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Mercury Matters 2021: A Science Brief for Journalists

09/09/2021 | Harvard Chan C-CHANGE

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Mercury and MATS in Context

Coal-fired power plants are the second largest source of mercury emissions in the U.S., accounting for approximately 4.4 tons of mercury emissions in 20171. The Mercury and Air Toxics Standards (MATS) were signed in 2011 and published in 2012 to regulate emissions of mercury, acid gases and other hazardous air pollutants (HAPs) from coal- and oil-fired power plants. As part of that action, EPA reaffirmed a prior decision under Section 112(n) of the Clean Air Act that it is “appropriate and necessary” to regulate HAPs from power plants.

Since MATS was promulgated, mercury emissions from coal-fired power plants have declined 84 percent; mercury levels measured in the environment are lower; and the estimated number of children who are prenatally exposed to unsafe levels of mercury has gone down by 50 percent. MATS  is also central to the U.S. meeting its commitments under the 2017 Minamata Convention on Mercury.

The Latest from EPA

On April 16, 2020, the Environmental Protection Agency (EPA) overturned the Agency’s prior determinations and announced a new position that it is not “appropriate and necessary” to regulate mercury and other HAPs from power plants. According to legal experts, this decision undermines the foundation of the MATS rule and invites challenges to emissions standards.

The EPA also issued a “Residual Risk and Technology Review” at that time which concluded that no further emissions reductions are warranted from affected power plants to protect human health.

In the near future, EPA is expected to release a new proposal, which may reverse EPA’s earlier decision by proposing to reinstate the appropriate and necessary finding for MATS. The proposal is expected to address the costs and benefits of regulating HAPs from power plants, and to consider whether additional protections against emissions of mercury and other HAP are warranted to protect public health and the environment.

The Issue

The anticipated proposal from EPA represents an opportunity to ensure that the Agency uses the best available science when it makes its “appropriate and necessary” and “residual risk” determinations.

In 2020 EPA failed to conduct a cost-benefit analysis to support its determinations and instead referenced estimates from the outdated and limited cost-benefit analysis in the 2011 MATS Regulatory Impact Assessment (RIA). Scientists have repeatedly pointed out that the methods and findings in this assessment are inconsistent with current science on mercury exposure, the societal impacts of mercury pollution in the U.S.2,3 and the full benefits of emissions controls. EPA’s Science Advisory Board issued a report urging the Agency to develop a new mercury exposure estimate before finalizing this residual risk assessment4.

The Impacts of Mercury Emissions on Health and the Environment Are Well-Understood

Mercury has been studied intensively for decades and its impacts are well-understood.

  • Mercury in the form of methylmercury is a potent neurotoxin.
  • Children exposed to methylmercury during a mother’s pregnancy can experience persistent and lifelong IQ and motor function deficits5.
  • High levels of methylmercury exposure in adults have been associated with adverse cardiovascular effects, including increased risk of fatal heart attacks6.
  • Other adverse health effects of methylmercury exposure that have been identified include endocrine disruption7, diabetes risk8, and compromised immune function9.
  • The societal costs of neurocognitive deficits associated with total methylmercury exposure in the U.S. were estimated at approximately $4.8 billion per year in 201710.
  • No known threshold exists for methylmercury exposure below which neurodevelopmental impacts do not occur11,12. Regulatory analysis should include estimates of the benefits of reducing mercury exposure below the established reference dose.
  • Wildlife that consume fish, such as common loons, bald eagles, otter and mink, and many marine mammals can also experience adverse effects from methylmercury exposure13.
  • The health of many songbird and bat species is threatened due to methylmercury exposure particularly in wetland habitats. The productivity of economically valuable game fish stocks can also be compromised14.

Mercury exposure in the U.S. occurs primarily through the consumption of seafood (finfish and shellfish). The most highly exposed individuals tend to be recreational fishers and consumers of freshwater fish. The consumption of marine fish, often harvested from U.S. coastal waters, accounts for greater than 80% of methylmercury intake by the U.S. population15. There are no treatments for methylmercury toxicity. Safe and consumable seafood is important for maintaining this otherwise healthy, low-cost source of protein and micronutrients that are essential for pregnant women, young children, and the general population.

Mercury is emitted following the combustion of coal and other fossil fuels. Coal has much higher mercury concentrations than other fossil fuels, leading to potentially large releases of mercury from coal-fired electricity generating units (power plants). After being released to the atmosphere, mercury is redeposited to the land or ocean where it can be converted by microbes into the only form of mercury that bioaccumulates in food webs, methylmercury. Methylmercury concentrations in predatory fish and other apex organisms are typically 10 to 100 million times greater than concentrations in water16.

