Diazinon

CAS RN:333-41-5

Exposure Summary

Diazinon's production and use as an insecticide is expected to result in its direct release to the environment; all residential uses have been cancelled. If released to air, a vapor pressure of 9.0X10-5 mm Hg at 25 deg C indicates diazinon will exist in both the vapor and particulate phases in the ambient atmosphere. Vapor-phase diazinon 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 1.8 hrs. Particulate-phase diazinon will be removed from the atmosphere by wet and dry deposition. Diazinon absorbs light in the environmental UV spectrum and therefore may be susceptible to direct photolysis by sunlight. The detection of diazinon in lake waters in Canada (arctic, sub-arctic and remote locations) at long distances (hundreds of kilometers) from known source areas indicates diazinon can undergo long range atmospheric transport. If released to soil, diazinon is expected to have moderate-to-low mobility based upon a range of experimental Kocs from 191-1842. Volatilization from moist soil surfaces is not expected to be an important fate process based upon a Henry's Law constant of 1.17X10-7 atm-cu m/mole. Biodegradation is a major fate process in soils. Half-lives of <1, 2, and 5 weeks have been reported in non-sterile soils as compared to half-lives of 6, 6.5, and 12.5 weeks in sterile soils; at 7.5 and 20 deg C, dissipation was approximately 65-95% in non-sterile soil and only 20-25% in sterile soil. Overall persistence in soils has been reported to be 3 to 14 weeks. Field dissipation half-lives are reported to range from 2.8 to 48 days with an average of 7 days. Disappearance of diazinon in soil is a combined effect of biotic and abiotic hydrolysis with hydrolysis rates depending on pH (especially important in acidic organic soil). The photolysis rate constant of diazinon on soil at 25 deg C is reported to be 0.151 per day which corresponds to a half-life of 4.6 days. Soil degradation of diazinon involves oxidation to the phosphate (diazoxon) and hydrolysis to 2-isopropyl-6-methyl-pyrimidinol. If released into water, diazinon is expected to adsorb to suspended solids and sediment based upon the experimental Kocs. Volatilization from water surfaces is not expected to be an important fate process based upon this compound's Henry's Law constant. Hydrolysis half-lives for diazinon at 25 deg C are 12 days (pH 5), 139 days (pH 7) and 77 days (pH 9). Photolysis may be an important degradation process; 36% of diazinon photolyzed in a pH 7 buffer solution exposed to light with wavelengths >290 nm in a period of 24 hrs. The photolysis rate constant in water at 25 deg C is reported to be 0.136 per day which corresponds to a half-life of 5.1 days. A range of experimental BCFs from 3-200 suggests bioconcentration in aquatic organisms is low to moderate. Occupational exposure to diazinon may occur through inhalation and dermal contact with this compound at workplaces where diazinon is produced or used. Monitoring data indicate that the general population may be exposed to diazinon during application following residential use as an insecticide, inhalation of ambient air, and ingestion of contaminated food and drinking water. (SRC)
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