VX

CAS RN:50782-69-9

Exposure Summary

VX's production may result in its release to the environment through various waste streams; its use as a nerve gas and chemical warfare agent will result in its direct release to the environment. If released to air, a vapor pressure of 8.78X10-4 mm Hg at 25 deg C indicates VX will exist solely as a vapor in the atmosphere. Below 7 deg C (45 deg F), VX's vapor pressure is less than 1X10-4 mm Hg, where it can exist in both the vapor and particulate phases. Vapor-phase VX 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 2.5 hours. Particulate-phase VX will be removed from the atmosphere by wet and dry deposition. VX does not absorb UV radiation above 290 nm and, therefore, is not expected to be susceptible to direct photolysis by sunlight. If released to soil, VX is expected to have moderate mobility based upon an estimated Koc of 187. The pKa of VX is 9.12, indicating that this compound will partially exist in cation form in the environment and cations generally adsorb more strongly to soils containing organic carbon and clay than their neutral counterparts. Adsorption to dry soil and sand has been observed. Rainfall on dry soil has been observed to desorb VX with resulting volatilization to air. However, volatilization from moist soil surfaces where VX is dissolved in water may not be important fate process based upon a estimated Henry's Law constant of 1.0X10-8 atm-cu m/mole. VX does volatilize from dry surfaces. VX is susceptible to hydrolysis and hydrolysis in moist soil is expected to be an important fate process. Degradation rates in soil will vary depending on soil type and conditions. Field and closed-container studies indicate that approximately 90% of VX is lost from soil in 15 days. When applied on soil, the concentration of VX may decline to low levels in a few days, after which the degradation is much slower. This behavior is observed on different types of soil and soils with different moisture contents. The half-life on vegetative surfaces may be as short as 1-2 days. If released into water, VX may adsorb to suspended solids and sediment based upon the estimated Koc. Volatilization from water surfaces is not expected to be an important fate process based upon this compound's estimated Henry's Law constant. An estimated BCF of 11 suggests the potential for bioconcentration in aquatic organisms is low. Hydrolysis rates vary with pH; at pH 5, the half-life is on the order of 100 days, whereas at pH 8 the half-life is roughly 9 days. Batch hydrolysis experiments demonstrated an increasing hydrolysis rate as pH increased, but also indicated that dissolved aqueous constituents (e.g. fulvic acids, carbonates, clays) can cause major differences in absolute hydrolysis rates; for example, addition of natural dissolved material at pH 7 increased the hydrolysis rate by a factor of 2. Hydrolysis rates in environmental systems where natural dissolved species are present can depend on pH, species and concentration, and other factors.In seawater, the half-life of VX was approximately 5-14 days at 25 deg C and may be several years at 4 deg C. Sensitized photooxidation may occur in natural water exposed to sunlight and has been suggested as a means of decontaminating VX. Occupational exposure to VX may occur through inhalation and dermal contact with this compound at workplaces where VX is produced or used. The general population will not be exposed to VX unless it is used as a weapon; exposure to VX, if used as a weapon, will be via inhalation of ambient air and dermal contact. (SRC)
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