Mercury Contamination in the U.S. is Widespread

Human activities have greatly increased levels of mercury in the environment. This means that all fish from U.S. waters have detectable levels of mercury. Some fish, such as swordfish, shark, orange roughy, tuna, and some freshwater game fish, can have concentrations of mercury that exceed safe consumption guidelines.

States post fish consumption advisories for waterbodies that are known to have elevated contaminants. In 2013, the most recent year for which summary data exist, consumption advisories for mercury were in effect in all 50 states, one U.S. territory, and three tribal territories, accounting for 81% of all U.S. consumption advisories17. Consumption advisories for mercury exceed advisories for all other contaminants combined.

Cutting Mercury Emissions Has Led to Improved Health Outcomes in the U.S.

Mercury controls on U.S. electric utilities have contributed to the following reductions in mercury emissions and associated environmental and human health improvements in the U.S.

  • Mercury emissions from U.S. coal-fired power plants declined 84% from 26.8 tons in 2011 to 4.4 tons in 201718 after states began setting standards and MATS was introduced in 2011. Eleven states implemented mercury emissions standards for power plants prior to 2011.
  • Concurrent with declines in mercury emissions, mercury levels in air, water, sediments, loons, freshwater fisheries, and Atlantic Ocean fisheries19 have decreased appreciably.
  • After mercury controls were instituted, the estimated number of children born in the U.S. each year with prenatal exposure to methylmercury levels that exceed the EPA reference dose has decreased by half from 200,000- 400,000 to 100,000-200,000, depending on the measure used20.

The Net Benefits of Reducing Mercury Are Much Larger Than EPA Previously Estimated

In its 2020 decision, EPA continued to use an estimate for the monetized health benefits of reducing mercury emissions of less than $10 million annually. However, studies that incorporate additional pathways of methylmercury exposure, the full range of populations experiencing exposure, and additional health effects, particularly cardiovascular effects, suggest that the monetized benefits of reducing power plant mercury emissions in the U.S. are likely in the range of several billion dollars per year21,22,23. These and other studies support the conclusion that the mercury-related benefits from MATS are more than one hundred times larger than EPA estimated in the 2011 RIA that was referenced in EPA’s 2020 decision to reverse the appropriate and necessary finding24.

In addition to the mercury-related benefits, MATS has also contributed to decreases in sulfur dioxide and nitrogen oxide emissions, improving air quality and public health by reducing fine particulate matter and ground-level ozone. The EPA estimated that the annualized value of these additional benefits is $24 to $80 billion; bringing the total annual benefits of MATS to tens of billions of dollars. Even with these more comprehensive estimates, substantial benefits of reducing mercury and other air toxics remain unquantified due to data limitations25.

On the cost side, new information suggests that the EPA’s original cost-estimate for MATS of $9.6 billion is much higher than the actual cost due to declines in natural gas prices and lower than expected control equipment and renewable energy costs26. Together, the larger benefits and lower costs lead to much higher net benefits from reducing mercury that EPA has previously estimated.

EPA Should Use the Best Available Science to Estimate Benefits and Need for Further Reductions

In its decision adopting MATS, EPA concluded that controlling emissions from power plants was critically important because of the serious harms that emissions of mercury and other HAPs from power plants pose to public health and the environment. Yet, the EPA’s quantification of those benefits was seriously incomplete, including only small subsets of the health impacts that are documented by high-quality scientific research.

In its 2020 decision reversing the appropriate and necessary finding, EPA conducted no analysis of the health impacts of deregulating mercury emissions. To the extent EPA considered the benefits of regulation at all, EPA relied on outdated science from the 2011 MATS RIA. Current science shows that U.S. mercury emissions have a larger impact on exposure in the U.S. than EPA previously estimated and that the benefits of reducing mercury emissions are also larger than what EPA claimed in 2020.

EPA assumed that mercury emissions from coal-fired utilities are mainly transported long distances from sources in the U.S. and that a substantial fraction of mercury in the U.S. originates from international sources. However, recent research on fate and transport shows that EPA markedly underestimated the contribution of U.S. coal-fired power plants to U.S. mercury contamination, particularly in the eastern U.S.27,28.

EPA quantified exposure to mercury from only a small subset of the total affected population —  consumers of recreationally caught freshwater fish. Studies show that many more people are exposed to mercury through the consumption of store-bought fish. The EPA Science Advisory Board submitted a letter in 2020 recommending that EPA should update the exposure estimate to account for other exposed people and pathways before completing the residual risk assessment for determining whether additional cuts in mercury emissions were needed to protect human health29.

