A Mercury Primer

Sources of Mercury

Most of the mercury that enters the environment comes from man-made sources. There are several large sources of mercury in the United States, as well as many smaller sources, and most of these are described in the Environmental Protection Agency's (EPA) comprehensive Mercury Study Report to Congress (EPA, 1997). Total mercury emissions in the U.S. are about 150 tons per year.

Coal-fired electric power plants comprise the largest single source, producing about one-third of U.S. emissions. Mercury is an impurity in coal, released into the air when the coal is burned. Because coal is cheap and abundant, these power plants enjoy widespread use both domestically and internationally. Most U.S. power plants are exempt from the pollution controls mandated by the Clean Air Act, and power plant mercury emissions are not yet specifically regulated.

Likewise, coal- and oil-fired industrial, commercial, and residential boilers also produce significant mercury emissions. Many industrial boilers are still in use in the U.S. While residential coal use has nearly vanished in the U.S., it is still prevalent in countries like China.

Municipal, medical, and hazardous waste incinerators are a large source of mercury emissions, due to the presence of mercury-containing items in their waste streams. The EPA has recently regulated most of these incinerators.

Chlor-alkali plants that use the mercury cell process to produce chlorine are a large source of mercury emissions. A related problem with these plants involves the disposition of excess mercury supplies when these plants convert to a membrane cell process that no longer requires mercury. Cement plants are another significant source, releasing mercury through fugitive dust emissions.

Manufacturing is still a large international source of mercury. Although the U.S. has cleaned up many of its manufacturing processes, many countries still permit widespread dumping of mercury and other toxins. Further, many products sold in the U.S. and abroad, such as thermometers, thermostats, and fluorescent lamps, still contain mercury. Breakage or burning of these products during disposal is a significant source of mercury emissions. Automobiles, appliances, and other heavy equipment still contain mercury components, which produce mercury emissions when these items are melted for scrap metal.

Some sources of mercury continue to haunt us for decades. Gold mining, for example, once used vast quantities of mercury, and most of this mercury ended up in nearby streams and rivers. Mercury from 150-year old gold mines in California still contaminates San Francisco Bay enough to prompt fish consumption advisories (USGS, 2000).

A lesser amount of mercury originates in outgassing and other natural processes. The natural cycling of mercury through soil, air, water, and sediments greatly extends the lifetime of both natural and anthropogenic mercury in the environment.

Controlling Mercury

There are many options for reducing these mercury emissions. For coal- and oil-fired power plants and boilers, improving the combustion heat rate efficiency saves fuel and reduces the emissions of all pollutants, including mercury. Coal cleaning is inexpensive and effective at removing mercury. Using coal with naturally lower mercury content reduces emissions, and it generally reduces sulfur dioxide emissions as well. In the long term, the transition to other fuels, such as natural gas, and the development of renewable energy sources will reduce coal and oil use and their associated mercury emissions.

For power plants and many other sources, direct emission control technologies already exist: scrubbers, selective catalytic reduction, particulate collectors, etc. This equipment can reduce mercury emissions while also helping sources to comply with other pollution reductions mandated by the Clean Air Act. More advanced technologies are under development.

For incinerators, the separation of mercury-containing items from the waste stream greatly reduces mercury emissions. Mandatory labeling of these items, which some states have implemented, greatly facilitates this process. Cost-effective alternatives exist for mercury in most consumer products. Using these alternatives reduces manufacturing emissions, cleans the output of incinerators and electric arc furnaces, eliminates the need for special disposal, reduces the global market for mercury, and reduces the need to mine more virgin mercury.

Finally, industrial process changes, such as switching from the mercury cell process to the more efficient membrane cell process in chlor-alkali plants, can eliminate large point sources of mercury emissions.

Mercury Transport

The movement of mercury pollution is far from simple. Some mercury is deposited locally, in the immediate vicinity of the source. Some mercury moves regionally, freely crossing state boundaries. The Northeast, for example, receives a significant amount of its mercury from the Midwest. Vermont produces virtually no mercury, yet the Vermont Department of Health is forced to issue statewide fish consumption advisories due to mercury contamination. Few states are immune from their own or someone else's mercury. Some mercury cycles globally, so that the U.S. and other countries receive some of each other's mercury.

This can have important policy implications. The EPA uses emissions trading in its Acid Rain Program, and some have suggested that mercury should be traded as well. However, a region that decides to reduce its mercury emissions and sell emissions credits to an upwind region could end up being contaminated by that other region's mercury emissions. Likewise, a power plant that purchases emissions credits and continues to emit mercury would continue to contaminate its neighbors through local deposition.

