Paraquat

CAS RN: 4685-14-7

Exposure Summary

Paraquat's production may result in its release to the environment through various waste streams; its use as a restricted use broad spectrum herbicide will result in its direct release to the environment. If released to air, a vapor pressure of <7.5X10-8 mm Hg at 25 deg C indicates paraquat will exist solely in the particulate phase in the atmosphere. Particulate-phase paraquat will be removed from the atmosphere by wet and dry deposition. Paraquat does not contain chromophores that absorb at wavelengths >290 nm and, therefore, is not expected to be susceptible to direct photolysis by sunlight. If released to soil, paraquat is expected to have no mobility based upon reported Koc values of 15,473-1,000,000. Field studies and soil column leaching studies have demonstrated that paraquat does not leach under environmental conditions. Volatilization from moist soil surfaces is not expected based upon an estimated Henry's Law constant of <3.0X10-14 atm-cu m/mole. Paraquat is not expected to volatilize from dry soil surfaces based upon its vapor pressure. The biodegradation of paraquat is very slow when strongly adsorbed to clay or organic matters in soils because it becomes unavailable. When not in the absorbed-state, paraquat will biodegrade. Some photodegradation may occur on plant leaves or soil surfaces exposed to sunlight. Field dissipation half-lives in soil are reported as 99-4747 days. If released into water, paraquat is expected to adsorb to suspended solids and sediment based upon the reported Koc values. Volatilization from water surfaces is not expected to be an important fate process based upon this compound's estimated Henry's Law constant. Reported BCFs of 0.05-6.9 suggest bioconcentration in aquatic organisms is low. Hydrolysis and direct photolysis in water are not expected based upon laboratory studies. Occupational exposure to paraquat may occur through inhalation and dermal contact with this compound at workplaces where paraquat is produced or used. However, the respiratory exposure to paraquat was found to be negligible compared to dermal exposure during field spraying of the herbicide. Monitoring and use data indicate that the general population may be exposed to paraquat via ingestion of food contaminated with paraquat. (SRC)
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