EPA’s benefits assessment associated with the MATS rule only considered the risks from utility-attributable mercury exposure. EPA should also account for cumulative exposure to mercury when estimating the benefits of reducing mercury emissions in order to capture the full health benefits of emission regulation. People are exposed to mercury from diverse sources that accumulate in seafood, and thus risks reflect our cumulative exposure. The benefits of reducing emissions should thus account for changes in potential health risks relative to cumulative mercury exposure rather than only the utility-derived portion. For example, an individual with high exposure from consuming seafood that reflects multiple sources may experience a reduction in exposure below the reference dose when an emission control policy is implemented. This methodology confers a higher monetized value than when benefits are estimated without accounting for cumulative exposure. Not accounting for cumulative exposure may perpetuate the disproportionate burden borne by some communities.

Since there is no safe threshold below which neurodevelopment effects do not occur, EPA’s regulatory analysis should also include estimates of the benefits of reducing mercury exposure below the established reference dose. EPA did not account for these effects in the prior RIA used in 2020.

Regarding health effects, EPA should account for additional health effects of mercury exposure beyond IQ, especially the cardiovascular impacts. Excluding these effects sharply underestimates the benefits of mercury controls and the assessment of the need for further regulation. Most studies and scientists agree that evidence is strong enough to include cardiovascular impacts in regulatory cost-benefit assessments. EPA’s own expert panel has agreed that evidence is strong enough to include cardiovascular impacts in regulatory cost-benefit assessments30.

The Bottom Line

The science is clear. The health impacts of mercury emissions in the U.S. are large and disproportionately affect children and other vulnerable populations. Mercury emission standards in the U.S. have markedly reduced mercury in the environment and improved human and wildlife health. The mercury-related benefits alone of the MATS rule greatly exceed values the EPA has estimated, the actual costs appear to be substantially lower than EPA has projected, and the total monetized benefits across all pollutants mitigated far outweigh the cost of the standards.

In upcoming regulatory processes for the appropriate and necessary finding and the residual risk assessment, EPA should fully account for local impacts of emissions in the U.S.; all sources of mercury exposure including recreational and commercial fisheries for the entire U.S. population across demographic groups; and all health effects including the well-documented cardiovascular impacts in addition to neurotoxicity when making policy decisions.


Charles Driscoll, Department of Civil and Environmental Engineering, Syracuse University

Elsie Sunderland, Harvard Paulson School of Engineering & Applied Sciences and Harvard T.H. Chan School of Public Health, Department of Environmental Health, Exposure, Epidemiology and Risk

Kathy Fallon Lambert, Harvard T.H. Chan School of Public Health, Center for Climate, Health, and the Global Environment

Literature Cited

[1] Streets, D.G.; Horowitz, H.M.; Lu, Z.; Levin, L.; Thackray, C.P.; Sunderland, E.M. 2019. Global and regional trends in mercury emissions and concentrations, 2010-2015. Atmospheric Environment. 201, 417-427.

[2] Sunderland, E.M.; Driscoll, Jr., C.T.; Hammitt, J.K.; Grandjean, P.; Evans, J.S.; Blum, J.D.; Chen, C.Y.; Evers, D.C.; Jaffe, D.A.; Mason, R.P.; Goho, S.; Jacobs, W. 2016. Benefits of Regulating Hazardous Air Pollutants from Coal and Oil-Fired Utilities in the United States. Environmental Science & Technology. 50 (5), 2117-2120. DOI: 10.1021/acs.est.6b00239.

[3] Giang, A.; Mulvaney, K; Selin, N.E. 2016. Comments on “Supplemental Finding That It Is Appropriate and Necessary to Regulate Hazardous Air Pollutants from Coal- and Oil-Fired Electric Utility Steam Generating Units”.

[4] Science Advisory Board (SAB) Consideration of the Scientific and Technical Basis of EPA’s Proposed Mercury and Air Toxics Standards for Power Plants Residual Risk and Technology Review and Cost Review, letter to Administrator Wheeler, U.S. Environmental Protection Agency, EPA-SAB-20-004, April 20, 2020.

[5] Grandjean, P. and Bellanger, M. 2017. Calculation of the disease burden associated with environmental chemical exposures: application of toxicological in health economic estimation. 16:123. DOI: 10.1186/s12940-017-0340-3.

[6] Genchi G., Sinicropi M.S., Carocci A., Lauria G., Catalano A. 2017. Mercury Exposure and Heart Diseases. Int J Environ Res Public Health. 2017;14(1):74. Published Jan 12. DOI:10.3390/ijerph14010074.

[7] Tan, S.W.; Meiller, J.C.; Mahaffey, K.R. 2009. The endocrine effects of mercury in humans and wildlife. Crit. Rev. Toxicol. 39 (3), 228−269.

