respiratory action. Methyl parathion is available in dust, emulsifiable concentrate, ULV liquid, and wettable powder formulations.

Formulation: Methyl parathion is available in dust, emulsifiable concentrate, ULV liquid, and wettable powder formulations.

Toxicological Effects:

• Acute toxicity: Methyl parathion is highly toxic via the oral route, with reported oral LD50 values of 6 to 50 mg/kg in rats, 14.5 to 19.5 mg/kg in mice, 420 mg/kg in rabbits, 1270 mg/kg in guinea pigs and 90 mg/kg in dogs [2,13]. It is highly toxic via the dermal route as well, with reported dermal LD50 values of 67 mg/kg in rats, 1200 mg/kg in mice, and 300 mg/kg in rabbits [2,13]. The 1-hour inhalation LC50 for methyl parathion in rats is 0.24 mg/L [2]. Effects associated with acute exposure to methyl parathion are similar to those associated with exposure to other organophosphate pesticides [8]. Symptoms of acute exposure to organophosphate or cholinesterase-inhibiting compounds may include the following: numbness, tingling sensations, incoordination, headache, dizziness, tremor, nausea, abdominal cramps, sweating, blurred vision, difficulty breathing or respiratory depression, and slow heartbeat. Very high doses may result in unconsciousness, incontinence, and convulsions or fatality. Persons with respiratory ailments, recent exposure to cholinesterase inhibitors, cholinesterase impairment, or liver malfunction are at increased risk from exposure to methyl parathion.
• Chronic toxicity: Studies with human volunteers have found that of 1 to 22 mg/person/day have no effect on cholinesterase activity. In a 4-week study of volunteers given 22, 24, 26, 28 or 30 mg/person/day, mild cholinesterase inhibition appeared in some individuals in the 24, 26 and 28 mg dosage groups. In the 30 mg/person/day (about 0.43 mg/kg/day) group, red blood cholinesterase activity was depressed by 37%. When methyl parathion was fed to dogs for 12 weeks, a dietary level of 1.25 mg/kg caused a significant depression of red blood cell and plasma cholinesterase. A dietary level of 0.125 mg/kg produced no effects [2].
• Reproductive effects: In a three-generation study with rats fed dietary levels of 0.5 or 1.5 mg/kg/day, reduced weanling survival and birthweight occurred at both doses, as well as an increase in the number of stillbirths at 1.5 mg/kg/day [2,8]. Single injections of 15 mg/kg in rats on day 12 of pregnancy, and single injections of 60 mg/kg on day 10 of pregnancy, in mice caused suppression of fetal growth and bone formation in the offspring that survived. In another study, there were no adverse effects observed in the offspring of rats given oral doses of 4 or 6 mg/kg on day 9 or 15 of pregnancy [2]. Reproductive effects in humans are not likely under normal circumstances.
• Teratogenic effects: In a three-generation study with rats fed dietary levels of 0.5 or 1.5 mg/kg/day, there were no compound-related teratogenic effects [2]. Single injections of 5 to 10 mg/kg in rats on day 12 of pregnancy and single injections of 20 mg/kg on day 10 of pregnancy in mice caused no statistically significant changes in the offspring [2,8]. Oral administration of 4 to 6 mg/kg on day 9 or 15 of pregnancy in rats resulted in no fetal anomalies. Available evidence indicates that methyl parathion does not cause teratogenic effects.
• Mutagenic effects: No signs of mutagenicity were seen in mice given dosages of 5 to 100 mg/kg, nor in mice fed methyl parathion for 7 weeks [2]. Available evidence suggests that methyl parathion is nonmutagenic.
• Carcinogenic effects: Available evidence suggests that methyl parathion is not carcinogenic [84].
• Organ toxicity: Methyl parathion primarily affects the nervous system.

• Fate in humans and animals: Methyl parathion is rapidly absorbed into the bloodstream through all normal routes of exposure. Following administration of a single oral dose, the highest concentration of methyl parathion in body tissues occurred within 1 to 2 hours [2]. Methyl parathion does not accumulate in the body, and is almost completely excreted by the kidneys (urine) within 24 hours as phenolic metabolites [2,13].

