Mercury Mercury
National Toxicology Program–Department of Health and Human Services


Mercury is a metal found in various forms. Metallic mercury is the silver colored liquid used in thermometers. Mercury combined with carbon is called organic mercury; methyl mercury is a common example of an organic mercury. Mercury compounds which contain non-carbon substances such as chlorine, oxygen, or sulfur are called inorganic mercurials. Mercury occurs naturally in the environment, and the levels are increased by certain human activities such as the burning of coal by power plants. Burning coal increases the amount of airborne mercury, which eventually falls back to earth into bodies of water. Mercury in water accumulates in fish. Common ways in which people are exposed to mercury include breathing contaminated air, eating contaminated fish, and through the use of mercury based amalgams (fillings) in dental treatments. Mercury can also enter the body through direct skin contact, and exposures to mercury may occur by contact with broken household items such as thermometers.

The effects of mercury on unborn children have been documented in cases of accidental poisonings and in scientific studies. During the 1950’s, large amounts of organic mercury were dumped into Minamata Bay in Japan, and mercury-contaminated fish were eaten by many pregnant women. Many of the children born to those women had severe nervous system damage, which was later referred to as Fetal Minamata Disease. In Iraq, studies showed that children born to mothers who ate grain contaminated with organic mercury may have learned to walk at a later age than non-exposed children. In the Faroe Islands, where mercury exposure occurs by eating contaminated whale meat, children born to mothers with higher body levels of mercury scored lower on brain function tests than children born to mothers with lower body levels of mercury. In contrast, no adverse effects were seen in children of the Seychelles Islands, where residents are routinely exposed to mercury by eating fish 12 times a week.

The Food and Drug Administration (FDA) and Environmental Protection Agency (EPA) recommend that young children and women who are pregnant, could become pregnant, or are nursing reduce their exposure to mercury by not eating fish containing high levels of mercury such as swordfish, shark, king mackerel, and tilefish. The FDA and EPA state that women and children can eat up to 12 ounces (2 average meals) per week of fish containing lower levels of mercury, such as shrimp, canned light tuna, salmon, Pollock, and catfish. Because albacore tuna contains higher levels of mercury, it is recommended that not more than 6 ounces be eaten per week.

Another possible source of mercury exposure to unborn children is the use of dental amalgams (mercury-based fillings) in pregnant women. Some special interest groups in the U.S. have attempted to place limits on the use of these amalgams. However, the U.S. Public Health Service and the FDA have taken the position that there is no scientific evidence to support limiting the use of mercury-based amalgams at this time. Similar conclusions were also reached by the World Health Organization and the government of New Zealand. German and Canadian governments concluded there was no evidence indicating that amalgams in pregnant women are harmful to unborn children, but suggested avoiding the use of, or even removing amalgams in pregnant women as a safety measure.

The EPA is proposing national standards for hazardous air pollutants and intends to require lower emissions of mercury and other pollutants from coal and oil-fired utility plants (EPA 2004).

Some individuals may also be exposed to mercury through religious practices. According to a health alert issued by the Agency of Toxic Substances and Disease Registry (ATSDR), mercury may be sprinkled around homes or cars by practitioners of religions such as Esperitismo, Santeria, or Voodoo. Broken thermometers may also be a source of mercury exposure in homes. Household exposures to mercury can be minimized by properly cleaning spills from items such as broken thermometers. Skin contact should be avoided and the spilled mercury should be placed into a vial which should then be tightly capped and properly disposed of.

The California Environmental Protection Agency (Cal/EPA) has classified mercury, methylmercury, and mercury compounds as developmental toxicants, which means there is evidence that these compounds can be harmful to unborn children. Acceptable levels of mercury have been established by other regulatory agencies. A limit of 2 parts mercury per billion parts of drinking water (2 ppb) has been established by the EPA. In addition, the EPA requires the reporting of spills or releases of 1 pound or more of mercury. The FDA has set a limit of 1 part per million (ppm) of mercury in seafood. The FDA also regulates the use of mercury in medical treatments such as dental amalgams. The Occupational Safety and Health Administration (OSHA) limits the level of mercury in workplaces to 1 milligram per 10 cubic meters of air (1 mg/10 m3).

Description of Mercury

According to the Agency of Toxic Substances and Disease Registry (ATSDR), "Mercury is a naturally occurring metal which has several forms. The metallic mercury is a shiny, silver-white, odorless liquid. If heated, it is a colorless, odorless gas. Mercury combines with other elements, such as chlorine, sulfur, or oxygen, to form inorganic mercury compounds or ‘salts.’ Most inorganic mercury compounds are white powders or crystals. Mercury also combines with carbon to make organic mercury compounds. The most common organic mercury compound is methylmercury. Methylmercury is produced mainly by small organisms in the water and soil, but more mercury in the environment can increase the levels of methylmercury that these small organisms make" (ATSDR ToxFAQ April 1999).