[8] He, K.; Xun, P.; Liu, K.; Morris, S.; Reis, J.; Guallar, E. 2013. Mercury exposure in young adulthood and incidence of diabetes later in life: the CARDIA trace element study. Diabetes Care. 36, 1584−1589.

[9] Nyland, J. F.; Fillion, M.; Barbosa, R., Jr.; Shirley, D. L.; Chine, C.; Lemire, M.; Mergler, D.; Silbergeld, E.K. 2011. Biomarkers of methylmercury exposure and immunotoxicity among fish consumers in the Amazonian Brazil. Env. Health Persp. 119 (12), 1733− 1738.

[10] Grandjean and Bellanger 2017.

[11] Rice, G.E.; Hammitt, J.K; and Evans, J.S. 2010. A probabilistic characterization of the health benefits of reducing methyl mercury intake in the United States. Environ Sci Technol. 1;44(13):516-24. DOI:10.1021/es903359u.

[12] Grandjean and Bellanger 2017.

[13] Chan, N.M.; Scheuhammer, A.M.; Ferran, A.; Loupelle, C.; Holloway, J.; and Weech, S. 2003. Impacts of Mercury on Freshwater Fish-eating Wildlife and Humans. Human and Ecological Risk Assessment. 9(4): 867-883.

[14] Sandheinrich, M.B.; Wiener, J.G. 2011. Methylmercury in freshwater fish: Recent advances in assessing toxicity of environmentally relevant exposures. In Environmental Contaminants in Biota: Interpreting Tissue Concentrations, 2nd; Beyer, W. N., Meador, J. P., Eds.; CRC Press/Taylor and Francis: Boca Raton, FL; pp. 169−190.

[15] Sunderland, E. M.; Li, M.; Bullard, K. 2018. Decadal Changes in the Edible Supply of Seafood and Methylmercury Exposure in the United States. Environ. Health Persp. DOI: 10.1289/EHP2644.

[16] Driscoll, C.T.; Han, Y-J; Chen, C.; Evers, D.; Lambert, K.F.; Holsen, T.; Kamman, N.; and Munson, R. 2007. Mercury Contamination on Remote Forest and Aquatic Ecosystems in the Northeastern U.S.: Sources, Transformations, and Management Options. BioScience. 57(1):17-28.

[17] U.S. Environmental Protection Agency. 2011 National Listing of Fish Advisories. 2013. EPA-820-F-13-058.

[18] U.S. Environmental Protection Agency. 2018. 2016-tri-national-analysis.

[19] Cross, F.A.; Evans, D.W.; Barber, R.T. 2015. Decadal declines of mercury in adult bluefish (1972−2011) from the mid- Atlantic coast of the U.S.A. Environ. Sci. Technol. 49, 9064−9072.

[20] Mahaffey K.R., Clickner R.P., Jeffries R.A. Adult women’s blood mercury concentrations vary regionally in the United States: association with patterns of fish consumption (NHANES 1999-2004). Environ Health Perspect. 2009 Jan;117(1):47-53. doi: 10.1289/ehp.11674.

[21] Rice et al. 2010.

[22] Giang, A.; Selin, N. E. Benefits of mercury controls for the United States. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 286.

[23] Sunderland et al. 2016.

[24] Giang et al. 2016.

[25] Sunderland et al. 2016.

[26] U.S. Environmental Protection Agency. Final Consideration of Cost in the Appropriate and Necessary Finding for the Mercury and Air Toxics Standards for Power Plants. 05/documents/20160414_mats_ff_fr_fs.pdf.

[27] Zhang, Y.; Jacob, D.; Horowitz, H.; Chen, L.; Amos, H.; Krabbenhoft, D.; Slemr, F.; St. Louis, V.; Sunderland, E. 2016. Observed decrease in atmospheric mercury explained by global decline in anthropogenic emissions. PNAS. 113 (3) 526- 531. DOI: 10.1073/pnas.1516312113.

[28] Lepak, R.F.; Yin, R.; Krabbenhoft, D.; Ogorek, J.; DeWild, J.; Holsen, T.; and Hurley, J. 2015. Use of Stable Isotope Signatures to Determine Mercury Sources in the Great Lakes. Environmental Science & Technology Letters. 2 (12), 335-34. DOI: 10.1021/acs.estlett.5b00277.

[29] EPA Science Advisory Board, 2020.

[30] Roman H.A., Walsh T.L., Coull B.A., et al. Evaluation of the cardiovascular effects of methylmercury exposures: current evidence supports development of a dose-response function for regulatory benefits analysis. Environ Health Perspect. 2011;119(5):607-614. doi:10.1289/ehp.1003012.