Mercury and Wildlife

Mercury eventually leaves the atmosphere through both wet and dry deposition. Most of it ends up in terrestrial soils. From an environmental perspective, the greatest concern is the mercury that enters rivers, lakes, and the upper layer of the ocean. Metallic and inorganic mercury are toxic in their own right. However, bacteria in water convert some of this mercury into methylmercury, which is a very powerful, persistent neurotoxin. A small amount can do great damage, and it is especially hard to eliminate. It bioaccumulates up the food chain, from plankton and insects to amphibians, fish, birds, and mammals. This means that animals at the top of the food chain have much higher mercury levels than those at the bottom.

Methylmercury is a serious problem in ecosystems across the U.S., and clear examples abound. Freshwater fish such as trout, pike, and perch suffer reproductive, immune, and endocrine system damage. Fish-eating loons in New England and Canada have exhibited highly elevated methylmercury levels, and a significant percentage of their population is at serious risk. Effects include reproductive problems, such as fewer young, and behavioral changes. Endangered Florida panthers that eat raccoons and alligators have also shown high methylmercury levels, and at least one has died from mercury poisoning. Mink, otters, frogs, ducks, oysters, and many other species are all at risk. Of course, this problem is not limited to the U.S. Mercury emissions threaten ecosystems around the world.

Some species of large, predatory ocean fish also harbor high levels of methylmercury. While it is less certain how these fish get their mercury, years of data show that they do have it. These include shark, popular seafood species such as swordfish and large tuna, and commonly caught sport fish such as king mackerel.

Mercury and People

Humans are exposed to methylmercury by eating contaminated fish, especially large predatory fish that are high on the food chain. This includes numerous freshwater species and several common seafood species. As a potent neurotoxin, methylmercury damages the human central nervous system. It also damages the cardiovascular system, the immune system, and other parts of the body. Abundant science confirms this hazard.

Historically, there are many examples of mercury toxicity. In past centuries, hatters went mad from handling the metallic mercury they used while making felt hats. In the twentieth century, large industrial mercury releases in Japan contaminated coastal waters, leading to widespread mercury poisoning in local fishing villages. Consumption of seed grain coated with a methylmercury fungicide induced a severe mercury poisoning episode in Iraq.

Outright methylmercury poisoning is rare in the U.S., although at least one man has been hospitalized from eating freshwater fish contaminated with methylmercury. The greatest concerns of methylmercury exposure are neurological dysfunction and developmental disorders. Levels of methylmercury that would not poison an adult can damage a child's developing brain, leading to deficiencies in IQ, attention, and motor function. Unborn children and young children are particularly vulnerable, especially among populations that practice subsistence fishing.

How Much Is Too Much?

There has been some controversy regarding what constitutes a dangerous level of methylmercury exposure. In 1969 the Food and Drug Administration (FDA), in response to growing health concerns, implemented an "administrative level" of 0.5 parts per million (ppm) of methylmercury in fish. The FDA converted this to an "action level" of 0.5 ppm in 1974. Partly in response to legal challenges, the FDA raised the action level to 1.0 ppm in 1979, and this level is still in effect. Unlike a formal tolerance, an action level does not require enforcement; consequently the methylmercury action level has seldom been enforced, and some easily available commercial seafood species (e.g., swordfish) commonly exceed this level.

In 1997 the EPA recommended a "reference dose" of 0.1 micrograms of methylmercury per kilogram of body weight per day (µg/kg/day) in their Mercury Study Report to Congress (EPA, 1997). When translated to comparable units, this is four to five times lower than the FDA action level.

In 1999, after considerable political maneuvering, the Department of Health and Human Services' Agency for Toxic Substances and Disease Registry published a "minimal risk level" of 0.3 µg/kg/day in their revised Toxicological Profile for Mercury (ATSDR, 1999).

In 2000 the National Research Council (NRC), at the request of Congress, thoroughly studied the issue of methylmercury and confirmed the EPA's reference dose of 0.1 µg/kg/day as "scientifically justifiable for the protection of public health" in their report Toxicological Effects of Methylmercury (NRC, 2000).

The National Academies are generally considered the last word in scientific assessments, but in this case the debate continues. Public health organizations and environmental groups have advocated for the more protective EPA level, with effective enforcement. Some, after conducting their own studies, have recommended even lower levels. On the other hand, industry groups prefer the higher, unenforced FDA action level, strenuously resisting any move to lower the level or make it more enforceable.

Limiting Mercury in Seafood

The federal government has been reluctant to set real limits on methylmercury in commercial seafood. Consequently, some fish species with levels commonly exceeding the FDA action level are readily available for sale across the U.S. There is strong disagreement regarding the need to set such limits and the form these limits should take. Fish is a good, low-fat source of protein with undeniable health benefits, and many people enjoy eating fish. Further, the fishing industry employs many Americans and forms the foundation of many coastal communities. On the other hand, the federal government has a responsibility to protect Americans from known toxins in their food supply.