Sources of Mercury exposure

People can be exposed to mercury by breathing in air, eating food, or drinking water contaminated with mercury compounds. Mercury can also enter the body through contact with the skin.

Mercury is released into the air from natural sources like volcanoes and off-gassing from the earth’s crust and bodies of water (EHP, Volume 104(8), August 1996). Mercury can also be released into the air through the incineration of waste, the burning of coal by power plants, and by industries that use mercury to manufacture products. Mercury in the air eventually falls back to earth and accumulates in bodies of water. Fish absorb mercury through their gills and by eating contaminated food sources. Mercury becomes most concentrated in fish that eat smaller fish.

Exposure to mercury is also possible through dental amalgams (fillings), which contain 50 percent mercury. Occasional exposure to mercury can also occur from broken thermometers. Lastly, practitioners of certain religions, such as Esperitismo, Santeria, or Voodoo have been known to use mercury in rituals.

Levels of Mercury in the body

According to ATSDR, "Tests are available to measure mercury levels in the body. Blood or urine samples are used to test for exposure to metallic mercury and to inorganic forms of mercury. Measurement of mercury in whole blood or in scalp hair is used to measure exposure to methylmercury. Your doctor can take samples and send them to a testing laboratory" (ATSDR ToxFAQ April 1999).

Body levels of mercury are often expressed in terms of parts per million (ppm), 1 part of mercury per million parts of body tissue. As an example, a hair concentration of 1 ppm mercury is equal to 1 milligram of mercury per kilogram of hair. In people who have not been exposed to high levels of mercury, the average level of mercury in the hair is 2 ppm (FDA Consumer, September 1994).

Mercury health effects in unborn children

According to ATSDR, "Exposure to high levels of metallic, inorganic, or organic mercury can permanently damage the brain, kidneys, and developing fetus" (ATSDR ToxFAQ April 1999). FDA toxicologist, Dr. Mike Bolger, stated that "Methyl mercury easily crosses the placenta, and the mercury concentration rises to 30 percent higher in fetal red blood cells than in those of the mother"(FDA Consumer, September 1994). The adverse health effects of mercury on unborn children were observed in cases of accidental poisonings and in studies of individuals who ate large amounts of seafood, a common source of mercury.

In the 1950’s, large of amounts of organic mercury were released into the Minamata Bay in Kyushu, Japan (Environmental Health Perspectives, Volume 104, April 1996). Many individuals who ate mercury-contaminated fish and shellfish from the bay developed severe damage to the nervous system and many eventually died. Symptoms caused by the mercury poisoning were referred to as Minamata Disease. Fetal Minamata Disease developed in about 7% of children whose mothers ate contaminated fish while pregnant. Symptoms of Fetal Minamata Disease include uncoordinated movements, abnormal reflexes, speech problems, and seizures. Fetuses were found to be much more sensitive to mercury than were their mothers. The nervous systems of some mothers of children with Fetal Minamata Disease were only mildly affected; a common symptom was a narrowing of the visual field. The level of mercury in the mothers who gave birth to affected children after eating mercury-contaminated fish is not certain, but generally, nerve damage was seen in adults with mercury levels in hair of 50 ppm or higher (EHP, Volume 104(8), August 1996).

The following excerpt describes a case of accidental mercury poisoning in Iraq (FDA Consumer, September 1994):

Studies of the poisoning incident in Iraq have provided limited data about what effects low levels of methyl mercury exposures to the fetus have on the infant. One possible effect, for example, is lateness in walking. In the fall and winter of 1971-72, wheat seed intended for planting–and which had therefore been treated with an alkyl mercury fungicide–was mistakenly used to prepare bread; more than 6,500 Iraqis were hospitalized with neurological symptoms and 459 died. The vast majority of the mothers experienced exposures that resulted in hair levels greater than the lowest levels associated with effects in adults. But there was no clear evidence that the fetus was more sensitive than the adult to methyl mercury.

