Tabun

CAS RN:77-81-6

Exposure Summary

Tabun's production may result in its release to the environment; its use as a chemical warfare agent and military nerve gas will result in its direct release to the environment. If released to air, a vapor pressure of 0.07 mm Hg at 25 deg C indicates tabun will exist solely as a vapor in the atmosphere. Vapor-phase tabun 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 8 hours. The UV absorption spectrum of tabun does not exhibit absorption above 295 nm; therefore, tabun is not expected to degrade through direct photolysis in the environment. If released to soil, tabun is expected to have very high mobility based upon an estimated Koc of 39. Volatilization from moist soil surfaces is not expected to be an important fate process based upon an estimated Henry's Law constant of 1.5X10-7 atm-cu m/mole and tabun's susceptibility to hydrolysis. Persistence depends on moisture and weather conditions. Liquid tabun may persist for 1 to 2 days under ambient weather conditions and it evaporates about 20 times more slowly than water. Field studies measuring the evaporation rate of tabun from soil found that 50% of applied tabun evaporated in 1.71 hr and that 90% evaporated in 4.66 hr. Several species of bacteria, such as Alteromonas sp. and Flavobacterium sp., are capable of degrading G-type nerve agents, but data are not available to assess the environmental importance of biodegradation. If released into water, tabun is not expected to adsorb to suspended solids and sediment based upon the estimated Koc. Tabun hydrolyzes in water at neutral or slightly acidic pH and more rapidly under strong acidic or alkaline conditions. The rate of hydrolysis also increases with increasing temperature. At neutral pH and 25 deg C, tabun persists in water for 14 to 28 hours; the half-life at 20 deg C and pH 7.4 is approximately 8 hours. The half-life in seawater at 20 deg C is shorter (4.5 hours) than in freshwater. Volatilization from water surfaces is not expected to be an important fate process based upon this compound's estimated Henry's Law constant and its hydrolysis in water. An estimated BCF of 3 suggests the potential for bioconcentration in aquatic organisms is low. Occupational exposure to tabun may occur through inhalation and dermal contact with this compound at workplaces where tabun is produced or used. Exposure to tabun occurs through vapor contact which is readily absorbable through not only the lungs and eyes but also through the skin and intestinal tract. The general population will not be exposed to tabun unless it is used as a weapon; exposure to tabun, if used as a weapon, will be via inhalation of ambient air and dermal contact. (SRC)
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