There is solid scientific support for the need to protect Americans from methylmercury exposure. In its report, the EPA estimated that 85,000 babies per year might be at risk from methylmercury (EPA, 1997). The NRC arrived at a similar number, estimating that "over 60,000 newborns annually might be at risk for adverse neurodevelopmental effects from in utero exposure to MeHg" (NRC, 2000). In 2001, the Centers for Disease Control and Prevention released data from the 1999 National Health and Nutrition Examination Survey, showing that 10% of women may have levels of mercury exceeding the EPA's safe level (CDC, 2001). This could put far more children at risk than the EPA or NRC estimated.

The FDA has not changed its action level since 1979, nor have they moved to enforce it. Indeed, they no longer even sample commercial seafood for methylmercury, even though their own data before 1998 confirmed that some fish species regularly exceeded their action level. However, in response to the growing scientific consensus, the FDA issued stronger consumer guidance in 2001, warning pregnant women and others to avoid the four fish with the highest methylmercury levels and to limit consumption of other fish. This was intended to protect those who are most sensitive to the damaging effects of methylmercury. The FDA has also committed to making mercury a high priority in 2001, including a reexamination of their risk assessment, their advisories, and their action level.

Mercury in Freshwater Fish

Methylmercury is also a serious problem in freshwater fish, including many species prized by sport fishermen. Across the country, many of these fish have dangerous levels of methylmercury, prompting state health agencies to warn their citizens against eating them. In 2000, 41 states issued more than 2,200 fish consumption advisories because of methylmercury contamination. 13 states issued statewide advisories for all their rivers and/or lakes, and 9 states issued statewide advisories for their coastal waters. These numbers continue to grow each year. The EPA works closely with state agencies to help develop these advisories. The FDA is not involved with freshwater fish advisories, as they are only responsible for commercial fish.

Regulating Mercury Emissions

The Clean Air Act amendments of 1990 required the EPA to categorize and regulate major sources of hazardous air pollutants such as mercury. In response, the EPA promulgated regulations for several major sources of airborne mercury pollution during the 1990's: municipal waste incinerators, commercial and industrial incinerators, medical waste incinerators, and hazardous waste incinerators. They have not regulated emissions from chlor-alkali plants, and their regulations for cement plants omitted mercury.

The most difficult and most important sources to regulate have been coal- and oil-fired electric power plants. Strong, well-funded industry opposition has produced numerous delays and generated extra obstacles for regulating these plants. The Clean Air Act itself includes extra requirements the EPA had to fulfill before regulating electric power plants. One of these requirements was the Mercury Study Report to Congress, which showed that coal-fired power plants contributed one-third of total U.S. mercury emissions (EPA, 1997). Another was a comprehensive study of hazardous air pollutant emissions from electric power plants, including a report to Congress. The EPA published this report in 1998, concluding that "mercury from coal-fired utilities is the HAP of greatest potential concern" (EPA, 1998).

In December 2000, the EPA issued a positive determination to regulate mercury from coal- and oil-fired electric power plants. They are currently developing these regulations, which are scheduled to be proposed in 2003 and issued in 2004, despite continued industry opposition.

Summary

Man-made sources in the U.S. pump more than 150 tons of mercury pollution into the air each year. The wind spreads this mercury across the country. Much of it returns to earth to contaminate our lakes and streams as methylmercury, a persistent bioaccumulative neurotoxin that poisons a wide variety of wildlife species. Humans are exposed to methylmercury by eating contaminated fish, and many states have been forced to warn their citizens against eating the fish they catch. Several commercially available seafood species also have high methylmercury levels. The FDA has warned women to limit their fish consumption, and they have set an "action level" to limit methylmercury in seafood, but this level is not enforced. The EPA and the NRC have recommended a much lower level, primarily to protect unborn children, who are the most vulnerable to methylmercury. The EPA has also regulated mercury emissions from some sources, and they are moving to regulate the largest source - electric power plants.

References

Agency for Toxic Substances and Disease Registry, 1999: Toxicological Profile for Mercury. (link)

Centers for Disease Control and Prevention, 2001: Blood and hair mercury levels in young children and women of childbearing age - United States, 1999. Morbidity and Mortality Weekly Report, 50, 140-143. (link)

Clean Air Act, 42 U.S.C. 7401 et seq. (link)

National Research Council, 2000: Toxicological Effects of Methylmercury. National Academy Press, 344 pp. (link)

U.S. Environmental Protection Agency, 1997: Mercury Study Report to Congress, EPA-452/R-97-006. (link)

U.S. Environmental Protection Agency, 1998: Study of Hazardous Air Pollutant Emissions from Electric Utility Steam Generating Units - Final Report to Congress, EPA-453/R-98-004a. (PDF)

U.S. Geological Survey, 2000: Mercury Contamination from Historic Gold Mining in California, FS-061-00. (link)