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Current Status of the Science of Mercury in the Environment

Melvin E. Keener, Ph.D.
February 10, 1999


Elemental mercury is a heavy, silvery liquid at ambient temperatures. Mercury easily vaporizes under the normal range of temperature experienced. The majority of the mercury in the atmosphere is elemental. The majority of mercury in the land and water is either in the form of an inorganic salt or an organic form of mercury (e.g., methylmercury).

Mercury cycles in the environment as a result of natural and human activities. Elemental mercury may remain in the atmosphere for up to a year, resulting in wide dispersion from its initial source. This makes mercury emissions an international issue, rather than a local one. Inorganic forms of mercury are readily removed from the atmosphere by precipitation or dry deposition. Mercury is commonly emitted back to the atmosphere either as a vapor or associated with particles. The complexity of the mercury cycle is not fully understood.

What is the Controversy over Mercury?

Mercury is a known human toxicant. Mercury poisonings have occurred in Japan and Iraq following high dose exposures to methylmercury. The controversy is based on how different public agencies use the available data to estimate a safe exposure level to mercury. A reference dose (RfD) is the amount of a substance that, if ingested daily over a lifetime, will have no adverse effect on human health. EPA has calculated a RfD for methylmercury of 0.1ug/kg body weight/day. The World Health Organization (WHO) estimates that a RfD of 0.48 ug/kg bw/day is a more appropriate value. The U.S. Food and Drug Administration (FDA) and the Agency for Toxic Substances and Disease Registry (ASTDR) agree with the World Health Organization. This has set up an interesting dilemma where different agencies within the U.S. government are at odds over what the action levels for mercury should be.

Sources of Mercury

While mercury is a natural element that has been cycling through the environment since the earth formed, there is little doubt that increased industrial activity has increased potential human exposure to mercury. However, there are very little historical data to compare recent levels of mercury in the soil and water columns. The short duration of recent data make it difficult to determine if mercury levels in the environment are increasing, decreasing, or remaining stable.

In a recent report to Congress (EPA, December 1997), EPA estimated that the world-wide emissions of mercury for 1995 were approximately 5000 Mg. These emissions come from natural sources (release of geologically bound mercury through normal weathering), anthropogenic sources (human activities), or re-emission (mass transfer of mercury already deposited on the earth's surface). This report estimated that anthropogenic sources account for between 50 and 75 percent of total yearly input to the atmosphere. The estimated U.S. anthropogenic emissions for 1995 were 144 Mg. Thus, the anthropogenic emissions from the United States contributed approximately 3 percent of the total mercury emitted in the world. Based on a computer simulation of long-range transport, EPA estimated that approximately one third of U.S. emissions (47 Mg) were deposited within the lower 48 states and the remainder was transported beyond U.S. borders. This simulation estimated that an additional 32 Mg were deposited in the U.S. from the global reservoirs. Thus, emissions from U.S. anthropogenic sources are responsible for approximately 60 percent of the mercury deposited in the lower 48 states.

Of the anthropogenic sources in the U.S., four specific sources account for approximately 80 percent of the total: coal-fired utility boilers (47 Mg or 33%); municipal waste combustion (27 Mg or 19%); commercial/industrial boilers (26 Mg or 18%); and medical waste incineration (14 Mg or 10%). EPA estimates that hazardous waste combustors added about 6 Mg to the atmosphere in 1995. This represents about 4% of the U.S. anthropogenic sources and about 0.1% of the total mercury deposited globally. It should be noted that two new Clean Air Act rules for municipal and medical waste incinerators are estimated to reduce mercury emissions from these two sources by 90 percent. In addition, several states and localities have begun programs to remove mercury containing materials from waste streams destined to be burned. The combustion of coal in either a utility boiler or a commercial/industrial boiler is the only identified major anthropogenic source of mercury where mercury emissions are not regulated.

Fate and Transport of Mercury

In their report to Congress, EPA relied heavily on the use of computer models to describe the environmental fate of mercury. The reason stated for this was the lack of monitoring data that conclusively demonstrates or refutes a relationship between an individual anthropogenic source and increased mercury concentrations in the soil or biota near that facility. Since the models have not been validated, all results from these analyses should be treated as qualitative rather than quantitative. Some of the important limitations found during their analysis includes a lack of fundamental understanding of how mercury moves from watershed soils to a local water body and inability to explain why the relationship between the concentration of mercury in a water body and the mercury concentration in the fish in that water body varies from water body to water body.

