Parathion

CAS RN:56-38-2

Exposure Summary

Parathion's former use as an insecticide and acaricide resulted in its direct release to the environment. If released to air, a vapor pressure of 6.68X10-6 mm Hg at 20 deg C indicates parathion will exist in both the vapor and particulate phases in the ambient atmosphere. Vapor-phase parathion will be degraded in the atmosphere by reaction with photochemically-produced hydroxyl radicals; the half-life for this reaction in air is estimated to be 4.2 hours. Particulate-phase parathion will be removed from the atmosphere by wet and dry deposition. Parathion absorbs radiation with wavelengths shorter than 320 nm, suggesting a potential for direct photolysis. If released to soil, parathion is expected to have moderate to no mobility based upon Koc values ranging from 314 to 15,860. Volatilization from moist and dry soil surfaces is not expected to be an important fate process based upon its Henry's Law constant of 2.98X10-7 atm-cu m/mole and vapor pressure, respectively. After 8 weeks of incubation in an organic and a mineral soil, <2 and 6%, respectively, of the applied parathion remained. Prior exposure of soils to p-nitrophenol resulted in increased mineralization of parathion to carbon dioxide. p-Nitrophenol, diethylthiophosphoric acid and paraoxon have been identified as metabolites under oxidative conditions; under low oxygen conditions reduction to aminoparathion occurs. The half-life for photodecomposition of parathion on 3 soils ranged from 31 to 70 hr. If released into water, parathion is expected to adsorb to suspended solids and sediment in the water column based upon sediment Koc values ranging from 3,086 to 38,000. Parathion biodegrades in acclimated natural waters within several weeks; it completely degraded to aminoparathion within 2 weeks in acclimated water from Holland Marsh, Ontario. After 30 days incubation in non-sterile coastal river water, only 6 to 21% of parathion remained. Volatilization from water surfaces is not expected to be an important fate process based upon this compound's Henry's Law constant. BCFs ranging from 63 to 462 suggest bioconcentration in aquatic organisms is moderate to high. Reported hydrolysis half-lives at 20 deg C at environmentally relevant pHs range from 3 weeks at pH 9 to 43 weeks at pH 5. The half-life for hydrolysis in sterile sea water has been reported to be approximately 1 yr at 4 deg C. Divalent cations may catalyze hydrolysis. 20% of parathion was lost by photolysis in 2 hr in Okeefenokee Swamp water. Occupational exposure to parathion may occur through inhalation of spray mists and dermal contact with this insecticide during or after its application. The general population may be exposed to parathion via inhalation of ambient air and ingestion of contaminated food. Parathion has been widely detected in soil and surface water. (SRC)
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