Lewisite

CAS RN: 541-25-3

Exposure Summary

Lewsite's production may result in its release to the environment through various waste streams; its former use as a war gas may have resulted in its direct release to the environment. If released to air, a vapor pressure of 0.58 mm Hg at 25 deg C indicates Lewsite will exist solely as a vapor in the atmosphere. Vapor-phase Lewsite 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 1.4 and 1.2 days for the cis- and trans-isomers, respectively. Reaction with ozone will also contribute to Lewsite's atmospheric degradation, but it will not be as rapid as reaction with hydroxyl radicals. Lewsite has a UV absorption spectrum at 200 to 350 nm, which suggests that direct photolysis in sunlight may occur. If released to soil, Lewsite is expected to have high mobility based upon an estimated Koc of 143. Volatilization from moist soil surfaces is expected to be an important fate process based upon an estimated Henry's Law constant of 3.2X10-4 atm-cu m/mole. While the Henry's Law constant for Lewsite indicates volatilization from moist soil surfaces, the rapid rate of hydrolysis may reduce the significance of this fate pathway. Lewsite is rapidly hydrolyzed by soil moisture, and minerals present in the soil would speed the process. Alkaline soils would neutralize Lewsite. Lewsite may volatilize slowly from dry soil surfaces based upon its vapor pressure. Lewsite has been shown to evaporate readily from soil surfaces, with 20% of applied Lewsite evaporated from warm soil in the first hour; however, evaporation quickly terminates as the Lewsite is converted to Lewsite oxide through hydrolysis. No direct information has been found regarding Lewsite biodegradation in soil, however, suggested pathways of microbial degradation in soil include epoxidation of the C=C bond and reductive dehalogenation and dehydrodehalogenation. If released into water, Lewsite is not expected to adsorb to suspended solids and sediment based upon the estimated Koc. No biodegradation studies are currently available for Lewsite in aquatic environments. Volatilization from water surfaces is expected to be an important fate process based upon this compound's estimated Henry's Law constant. Estimated volatilization half-lives for a model river and model lake are 10 hours and 7 days, respectively. While the Henry's Law constant for Lewsite indicates volatilization from water surfaces, the rapid rate of hydrolysis may reduce the significance of this fate pathway. An estimated BCF of 19 suggests the potential for bioconcentration in aquatic organisms is low. Lewsite is reported to have a fast hydrolysis rate and is quickly converted to 2-chlorovinyl arsonous acid and HCl; however, a specific environmental hydrolysis rate constant was not reported. Occupational exposure to Lewsite may occur through inhalation and dermal contact with this compound at workplaces where Lewsite is produced or used. (SRC)
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