Human Health Effects

Mercury primarily effects the nervous system. Symptoms in adults include impairment of the peripheral vision, disturbance of sensations (numbness or "pins and needles") usually in the hands and feet, lack of coordination of movements such as writing, impairment of speech, impairment of walking, impairment of hearing, and mental disturbances. Developing fetus and infants, when exposed to high levels of methylmercury, develop symptoms similar to congenital cerebral palsy. These include the inability to walk two steps without support by the age of two, the inability to respond to verbal communications by age two (with good hearing), lower neurological test scores, and presence of seizures.

These symptoms were first recognized in 1956 in Minamata City, Japan. A chemical factory released methylmercury into Minamata Bay. Once released, the methylmercury accumulated in the fish (mercury concentrations as high as 30 ppm). A second methylmercury poisoning outbreak occurred in 1965 in Niigata, Japan, when a chemical factory released mercury into the Agano River. These two events were long-term exposures to mercury via the consumption of contaminated fish. There have been two outbreaks of mercury poisoning from the consumption of mercury contaminated grain in Iraq. The first occurred prior to 1960 and the second in the early 1970s. In the second event, mercury-treated seed grain arrived after the planting season (at that time, seeds were routinely treated with mercury-based chemicals to maintain seed germination). Instead of being planted or waiting for the next growing season, the treated grain was ground for flour and baked into bread. Unlike the exposures in Japan, the mercury poisoning in Iraq was a short-term, higher exposure event. However, the human health symptoms were similar for both types of exposure.

Necessarily, the studies on these mercury exposure events were retrospective. In retrospective studies, it can be difficult to determine the exact exposure to mercury at critical times. These exposures must estimated from interviews on consumption and back-calculations for current body levels of mercury. Two prospective studies are currently being conducted on the fish-eating populations of the Seychelles Islands and the Faeroe Islands. These studies are monitoring the exposure of a large number of infants to lower levels of mercury and how this exposure effects neurological development. A number of papers have been published in the last three years that chronicle the development of these children. The most recent paper (Davidson, et. al.,1998) reported no significant developmental effects in children through the age of five and a half years old with mercury exposure that produced 10 ppm mercury in hair samples.

Human Exposure Pathways

All forms of mercury can cause adverse health effects in humans at sufficiently high doses. Elemental mercury is rapidly absorbed through the lungs but is poorly absorbed in the gastrointestinal tract. Inorganic mercury is also poorly absorbed by the gastrointestinal tract. Because the gastrointestinal tract rapidly absorbs methylmercury, the primary pathway for human exposure is eating food that contains methylmercury.

The methylation of mercury is a key step in the entrance of mercury into the food chain. This transformation can occur in the sediment or the water column of a water body. All mercury entering an aquatic ecosystem is not methylated. Demethylation reactions as well as the volatilization of dimethylmercury reduced that amount of methylmercury in an ecosystem. There is considerable variability between bodies of water on rates of methylization, demethylization, and conversion to dimethylmercury.

Since nearly 100% of the mercury that accumulates in the aquatic food chain is methylated, consumption of fish is the considered to be the primary pathway for mercury exposure to humans.

Development of Safe Exposure Levels

The current EPA RfD for methylmercury is based on neurological changes of 81 Iraqi children who were exposed when their mothers ate methylmercury contaminated bread during pregnancy. The data were collected by interviewing the mothers and by clinical examination of the children by pediatric neurologists approximately 30 months after the poisoning. Based on this data and an uncertainty factor of 10, EPA calculated an RfD for methylmercury to be 0.1 ug methylmercury/kg body weight/day. This corresponds to a mercury hair content of 11 ppm and a mercury blood level of 44ug/L.

In 1990, the World Health Organization, using the same data base, concluded that a daily uptake of 0.48 ug/kg bw/day would not cause any adverse effect to adults. In addition, they concluded that methylmercury uptakes of 3 to 7 ug/kg bw/day would result in less than a 5% increase in neurological effects in adults. These uptake rates correspond to mercury hair levels of 50 to 125 ppm.