From an analysis of the Iraqi poisoning case, it was estimated that there is about a 5% chance that fetuses could be affected if their mothers have a hair level of 10-20 ppm mercury (EHP, Volume 104(8), August 1996)

The U.S. National Institute of Environmental Health Sciences (NIEHS) and the European Commission sponsored a mercury health study in the Faroe Islands in the North Atlantic. People on these islands are exposed to mercury mainly by eating whale meat (NIEHS PR #24-97, November 1997). Pregnant women in the study had hair mercury levels of 10-20 ppm (EHP, Volume 107(1), January 1999). To determine the effects of mercury intake by pregnant women, 7-year-old children were tested for brain function by measuring their attention span, memory, and speaking ability. Children born to mothers who had higher concentrations of mercury in their hair and umbilical cords scored lower on the brain function tests. Professor Roberta F. White of Boston University looked at the study results and concluded that "Several domains of brain function may be affected by prenatal methylmercury exposure. Most of the results remained within normal ranges, but any developmental delay in young children may be a concern.

The NIEHS also sponsored a study to examine the effects of mercury on unborn children from the Seychelles Islands in the Indian Ocean (NIEHS, February 7, 1996). The residents of the islands eat fish about 12 times a week and the average concentration of mercury in the hair of women is 10 ppm. However, some woman had hair levels of mercury as high as 36 ppm. Children were tested for mental and physical ability from the ages of 6 months to five years. The researchers concluded that "Cognitive developmental outcomes up to 2 years of age appear essentially normal following intrauterine exposure to a maternal hair mercury level of about 6 ppm through maternal fish consumption." They went on to say that "although data at older ages is [sic] needed, the results available so far indicate that low-level mercury exposure from eating fish during pregnancy shows no harmful effect." Philip Davidson of the University of Rochester stated, "If somebody who eats fish twice a day does not show effects from mercury exposure, it’s unlikely that somebody who eats fish twice a week will be affected, and the fish they eat in the Seychelles contains the same amount of mercury as fish sold at supermarkets and eaten in the United States" (NIEHS Center Program, September 1998).

Reviews of the Faroe Islands and Seychelles Islands studies are continuing as scientists try to determine why different results were obtained. One scientific panel concluded that discrepancies may have resulted from dietary differences or ethnic variability (EHP Volume 107(1), January 1999). The whale meat eaten by residents of the Faroe Islands also contained PCBs, which may have affected study results. However, scientists did take this into account when evaluating the Faroe Islands study results (NIEHS PR #24-97, November 1997).

Mercury in fish

Mercury levels in fish are expressed in terms of parts per million (ppm), which is equivalent to 1 part mercury per 1 million parts fish (1 mg mercury/kg fish). According to the FDA (FDA Consumer, September 1994),

Fish absorb methyl mercury from water as it passes over their gills and as they feed on aquatic organisms. Larger predator fish are exposed to higher levels of methyl mercury from their prey.

Methyl mercury binds tightly to the proteins in fish tissue, including muscle. Cooking does not appreciably reduce the methyl mercury content of the fish. Nearly all fish contain trace amounts of methyl mercury, some more than others. In areas where there is industrial mercury pollution, the levels in the fish can be quite elevated. In general, however, methyl mercury levels for most fish range from less than 0.01 ppm to 0.5 ppm. It’s only in a few species of fish that methyl mercury levels reach FDA limit for human consumption of 1 ppm. This most frequently occurs in some large predator fish, such as shark and swordfish. Certain species of very large tuna, typically sold as fresh steaks or sushi, can have levels over 1 ppm. (Canned tuna, composed of smaller species of tuna such as skipjack and albacore, has much lower levels of methyl mercury, averaging only about 0.17 ppm.) The average concentration of methyl mercury for commercially important species (mostly marine in origin) is less than 0.3 ppm. Spot-caught predator fresh-water species like pike and walleye sometimes have methyl mercury levels in the 1 ppm range. Other fresh-water species also have elevated levels, particularly in areas where mercury levels in the local environment are elevated.

In a recent consumer advisory, The FDA and EPA state (FDA 2004),

Fish and shellfish are an important part of a healthy diet. Fish and shellfish contain high-quality protein and other essential nutrients, are low in saturated fat, and contain omega-3 fatty acids. A well-balanced diet that includes a variety of fish and shellfish can contribute to heart health and children’s proper growth and development. So, women and young children in particular should include fish or shellfish in their diets due to the many nutritional benefits.

However, nearly all fish and shellfish contain traces of mercury. For most people, the risk from mercury by eating fish and shellfish is not a health concern. Yet, some fish and shellfish contain higher levels of mercury that may harm an unborn baby or young child’s developing nervous system. The risks from mercury in fish and shellfish depend on the amount of fish and shellfish eaten and the levels of mercury in the fish and shellfish. Therefore, the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) are advising women who may become pregnant, pregnant women, nursing mothers, and young children to avoid some types of fish and eat fish and shellfish that are lower in mercury.

