Mercury exists in various forms, and people are exposed to each in different ways. The most common way people in the U.S. are exposed to mercury is by eating fish containing methylmercury. Other exposures may result from using or breaking products containing mercury. The health effects of these exposures are discussed in a separate section.
- How mercury enters the environment
- Moving up the food chain
- Elemental mercury
- Other mercury compounds (inorganic and organic)
People who use mercury in the workplace need to take special precautions.
Outbreaks of methylmercury poisoning have made it clear that adults, children, and developing fetuses are at risk from dietary exposure to methylmercury. During these poisoning outbreaks some mothers with no symptoms of nervous system damage gave birth to infants with severe disabilities and it became clear that the developing nervous system of the fetus may be more vulnerable to methylmercury than is the adult nervous system. Mothers who are exposed to methylmercury and breast-feed their babies may also expose their infant children through their milk.
In 2004 EPA and FDA issued the first-ever joint consumer advice about methylmercury in fish and shellfish. This advice was for women who might become pregnant; women who are pregnant; nursing mothers; and young children. The advisory provides three recommendations for selecting and eating fish or shellfish to ensure that women and young children will receive the benefits of eating fish and shellfish and be confident that they have reduced their exposure to the harmful effects of methylmercury. EPA also hosts a web-based compilation of fish advisories issued by States, tribes, territories and local governments. Fish Consumption Advisories
Reference Dose (RfD): An estimate (with uncertainty spanning perhaps an order of magnitude) of a daily oral exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime. It can be derived from a NOAEL, LOAEL, or benchmark dose, with uncertainty factors generally applied to reflect limitations of the data used. Generally used in EPA's noncancer health assessments.
No-Observed-Adverse-Effect Level (NOAEL): The highest exposure level at which there are no biologically significant increases in the frequency or severity of adverse effect between the exposed population and its appropriate control; some effects may be produced at this level, but they are not considered adverse or precursors of adverse effects.
Lowest-Observed-Adverse- Effect Level (LOAEL): The lowest exposure level at which there are biologically significant increases in frequency or severity of adverse effects between the exposed population and its appropriate control group
Recent human biological monitoring by the Centers for Disease Control and Prevention in 1999 and 2000 (PDF) (3 pp., 42 KB, About PDF) shows that most people have blood mercury levels below a level (5.8 µg/L of whole blood) associated with possible health effects. Consumption of fish with higher methylmercury levels can lead to elevated levels of mercury in the bloodstream of unborn babies and young children and may harm their developing nervous system. These disabilities have been documented in ability to use language, to process information, and in visual/motor integration. U.S. EPA's 2001 Reference Dose (RfD) for methylmercury was calculated to protect the developing nervous system. Currently, U.S. EPA uses a RfD of 0.1 µg/kg body weight/day as an exposure without recognized adverse effects. A description of EPA’s Reference Dose for methylmercury may be found at http://www.epa.gov/iris/subst/0073.htm.
In U.S. EPA’s Mercury Study Report to Congress (1997) EPA estimated that 7% of women of childbearing age would have blood mercury concentrations greater than those equivalent to the RfD. The estimate of 7% of women of childbearing age above the RfD was based on patterns of fish and shellfish consumption and methylmercury concentrations present in fish and shellfish. Blood mercury analyses in the 1999-2000 National Health and Nutrition Examination Survey (1999-2000 NHANES) for 16-to-49 year old women showed that approximately 8% of women in the survey had blood mercury concentrations greater than 5.8 ug/L ( which is a blood mercury level equivalent to the current RfD). Based on this prevalence for the overall U.S. population of women of reproductive age and the number of U.S. births each year, it is estimated that more than 300,000 newborns each year may have increased risk of learning disabilities associated with in utero exposure to methylmercury. More recent data from the CDC support this general finding.
Nearly all methylmercury exposures in the U.S. occur through eating fish and shellfish. Microscopic organisms convert inorganic mercury into methylmercury, which accumulates up the food chain in fish, fish-eating animals, and people.
This process is explained below.
Mercury is emitted to the air by human activities, such as manufacturing or burning coal for fuel, and from natural sources, such as volcanoes.
Typically, mercury is released into the atmosphere in one of three forms:
- elemental mercury: can travel a range of distances, may remain in the atmosphere up to one year and may travel globally before undergoing transformation
- particle-bound mercury: can fall out of the air over a range of distances
- oxidized mercury (sometimes called ionic or reactive gaseous mercury (RGM)): found predominantly in water-soluble forms, which may be deposited at a range of distances from sources depending on a variety of factors including topographic and meteorologic conditions downwind of a source.
What happens to mercury after it is emitted depends on several factors:
- the form of mercury emitted
- the location of the emission source
- how high above the landscape the mercury is released (e.g., the height of the stack)
- the surrounding terrain
- the weather.
