Use of Mercury Vapor Flux Measurements for Reducing Uncertainty in the Evaluation of Inhalation Exposures. T. C. Bernhardt and D. B. Mathur, ChemRisk Service, McLaren/Hart, Inc., 11320 Harbor Bay Parkway, Suite 100, Alameda, CA 94502-6578; and P. S. Scott, North Shore Center, Suite 100, Pittsburgh, PA 15212
As part of a comprehensive risk assessment program, a health risk assessment was conducted to evaluate occupational exposures to mercury vapors emitted from soil and groundwater at a large chlor-alkali plant. Evaluation of exposure due to unregulated releases of mercury to ambient air from subsurface media was complicated by permitted releases to air from ongoing plant operations. To distinguish the contribution of subsurface and surface contamination to ambient air from the permitted component, mercury vapor flux chamber samples were collected from several locations across the facility. Both long term and short term and day and night flux chamber samples were collected to account for diurnal variability in mercury fluxes, and to obtain realistic estimates of exposure due to the 24-hour operations at the facility. Mercury vapor fluxes varied widely between daylight and nighttime measurements and short and long term measurements, but correlated well with known chemical concentrations in shallow soils. The measured emission rates were modeled using indoor and outdoor box models to obtain concentrations in the breathing zone. The calculated air concentrations of mercury due to subsurface contamination were found to be significantly less than those measured in ambient air. Potential inhalation exposures and adverse non-cancer health effects were estimated for different on-site worker groups using both the point estimate approach and Monte Carlo Analysis (MCA). The results indicate that adverse non-cancer health effects due to potential exposure to mercury were below the range considered acceptable for occupational settings, and that site-specific flux chamber sampling for mercury provides more meaningful information to assess exposures to mercury vapor emissions from subsurface media by reducing the uncertainty inherent in the use of models.
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