Given that mercury emissions from coal power plants will almost certainly be limited by some form of national regulation or legislation, Brookhaven National Laboratory (BNL) is performing an assessment of the reduction in human health risk that may be achieved through reduction in coal plant emissions of mercury. The primary pathway for mercury exposure is through consumption of fish. The most susceptible population to mercury exposure is the fetus. Therefore, the risk assessment focuses on consumption of fish by women of child-bearing age.

Preliminary Risk Assessment
A preliminary risk assessment was conducted using a simplified approach based on three major topics: Hg emissions and deposition (emphasizing coal plants), Hg consumption through fish, and dose-response functions for Hg. Using information available from recent literature, dose response factors (DRFs) were generated from studies on loss of cognitive abilities (language skills, motor skills, etc.) by young children whose mothers consumed large amounts of fish with high Hg levels. Population risks were estimated for the general population in three regions of the country, (the Midwest, Northeast, and Southeast) that were identified by EPA as being heavily impacted by coal emissions.

Three scenarios for reducing Hg emissions from coal plants were considered: (1) A base case using current conditions; (2) A 50% reduction; and, (3) A 90% reduction. These reductions in emissions were assumed to translate linearly into a reduction in fish Hg levels of 8.6% and 15.5%, respectively. Population risk estimates were also calculated for two subsistence fisher populations. These groups of people consume substantially more fish than the general public and, depending on location, the fish may contain higher Hg levels than average. Risk estimates for these groups were calculated for the three Hg levels used for the general population analyses.

The general population risks for exposure of the fetus to Hg were found to be small. The estimated risk reductions from a 90% reduction in coal plant Hg emissions ranged from 25% – 68%, which is greater than the assumed reduction in Hg levels in fish, (15.5%). To place this risk in perspective, there are approximately 4 x 10⁶ births/year in the U.S (National Vital Statistics Report, 2000). Assuming that the Southeast risk level (the highest of the regions) is appropriate for the entire U.S., an estimate of 80 newborn children per year have a 5% chance of realizing any of the 16 adverse effects used to generate the DRF. If Hg emissions from power plants are reduced 90%, the number of children at risk is reduced to 60.

Risk assessment including impacts of local Hg deposition
One of the greatest uncertainties associated with mercury risk assessment is the unknown extent to which local deposition of mercury in the vicinity of coal power plants is occurring, and the effect this deposition may have on mercury exposure experienced by local subsistence fisher populations. This uncertainty is also driving an important policy debate over whether mercury emission limits on coal plants should be on a plant-by-plant basis, or whether a nationwide cap-and-trade program (similar to the successful acid rain program) could be implemented to achieve a more cost-effective form of protection from mercury exposure. In an effort to resolve this uncertainty, BNL is attempting to determine the amount by which local mercury deposition is enhanced directly downwind of coal-fired power plants, and is refining its risk assessment to account for the effects of local mercury deposition.

The current BNL effort is focusing on two major areas: (1) using atmospheric modeling and field measurements of mercury deposition in soils in the vicinity of coal-fired power plants to characterize the increase in local mercury deposition as compared to literature-based deposition processes and amounts from other mercury sources; and (2) creating a database that incorporates coal plant locations and emissions, local population density, and nearby waterbodies as a means of identifying and evaluating the impacts of nationwide mercury emissions trading on human health.

Probabilistic risk assessments were performed for two power plants, Bruce Mansfield in western Pennsylvania and Monticello in eastern Texas. Local hourly meteorological data was obtained for these sites and deposition modeling was performed for a region 50 Km around the site. Risk assessments were performed for two population groups (general and subsistence fishers) and the modeled deposition patterns. The risk assessments indicated that for the general population local deposition associated with the emissions from the coal-fired power plant were small (<10^-5 risk of observed neurological effects) but risks could be two orders of magnitude higher for subsistence fisher populations. Estimated risks were more highly dependent on consumption patterns than increases in deposition due to coal-fired power plant emissions.

Future work on this project will consist of detailed sampling in the vicinity of several power plants to verify model predictions of local mercury deposition.