Depending on these factors, atmospheric mercury can be transported over a range of distances before it is deposited, potentially resulting in deposition on local, regional, continental and/or global scales. Mercury that remains in the air for prolonged periods of time and travels across continents is said to be in the "global cycle."
Recent emissions estimates of annual global mercury emissions from all sources, natural and anthropogenic (human-generated), which are highly uncertain, are about 4800-8300 tons per year.
U.S. anthropogenic mercury emissions are estimated to account for roughly 3 percent of the total global emissions, and the U.S. power sector is estimated to account for about 1 percent the total global emissions. EPA has estimated that about one third of U.S. emissions are deposited within the contiguous U.S. and the remainder enters the global cycle.
Current estimates are that less than half of all mercury deposition within the U.S. comes from U.S. sources, although deposition varies by geographic location. For example, compared to the country as a whole, U.S. sources represent a greater fraction of the total deposition in parts of the Northeast because of the direction of the prevailing winds.
When mercury falls in rain or snow, it may flow into bodies of water like lakes and streams. When it falls out of the air as dry deposition, it may eventually be washed into those bodies by rain. Bacteria in soils and sediments convert mercury to methylmercury. In this form, it is taken up by tiny aquatic plants and animals. Fish that eat these organisms build up methylmercury in their bodies. As ever-bigger fish eat smaller ones, the methylmercury is concentrated further up the food chain. This process is called "bioaccumulation".
Methylmercury concentrations in fish depend on many factors, including mercury, the concentration in water, water pH and temperature, the amount of dissolved solids and organic matter in the water, and what organisms live in the water. Methylmercury concentrations in fish may also be affected by the presence of sulfur and other chemicals in the water. Because of these variables, and because food webs are very complex, bioaccumulation is hard to predict and can vary from one water body to another.
However, in a given water body, the highest concentrations of methylmercury are generally found in large fish that eat other fish. The concentrations of methylmercury in large fish can be over a million-fold larger than in the surrounding water. EPA discussions of estimates bioaccumulation can be found in Chapter 6 and Appendix A of the Water Quality Criterion for the Protection of Human Health: Methylmercury.
When elemental mercury is spilled or a device containing mercury breaks, the exposed elemental mercury can evaporate and become an invisible, odorless toxic vapor. This is especially true in warm or poorly-ventilated rooms or spaces. Sources of potential exposure to elemental mercury are described below.
Elemental or metallic mercury is the liquid metal used in thermometers, barometers, and thermostats and other electrical switches. Metallic mercury is often found in school laboratories as well as in thermometers, barometers, switches, thermostats, and other devices found in school science labs.
It is not uncommon for children to break fever thermometers in their mouths. Mercury that is swallowed in such cases poses low risk comparison to the risk of breathing mercury vapor.
There are some necklaces imported from Mexico that contain a glass pendant that contains mercury. The mercury-containing pendants can come in various shapes such as hearts, bottles, balls, saber teeth, and chili peppers. If broken, they release metallic mercury to the environment.
Dental amalgam - Mercury is used in dentistry in dental amalgam. Dental amalgam is a direct filling material used in restoring teeth. It is made up of approximately 40-50% mercury, 25% silver and 25-35% a mixture of copper, zinc and tin. Amalgam use is declining because the incidence of dental decay is decreasing and because improved substitute materials are now available for certain applications.
Dental amalgams are considered medical devices and are regulated by U.S. Food and Drug Administration (FDA). Since the 1990s, FDA, the Centers for Disease Control and Prevention (CDC) and other government agencies have reviewed the scientific literature looking for links between dental amalgams and health problems. CDC reported in 2001 that there is little evidence that the health of the vast majority of people with dental amalgam is compromised, nor that removing amalgam fillings has a beneficial effect on health. In 2002, FDA published a proposed rule to classify dental amalgam as a class II medical device with special controls. On April 28, 2008, FDA reopened the comment period for that proposed rule. After reviewing all comments, FDA intends to issue a final rule classifying dental amalgam.
- June 2008 FDA consumer update
- September 2001 CDC information on dental amalgam use
- Best management practices for dental offices
Ritual Use of Mercury - Persons who use metallic mercury in ethnic folk medicine and for religious practices may be at risk of exposure to mercury. 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.
Inorganic mercury compounds take the form of mercury salts. They are generally white powders or crystals, with the exception of mercuric sulfide (cinnabar) which is red. Inorganic compounds and organic compounds (such as phenylmercury acetate and ethylmercury), have been commonly used as fungicides, antiseptics or disinfectants. They have also been used in a variety of products. Most of these uses have been discontinued, but small amounts of these compounds can still be found as preservatives in some medicines. The U.S. Food and Drug Administration maintains a list of medicines that contain mercury.
Excessive exposure to inorganic and organic mercury compounds can result from misuse or overuse of mercury-containing products, especially outdated products containing more mercury. Exposure to mercury compounds is primarily through ingestion, but can occur through other pathways. Ingested organic mercury compounds are more readily absorbed through the gastrointestinal tract than are inorganic compounds.