To reduce exposure to mercury while obtaining benefits of eating fish, the FDA and EPA recommend that women and children (FDA 2004):

1. Do not eat Shark, Swordfish, King Mackerel, or Tilefish because they contain high levels of mercury.

2. Eat up to 12 ounces (2 average meals) a week of a variety of fish and shellfish that are lower in mercury.

Five of the most commonly eaten fish that are low in mercury are shrimp, canned light tuna, salmon, pollock, and catfish.

Another commonly eaten fish, albacore tuna has more mercury than canned light tuna. So, when choosing your two meals of fish and shellfish, you may eat up to 6 ounces (one average meal) of albacore tuna per week.

For fish caught by sports fishermen (FDA 2004) FDA and EPA state, "Check local advisories about the safety of fish caught by family and friends in your local lakes, rivers, and coastal areas. If no advice is available, eat up to 6 ounces (one average meal) per week of fish you catch from local waters, but don’t consume any other fish during that week."

The FDA has determined that mercury levels in fish sold in the United States must not exceed 1 ppm (FDA Consumer, September 1994). To ensure a safe fish supply (FDA Consumer, September 1994), "FDA works with state regulators when commercial fish, caught and sold locally, are found to contain methyl mercury levels exceeding 1 ppm. The agency also checks imported fish at ports and refuses entry if methyl mercury levels exceed the FDA limit."

Mercury in dental amalgams (fillings)

Dental amalgams contain about 50% mercury and there are concerns that unborn children can be exposed to mercury vapors given off by the amalgams in mothers. Some special interest groups have tried to ban or limit the use of mercury based amalgams. In order to investigate this issue, the U.S. Public Health Service (PHS) assembled a group of scientists from various governmental agencies. The following excerpts, taken directly from the report generated in 1993, summarize some of the primary conclusions reached by the scientists (U.S. Department of Health and Human Services Public Health Service, January 1993):

Dental amalgam, an inter-metallic compound, contains elemental mercury that is emitted in minute amounts as vapor. Because vapor emitting from amalgam restorations can be absorbed by the patient through inhalation, ingestion, or other means, concerns have been raised about possible toxicity. At present, there is scant evidence that the health of the vast majority of people with amalgam is compromised, nor that removing amalgam fillings has a beneficial effect on health. It also is recognized that a total conversion from dental amalgam to alternative materials would cause a significant increase in U.S. health care costs. Nonetheless, the possibility that this material, as well as currently available alternatives, could pose health risks cannot be totally ruled out because of the paucity of definitive human studies.

Given the limitations of existing scientific data, a research program should be designed and implemented to fill as many gaps as possible in current knowledge about the potential long-term biological effects of dental amalgam and alternative restorative materials. The PHS should be a leader in this effort.

The PHS should also educate dental personnel and consumers about the risks and benefits of dental amalgam. An educational program should include information on all restorative materials to help dentists and their patients make informed dental treatment decisions, and encourage dental care providers to report adverse reactions. Such a program should promote the use of preventive measures such as fluoride and dental sealants to prevent caries and thus further reduce the need for dental restorations.

The U.S. Public Health Service believes it is inappropriate at this time to recommend any restrictions on the use of dental amalgam, for several reasons. First, current scientific evidence does not show that exposure to mercury from amalgam restorations poses a serious health risk in humans, except for an exceedingly small number of allergic reactions. Second, there is insufficient evidence to assure the public that components of alternative restorative materials have fewer potential health effects than dental amalgam, including allergic-type reactions. Third, there are significant efforts underway in the U.S. to reduce the amount of mercury in the environment. And finally, as stated previously, amalgam use is declining due to a lessening of the incidence of dental caries and the increasing use of alternative materials.

The U.S. Public Health Service reassembled the group in 1997 to discuss new information published since the original report in 1993. According to the 1997 report (U.S. Department of Health and Human Services Public Health Service, October 1997), "In 1997, with input from a broad cross-section of scientists and dental professionals within USPHS, the FDA completed a review of nearly 60 studies that were published in peer reviewed scientific literature and were cited by citizen groups that petitioned the agency for stringent regulatory actions against dental amalgam. The analysis of the cited studies indicated that the current body of data does not support claims that individuals with dental amalgam restorations will experience adverse effects, including neurologic, renal or developmental effects, except for rare allergic or hypersensitivity reactions." The FDA concluded that "the agency does not believe there is scientific justification for discontinuing or curtailing amalgam use."