Ecological Effects:

• Effects on birds: Methyl parathion is very highly to highly toxic to birds. Reported acute oral LD50 values are 3 mg/kg in American kestrels, 7.5 mg/kg in European starlings, 6 to 10 mg/kg in mallards, 8 mg/kg in northern bobwhites, 10 to 24 mg/kg in red-wing blackbirds,and 8 mg/kg in ring-neck pheasants [6]. The 5- to 8-day dietary LC50 values reported for methyl parathion include 69 ppm in Japanese quail, 330 to 680 ppm in mallard, 90 ppm in northern bobwhite, and 91 ppm in ring-neck pheasant [6].
• Effects on aquatic organisms: Methyl parathion is moderately toxic to fish and to animals that eat fish [13,8]. Reported 96-hour LC50 values are from 1.9 to 8.9 mg/L in the following fish species: coho salmon, cutthroat trout, rainbow trout, brown trout, lake trout, goldfish, carp, fathead minnow, black bullhead, channel catfish, green sunfish, bluegill, largemouth bass, and yellow perch. Reported 96-hour LC50 values indicate very high toxicity for aquatic invertebrates such as Daphnia spp., scuds, and sideswimmers [8].
• Effects on other organisms: Mthyl parathion is toxic to bees [13].

Environmental Fate:

• Breakdown in soil and groundwater: Methyl parathion is of low persistence in the soil environment, with reported field half-lives of 1 to 30 days [19]. A representative value is estimated to be 5 days [19]. The rate of degradation increases with temperature and with exposure to sunlight. Methyl parathion is moderately adsorbed by most soils, and is slightly soluble in water [19]. Due to its low residence time and soil binding affinity, it is not expected to be significantly mobile. 4-Nitrophenol, a breakdown product of methyl parathion, does not adsorb well to soil particles and may contaminate groundwater. When large concentrations of methyl parathion reach the soil, as in an accidental spill, degradation will occur only after many years, with photolysis being the dominant route [12]. Some volatilization of applied methyl parathion may occur.
• Breakdown in water: Methyl parathion degrades rapidly in seawater, lake, and river waters, with 100% degradation occurring within 2 weeks to 1 month or more. Degradation is faster in the presence of sediments, and is faster in fresh water than in salt water. Complete breakdown occurs at a rate of 5 to 11% in 4 days in rivers, and more slowly in marine waters. In water, methyl parathion is subject to photolysis, with a half-life of 8 days during the summer and 38 days in winter [12].
• Breakdown in vegetation: Uptake and metabolism of methyl parathion in plants is fairly rapid. Within 4 days after applying methyl parathion to the leaves of corn, it was almost completely metabolized [12].

Physical Properties:

• Appearance: Pure methyl parathion is a colorless crystalline solid. The technical product is light to dark tan, with about 80% purity [13].
• Chemical Name: O,O-dimethyl O-4-nitrophenyl phosphorothioate [13]
• CAS Number: 298-00-0
• Molecular Weight: 263.21
• Water Solubility: 55-60 mg/L @ 25 C [13]
• Solubility in Other Solvents: s. in dichloromethane, 2-propanol, toluene, and most organic solvents; i.s. in n-hexane [13]
• Melting Point: 35-36 C [13]
• Vapor Pressure: 1.3 mPa @ 20 C [13]
• Partition Coefficient: 3.5185-3.8388 [8]
• Adsorption Coefficient: 5100 [19]

Exposure Guidelines:

• ADI: 0.02 mg/kg/day [38]
• MCL: Not Available
• RfD: 0.00025 mg/kg/day [53]
• PEL: 0.2 mg/m3 (8-hour) (skin) [39]
• HA: 0.002 mg/L [8]
• TLV: Not Available

Basic Manufacturer:

Drexel Chemical Company
1700 Channel Avenue
Memphis, TN 38113

• Phone: 901-774-4370
• Emergency: Not Available
  

References:

References for the information in this PIP can be found in Reference List Number 5

DISCLAIMER: The information in this profile does not in any way replace or supersede the information on the pesticide product labeling or other regulatory requirements. Please refer to the pesticide product labeling.