The U.S. Food and Drug Administration (FDA) takes a somewhat different approach but comes to a similar conclusion as the World Health Organization. FDA's action levels are based on a tolerable daily intake (TDI) which combines the concentration of methylmercury in food (primarily fish) and information on consumption. FDA's action level of 0.2 mg per person per week translates to an RfD of 0.47 ug/kg bw/day. This level was based on data from the Japanese poisoning episodes. FDA has chosen not to include the data from the Iraq study but to wait until the data from the Seychelles Islands and the Faeroe Islands are available before revising their action levels.

Future Steps

Control Mechanisms. There is no single control mechanism that can eliminate anthropogenic mercury emissions. Since mercury is highly volatile, any elimination of mercury before combustion will reduce the amount emitted to the atmosphere. The municipal and medical incineration industries are attempting to remove as much mercury in the feed as possible to meet the new MACT standards. EPA is currently evaluating techniques to remove mercury from coal in an effort to reduce the amount emitted from the largest sources. End-of-the-pipe controls for mercury are not very effective and simply transfer the contamination from the air to another medium (solid waste).

New Clean Air Act rules will result in significant reduction in emissions from municipal and medical waste incinerators. Additional reductions can be expected when the Clean Air Act rules for hazardous waste combustors are finalized. However, since these units as such a small source, the amount of reduction in mercury emissions will be small. There are no currently proposed regulations to reduce mercury emissions from coal-fired utility boilers or coal-fired industrial/commercial boilers.

However, even if the U.S. were capable of eliminating mercury emissions, it may not significantly impact the amounts of methyl mercury in fish, the primary source of human exposure (EPA Mercury Study Report to Congress, 1997) because the U.S. emissions represent less than 3 % of the global emissions of mercury. Fitzgerald and Mason (1996) estimated that if all anthropogenic emissions of mercury were ceased, it would take about 15 years for the ocean and soil reservoirs to return to pre-industrial levels. When the Science Advisory Board panel reviewed the EPA's Mercury Study Report to Congress, they concluded that the process would take much longer.

Revisions of RfD. When the Science Advisory Board panel reviewed EPA Mercury Study Report to Congress, they advised that the current RfD not be revised with the new data from the Seychelles Islands and the Faeroe Island. This was based on the concept that time was needed to further analyze the data to determine its exact meaning. The panels suggested that EPA add the new data and re-estimate the RfD within the next two years.

In December 1998, the ASTDR indicated that they would revise their recommendation for daily exposure to 0.4 ug/kg bw/day, based on the most recent data from the Seychelles study. In January 1999, ASTDR decided to postpone that decision for a couple of months until EPA could comment on their methods of determining an RfD.

While on the surface, it appears that the Iraqi and Seychelles data sets give totally different answers, it is probable that when all are examined together, some consistency will be found. The development of past action levels was based on high exposure events and estimating levels where no adverse effects would be seen. The populations of the newest studies were selected based on identifying subtle effects at the levels of mercury exposure that were thought to be problematic. The preliminary results from the new studies indicate that the estimations of no adverse effect levels by EPA were too conservative. The new studies also more closely mimic normal exposure pathways rather than short-term, high exposure events.

Rectification of U.S. Agencies Positions. The rectification of the differences between the agencies within the U.S. government will be difficult primarily because there is so much at stake. Should everyone agree that FDA's and ASTDR's RfD is correct, this would eliminate most of the fish advisories in the United States with a stroke of a pen. It would also reduce the need to regulate mercury emissions from coal fired boilers, the largest source of mercury emissions in the U.S. EPA staff believes that mercury emissions from these units need to be regulated to protect human health and the environment. In all likelihood, resolution of this difference will take place at the highest levels within the U.S. Government.


Davidson, P.W., G. J. Myers, C. Cox, C. Axtell, C. Shamlaye, J. Sloane-Reeves, E. Cernichiari, L. Needham, A. Choi, Y. Wang, M. Berlin, and T. W. Clarkson. 1998. Effects of Prenatal and Postnatal Methylmercury Exposure from Fish Consumption on Neurodevelopment: Outcomes at 66 Months of Age in the Seychelles Child Development Study. Journal of the American Medical Association, 280:701-707.

EPA. Mercury Study Report to Congress. December 1997. EPA-452/R-97-003.

Fitzgerald, W. F., and R. P. Mason. 1996. The Global Mercury Cycle: Oceanic and Anthropogenic Aspects. Pp. 185-208. In Baeyens, W., R. Ebinghaus, and O. Vasiliev, eds. Global and Regional Mercury Cycles: Sources, Fluxes, and Mass Balances.

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