The 1997 U.S. Public Health Service Report also discussed opinions of foreign governments regarding the use of mercury amalgams. According to the report U.S. Department of Health and Human Services Public Health Service, October 1997, "The governments of Sweden and Denmark have recommended against the use of mercury-containing materials as part of national environmental protection initiatives provided that suitable non-amalgam materials are available. The German government has recommended against the placement of dental amalgam and dental restorative materials in general in patients with demonstrated allergy to such materials, as well as patients with severe renal dysfunction. Germany has also advised against the placement of dental amalgam and the removal of amalgam fillings in pregnant women as a precautionary measure while at the same time acknowledging the lack of evidence that exposure of the unborn to mercury released from the mother’s amalgam fillings causes any health damage to the child. The European Commission, the governments of Canada, Quebec and New Zealand, and the World Health Organization have independently evaluated the current body of science relating to dental amalgam safety and universally concluded that the vast majority of people treated with dental amalgam are not at risk. Notwithstanding this conclusion, Canada and its province of Quebec have recommended prudence in dental intervention therapies for certain patient sub-populations such as pregnant women."

Research to address health concerns of dental amalgams continues.

Mercury in religious practices

The following national alert was issued by the Agency of Toxic Substances and Disease Registry (ATSDR) (ATSDR National Alert: A Warning About Continuing Patterns of Metallic Mercury Exposure):

Persons who use metallic mercury in ethnic folk medicine and for religious practices are at risk. Metallic mercury is sold under the name "azogue" in stores (sometimes called botanicas), which specialize in religious items used in Esperitismo (a spiritual belief system native to Puerto Rico), Santeria (a Cuban-based religion that venerates both African deities and Catholic saints), and voodoo.

The use of azogue in religious practices is recommended in some Hispanic communities by family members, spiritualists, card readers, and santeros. Typically, azogue is carried on one’s person in a sealed pouch prepared by a spiritual leader or sprinkled in the home or automobile. Some botanica owners suggest mixing it in bath water or perfume and placing it in devotional candles.

Mercury spills in households

The following information about cleaning small household mercury spills is taken from the Louisiana Department of Environmental Quality web page (Louisiana Department of Environmental Quality, 2000):

If mercury escapes into the environment, evacuate children and pregnant women. Remove all jewelry, especially gold. Handle the mercury carefully. Wear rubber gloves and scoop it onto a sheet of paper or suck it up with an eyedropper. Place the mercury in a medicine vial or similar airtight container. The scoop, paper or eyedropper should also be bagged and disposed properly according to guidance provided by environmental officials or your local health department. Ventilate the room to the outside and close off the rest of the home. Use fans for a minimum of one hour to speed the ventilation. Do not simply throw the mercury away. Seek professional guidance from local recycling, solid waste or hazardous waste agencies. Large retailers and building centers may accept glass-encapsulated mercury, as in thermostats, for recycling.

Keep any objects containing mercury out of the reach of children. Children found to be playing with liquid mercury or broken fluorescent lamps should be referred to a physician or poison control center immediately.

Mercury contaminated gold jewelry must be taken to a jeweler to have them professionally cleaned.

While handling mercury, or any other hazardous substance, one should always wear protective gloves. If mercury contacts with the skin, wash the area(s) thoroughly and immediately with soap and warm water. If you believe that you have absorbed mercury though your skin or inhaled mercury vapors, you should contact your physician or poison control center immediately.

A vacuum cleaner should never be used to clean mercury spills. According to ATSDR (ATSDR National Alert: A Warning About Continuing Patterns of Metallic Mercury Exposure), "Using a vacuum cleaner causes metallic mercury to vaporize in the air, creating greater health risks. It also ruins the vacuum cleaner."

Federal and State government regulatory limits

The California Environmental Protection Agency (Cal/EPA) has added methyl mercury, and mercury compounds to its Proposition 65 list of developmental toxins. This means there is evidence that the compounds can be harmful to unborn children (Cal/EPA Proposition 65 List).

The following are government limits (ATSDR ToxFAQs, April 1999):

The EPA has set a limit of 2 parts of mercury per billion parts of drinking water (2 ppb). The EPA requires that discharges or spills of 1 pound or more of mercury be reported.

The Food and Drug Administration (FDA) has set a maximum permissible level of 1 part of methylmercury in a million parts of seafood (1 ppm).

The Occupational Safety and Health Administration (OSHA) has set a limit of 1 milligram of mercury per 10 cubic meters of workplace air (1 mg/10 m³) that should not be exceeded during any part of the workday.