The precise effect of emissions controls on the production of methylmercury is unknown. There is no model, at present, to imitate the complex chemical and physical processes that contribute to the formation of methylmercury and its uptake in the food chain. But enough is known to establish that conditions in the Great Lakes are not particularly conducive to methylmercury formation and bioaccumulation, and conditions vary from lake to lake. Tributaries and surrounding wetlands may support methylation.
In developing the federal mercury regulations, the EPA used a model (Mercury Maps) that assumed the reduction in mercury levels in fish would be proportionate to the reduction in mercury deposition in the watershed. Given the minimal impact of Michigan utility emissions on mercury depositions in Michigan and the Great Lakes, more stringent controls would only minimally reduce the amount of methylmercury in fish over the span of several decades.
Contrary to the conclusions of the EPA, independent researchers have determined that coal-fired power plants do not represent a major source of mercury depositions. For example, the density of coal-fired power plants is highest in the Ohio Valley and Appalachia. But while the deposition of mercury from power plants is highest within and downwind of this region, the levels of mercury in fish are among the lowest nationwide.
The EPA has evaluated the potential health effects of more stringent reductions in mercury emissions. The agency concluded that the average IQ impairment due to all power plant emissions of mercury in the United States for children exposed in vitro to a mother’s consumption of freshwater fish (caught recreationally) was inconsequential — a mere 0.009 of an IQ point. Likewise, the average IQ impairment to 25,000 Michigan children born to mothers who consumed freshwater fish was 0.008 of an IQ point. The agency also evaluated the effects of mercury exposure from consumption of commercial freshwater fish, estuarine fish and marine fish, and concluded the risk of IQ impairment to be even less.
Overall, the health benefit of more stringent mercury controls was extremely small in all the cases evaluated by the EPA. When these results are monetized, which is necessary to measure regulatory efficacy, the benefits of stricter mercury controls were in the range of a few million dollars per year. The cost of stricter controls, however, was estimated by the EPA to be on the order of several billion dollars annually.
Despite this unbalanced cost-benefit ratio, the EPA still has required a 70 percent reduction in mercury emissions by coal-fed power plants.
This does not mean that reducing mercury emissions won’t reduce mercury levels in fish. But it does mean that the amount of deposited mercury that ends up in fish varies substantially across the country, depending on a variety of factors such as storage and release from aquatic sediments, as well as differences in the acidity and oxygenation of lakes.