Mercury Emissions

Power Plant Mercury Emissions and Human Health: What are the Central Issues?

Why all this concern about mercury?

At high doses, mercury can cause serious damage to the central nervous system, brain, and/or kidneys. However, such high doses are extremely rare in the world today. Considerable scientific debate continues regarding the health effects of low mercury doses, which may have some developmental effects on fetuses and infants exposed via the mother's diet. Today, mercury enters the body primarily through consumption of fish—especially large predatory fish at the top of the food chain. These fish accumulate the most toxic form of mercury, i.e., methylmercury. In many bodies of water, microorganisms convert some of the inorganic forms of mercury to this more toxic compound. Methylmercury can accumulate over time if the ingestion rate exceeds the rate of excretion. Health concerns focus mainly on those who eat fish frequently and continuously, especially pregnant women.

What are the major sources of mercury in the world and in the United States and what is TVA's contribution?

Natural sources of mercury—such as volcanic eruptions and emissions from the ocean—have been estimated to contribute about 40 percent of current worldwide mercury air emissions, whereas human-made sources account for the remaining 60 percent. It is estimated that the United States presently accounts for 3 to 5 percent of total global mercury air emissions, whereas Asian countries are estimated to contribute about one-half of the human-made air emissions. Because of federal bans on mercury additives in paints and pesticides, reduced mercury use in batteries, and improved battery recycling, the U.S. industrial demand for mercury has dropped 75 percent since 1988. Based on current preliminary data from the U.S. Environmental Protection Agency (EPA), the principal human-caused sources of mercury emissions in the United States are coal-fired power plants (33 percent of the total emitted), municipal waste incinerators (19 percent), and medical waste incinerators (10 percent). (See Figure 1.)

During 1999, according to a recent EPRI report using fossil plant stack sample data, TVA plants accounted for approximately 4 percent of all U.S. power plant mercury emissions. Therefore, on an annual basis, TVA plants account for about 1.5 percent of all human-made emissions in the United States. Mercury air emissions from individual power plants vary greatly. The major causes of variation are: (1) the concentration of mercury in coal, (2) the chemical form of the mercury, and (3) the efficiency of pollution control devices in removing mercury.

Mercury is a global problem to which re-emissions from past activities contribute.

Once released into the environment, the cycling of mercury is very complex and not well understood. Mercury never breaks down into another element; it always remains as mercury. Mercury is a volatile, heavy metal, and, as such, can be re-emitted into the atmosphere from land and water surfaces repeatedly after its initial release into the environment. Estimates of the magnitude of re-emission are very difficult to quantify. But there is no disputing that re-emission is a major source of total modern-day mercury emissions. Consequently, much of the mercury circulating through today's environment is mercury that was released decades or centuries ago, when mercury was commonly used in many industrial, commercial, and residential products and processes.

Mercury also can travel great distances in the atmosphere before it is eventually deposited back to the earth in rainfall or in dry gaseous forms. Thus, mercury is a global problem that knows no national or continental boundaries. And since little is known about the dry deposition of mercury compounds, considerable research is needed to determine the quantity or fate of this dry deposition. After being deposited, a considerable amount of mercury is retained by plants and soil, but some is directly deposited into or washed into streams, lakes, and oceans. As long as this mercury stays in its inorganic forms, it remains in a relatively inactive biological state. However, once in an aquatic ecosystem, inorganic mercury forms may be converted to methylmercury by microorganisms in the sediment and water. And methylmercury may enter the aquatic food chain and work its way up to the larger fish, which eventually may be consumed by humans and other animals.

How are fish consumption advisories determined and what do they mean?

Both saltwater and freshwater fish, depending on their relative position in the food chain, contain varying amounts of methylmercury. "Bottom-feeding" fish, such as catfish, flounder, and shellfish, have some of the lowest mercury concentrations; whereas, large carnivorous fish, such as shark, swordfish, and king mackerel, have the highest mercury concentrations. Smaller carnivorous fish, such as tuna and bass have intermediate mercury levels. Within fresh water bodies in the Southeast, bass generally have higher mercury concentrations than catfish. And farm-raised fish generally test lower in mercury than those caught in "natural" surface waters.

