Urbanization, Energy, and Air Pollution in China The Challenges Ahead: Proceedings of a Symposium (2004) / Chapter Skim
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The Power Sector Analysis of Emissions, Exposures, and Risks of Toxic Air Emissions from U.S. Coal-Fired and Oil-Fired Power Plants--CHRIS G. WHIPPLE
The Power Sector Analysis of Emissions, Exposures, and Risks of Toxic Air Emissions from U.S. Coal-Fired and Oil-Fired Power Plants--CHRIS G. WHIPPLE
Pages 169-186
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The Power Sector
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was formed, and significant changes were made to the Clean Air Act. Previous versions had provided mainly for research and development and left the regulation of air quality to the states, but the 1970 Act required that national primary and secondary ambient air quality standards be set.
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that serves a generator that produces electricity for sale." The amendments also required that "EPA proceed with rulemaking activities under section 112 to control HAP [hazardous air pollutant] emissions from utilities if EPA finds such regulation is appropriate and necessary after considering the results of the study." The 1990 Clean Air Act Amendments also directed EPA to prepare "a study of mercury emissions from electric utility steam generating units, municipal waste combustion units, and other sources, including area sources.
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The results of the analyses are described in the remainder of this paper, based on the major elements of the analyses by EPA and EPRI listed below: · scenarios for future power-plant inventories and use, including fuel mix, coal sources, and emission controls · identification of substances included in the study · emission factors for each fuel type and plant configuration · air dispersion modeling · exposure modeling for various exposure scenarios calculations of inhalation exposures for all U.S. plants calculations of multipathway exposures for selected chemicals for model plants (EPA)
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It is too soon to tell how accurate the estimates are of the mix of power plant controls in 2010, because required reductions in SO2 and NOx have not taken full effect. Identification of Substances The 1990 Clean Air Act Amendment's list of air toxics includes 189 specific substances or classes of substances.
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Figure 1 indicates the observed release rates, in pounds of particulate emissions per pound of fuel input; EPRI used these and similar data to estimate source terms. Determining the content of organic pollutants in fuels was not as useful as for trace metals because organic hazardous air pollutants (HAPs)
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EPRI based the radionuclide emission estimate on the annual mass of particulate emissions and representative concentrations of radionuclides in fly ash for the type of plant being analyzed. Air-Dispersion Models and Exposure and Risk Assessment The usual approach to assessing exposures to hazardous substances is to calculate an "exposure point concentration" and then apply assumptions or data regarding individual factors that would affect exposure.
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Figure 2, which shows the results of the EPA study for coal-fired plants, indicates that the calculated lifetime cancer risk to the maximally exposed individual exceeded one in a million for only two plants. As a point of comparison, EPA frequently uses an acceptable risk range for lifetime cancer risks of one in ten thousand to one in a million.
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Source: EPA, 1998. FIGURE 3 EPA results of inhalation cancer risks by chemical for coal-fired power plants.
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A multipathway risk assessment is much more complex and requires much more input data than an inhalation risk assessment. The EPA study noted that power-plant emissions of lead and cadmium are roughly 1 percent of the total annual U.S.
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Neither RfD was based on exposure through fish consumption. Since the publication of the EPA electric utility air toxics study, EPA has updated the methylmercury RfD based on epidemiological studies of populations that consume large amounts of fish (relative to the United States)
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Although the current basis for the RfD is concerns about exposures during pregnancy, the RfD is the same, 0.1 µg/kg-day. In EPA's case studies, exposures to methylmercury from power plants alone did not exceed the RfD for the typical recreational angler (assumed to consume 8 grams per day of freshwater fish)
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consumer. Although mercury exposures occur mainly through the consumption of marine fish, incremental exposures from power-plant emissions added to other exposures could exceed the RfD for a significant number of people.
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, the lifetime cancer risks from arsenic were typically in the range of one in a million or less. EPA further noted that, even after 30 years of accumulated deposition, the arsenic contributions to soils near power plants would be around 10 percent of average background concentrations.
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UARG's review revealed that in several cases the point of maximum risk in EPA's analysis was inside the plant boundary, for example, on a coal pile or cooling pond. In discussions between the industry and EPA, it was agreed that the EPA analyses were conservative, but because EPA showed no interest in regulating radionuclide emissions, it was not considered important to get more refined and accurate risk estimates.
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In EPA's analyses of mercury exposures, data on fish consumption, when combined with data on the mercury concentration in fish, indicated that a significant number of people in the United States were exposed to methylmercury at higher levels than the EPA RfD. Most of this exposure was from the consumption of commercially purchased marine fish, but some people were thought to be overexposed through the consumption of self-caught freshwater fish.
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The fish consumption survey of the NHANES study did not have the resolution necessary to determine the contribution of wild freshwater fish to exposures at the upper tail of the exposure distribution. REFERENCES EPA (Environmental Protection Agency)
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