Assessing and Managing Methylmercury Risks Associated With Power Plant Mercury Emissions in the United States

Gail Charnley, PhD

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In This Article

Abstract and Introduction

Abstract

Until the Clean Air Mercury Rule was signed in March 2005, coal-fired electric utilities were the only remaining, unregulated major source of industrial mercury emissions in the United States. Proponents of coal-burning power plants assert that methylmercury is not a hazard at the current environmental levels, that current technologies for limiting emissions are unreliable, and that reducing mercury emissions from power plants in the United States will have little impact on environmental levels. Opponents of coal-burning plants assert that current methylmercury exposures from fish are damaging to the developing nervous system of infants, children, and the fetus; that current technology can significantly limit emissions; and that reducing emissions will reduce exposure and risk. One concern is that local mercury emissions from power plants may contribute to higher local exposure levels, or "hot spots." The impact of the Mercury Rule on potential hot spots is uncertain due to the highly site-specific nature of the relationship between plant emissions and local fish methylmercury levels. The impact on the primary source of exposure in the United States, ocean fish, is likely to be negligible due to the contribution of natural sources and industrial sources outside the United States. Another debate centers on the toxic potency of methylmercury, with the scientific basis of the US Environmental Protection Agency's (EPA's) recommended exposure limit questioned by some and defended by others. It is likely that the EPA's exposure limit may be appropriate for combined exposure to methylmercury and polychlorinated biphenyls (PCBs), but may be lower than the available data suggest is necessary to protect children from methylmercury alone. Mercury emissions from power plants are a global problem. Without a global approach to developing and implementing clean coal technologies, limiting US power plant emissions alone will have little impact.

Introduction

The extent to which mercury in the environment poses a risk to human health has been under debate for a number of years. Although few would question the toxicity of methylmercury at high exposure levels, such as from poisoning, the debate centers on low environmental exposure levels and the extent to which low levels pose a risk to infants, children, and the developing fetus. This concern has been highlighted because regulations have been promulgated for the first time by the US Environmental Protection Agency (EPA) that would limit mercury emissions from electric power generating plants that burn coal.

Coal-fired electric utilities are the only remaining, unregulated commercial source of mercury emissions in the United States. Proponents of coal-burning plants assert that methylmercury is not a hazard at the current environmental levels, that current technologies for limiting emissions are unreliable, and that reducing mercury emissions from power plants in the United States will have little impact on environmental levels. Opponents of coal-burning plants assert that current methylmercury exposures are damaging to the developing nervous system of infants, children, and the fetus; that current technology can significantly limit emissions; and that reducing emissions will reduce exposure and risk. Also debated is the toxic potency of methylmercury, with the scientific basis of the EPA's recommended exposure limit questioned by some and defended by others. If the exposure limit is correct, some people may be at risk from current environmental levels. If the exposure limit is more stringent than is necessary to protect public health, risk at current exposure levels is less likely.

This article critically evaluates the scientific evidence that is relevant to the assertions of both proponents and opponents of limiting mercury emissions and the impact that such limits are likely to have on public health. This article focuses on the health effects associated with methylmercury -- not all forms of mercury -- and provides comments on the nature and likely effectiveness of the actions proposed to limit mercury emissions from power plants that burn coal. The basis for the EPA's methylmercury exposure limit and the various assertions in regard to the number of people "at risk" for the effects of methylmercury are discussed. This article concludes that until a global approach to limiting mercury emissions and methylmercury exposures is undertaken, limiting power plant emissions in the United States alone may be good public policy as part of the EPA's efforts to improve air quality, but the wider public health benefits should not be oversold. The nature and extent of the anticipated impact, if any, on people who consume locally caught fish are difficult to predict due to the highly site-specific nature of the relationship between plant emissions and local fish methylmercury levels.

 


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