The EPA and the World Health Organization take a relatively conservative approach to setting "safe" exposure levels for mercury, whereas other federal agencies, such as the Agency for Toxic Substances and Disease Registry (ATSDR) and the Food and Drug Administration (FDA), traditionally have set substantially higher "safe" exposure levels. Recently, the EPA adjusted its guidelines to include specific recommendations on the amounts of fish that could safely be eaten per month. Amounts were based on the estimated average level of mercury contained in that fish species. Pregnant women are advised to consume no more than one-half the amount recommended for other adults.

Fish consumption advisories for specific bodies of water in the United States are issued by individual state agencies and are based on widely-differing standards. Some states base their standard on the more conservative EPA "safe" dose, whereas others use the ATSDR or FDA standards, which are typically used for fish sold commercially. As of December 2000, more than 2,200 bodies of water in 41 states had fish consumption advisories related to mercury. Approximately 80 percent of all the fish consumption advisories in the United States were related to mercury. In 2000, 13 states issued state-wide advisories, including North Carolina. Many others, including Georgia, have issued advisories for numerous individual streams and reservoirs across the state, including mercury advisories for bass from two TVA reservoirs (Blue Ridge Reservoir and Chatuge Reservoir) and one Tennessee River tributary (Chickamauga Creek-west branch). To date, Tennessee has issued fish consumption advisories for mercury in only two streams (North Fork of Holston River and East Fork Poplar Creek), while Alabama and Kentucky have issued no advisories within the Tennessee River Valley.

What are fish mercury levels in the Tennessee River Valley?

TVA has been monitoring mercury levels in fish and sediments since 1970. The current TVA fish monitoring program was begun in 1987. Mercury levels in sediments at the bottom of TVA reservoirs have declined substantially in 21 of 26 reservoirs since 1973 (Figure 2).

Fish mercury levels tend to be quite low in largemouth bass and catfish in the reservoirs of the main Tennessee River downstream from Knoxville. On average, mercury levels are higher in tributary reservoirs and in streams further upriver. Data from channel catfish indicate, on average, no apparent change in mercury levels—up or down—between 1987 and 1999. And four states (Tennessee, Virginia, Alabama, and Kentucky) combined have issued fish advisories for only two streams in the Tennessee River Valley. Nevertheless, mercury levels in Valley fish (as well as fish from other sources) remain a potential health issue for pregnant women who eat certain types of fish frequently. Notwithstanding the ongoing disagreement that exists between federal agencies on "safe" levels of consumption, TVA is continuing to monitor mercury levels in sediments and fish.

Summary and Conclusions

Most health concerns regarding mercury center around the potential for developmental damage to infants resulting from mercury consumed in the mother's diet, particularly fish consumption.
Mercury is a global problem, with much of the mercury in the environment today coming from outside the United States and from the re-circulation of past emissions from both natural and human-made sources.
According to current estimates, the United States accounts for about 4 percent of the world's "new" mercury emissions, with about 60 percent of those emissions coming from human-made sources.
Current preliminary data indicate that U.S. coal-fired power plants account for about one-third of national mercury emissions, and TVA coal-fired power plants account for approximately 4 percent of all U.S. power plant emissions.
U.S. power plants, overall, are responsible for about 1 percent of current global emissions.
Mercury levels in TVA reservoir sediments have declined substantially since 1973.
Mercury levels in channel catfish in the Tennessee River System, on average, have remained unchanged over the past 13 years.

Information Contacts

Thomas A. Burnett, (423) 751-3938, taburnett@tva.gov

Donald L. Dycus, (423) 751-7322, dldycus@tva.gov

William J. Parkhurst, (256) 386-2793, wjparkhurst@tva.gov

Last updated on 1-17-2002.
Inquiries and comments should be sent to wjparkhurst@tva.gov.