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Frequently Asked Questions about Mercury and Thimerosal

What is mercury?
Mercury occurs naturally in the environment and exists in several forms. These forms can be organized under three headings: metallic mercury (also known as elemental mercury), inorganic mercury, and organic mercury.

Metallic mercury is a shiny, silver-white metal that is a liquid at room temperature. Metallic mercury is the elemental or pure form of mercury (i.e., it is not combined with other elements). Metallic mercury metal is the familiar liquid metal used in thermometers and some electrical switches. At room temperature, some of the metallic mercury will evaporate and form mercury vapors. Mercury vapors are colorless and odorless. The higher the temperature, the more vapors will be released from liquid metallic mercury. Some people who have breathed mercury vapors report a metallic taste in their mouths. Metallic mercury has been found at 714 hazardous waste sites nationwide.

Inorganic mercury compounds occur when mercury combines with elements such as chlorine, sulfur, or oxygen. These mercury compounds are also called mercury salts. Most inorganic mercury compounds are white powders or crystals, except for mercuric sulfide (also known as cinnabar) which is red and turns black after exposure to light.

When mercury combines with carbon, the compounds formed are called "organic" mercury compounds or organomercurials. By far the most common organic mercury compound in the environment is methylmercury. Methylmercury is of particular concern because it can build up in certain edible freshwater and saltwater fish and marine mammals to levels that are many times greater than levels in the surrounding water. Thimerosal is an organic mercury compound that is metabolized to ethylmercury and thiosalicylate (Institute of Medicine, 2001).

For more information, visit the Agency for Toxic Substances and Disease Registry's Public Health Statement for Mercury.

How are recommended limits for mercury exposure established?
Several federal agencies, including the Agency for Toxic Substances and Disease Registries (ATSDR), the Food and Drug Administration (FDA), and the Environmental Protection Agency (EPA), have established guidelines for levels of mercury exposure that are thought to be safe. Federal safety standards for mercury are based on research performed on methylmercury. More information is available about methylmercury than about a related form called ethylmercury because methylmercury binds to tissue more easily than ethylmercury, and remains there a longer time. Methylmercury is also believed to be more toxic than ethylmercury (Magos, 2001) and is the form of mercury of greatest public health concern (Mahaffey, 1999).

Recommended limits on methylmercury exposure are not “set lines” below which there is safety and above which adverse health effects will occur immediately (Mahaffey, 1999). A significant safety margin is incorporated into all acceptable mercury exposure limits. Such guidelines are meant to be starting points for evaluation of mercury exposure, and should not be viewed as absolute levels above which harm can be expected to occur.

What happens if your exposure exceeds the recommended levels?
The nervous system is very sensitive to all forms of mercury. Methylmercury and metal vapors are often more harmful than other forms, because more mercury in these forms reaches the brain. Exposure to high levels of metallic, inorganic, or organic mercury can permanently damage the brain, kidneys, and developing fetus. Effects on brain functioning may include irritability, shyness, tremors, changes in vision or hearing, attention, language, and memory problems. Effects of short-term exposure to high levels of metallic mercury vapors may include lung damage, nausea, vomiting, diarrhea, increases in blood pressure or heart rate, skin rashes, and eye irritation.

It is important to keep in mind that being exposed to more than the recommended mercury limits does not necessarily mean you will experience adverse health effects. A significant safety margin is incorporated into all acceptable mercury exposure limits; they should not be viewed as absolute levels above which harm can be expected to occur.

Who is most vulnerable to mercury?
Unborn babies (developing fetus) are more sensitive to the effects of many chemicals, including mercury. Premature babies are also more vulnerable because they tend to be very small and their brain is not as developed as that of a full-term baby. Children may be at higher risk of mercury exposure than are adults because they eat more per pound of body weight and because they may be inherently more sensitive than adults, since their nervous systems are still developing. Thus, the FDA and EPA advise women who are pregnant or may become pregnant, nursing mothers, and young children to avoid some types of fish and eat fish and shellfish that are lower in mercury. For more information, visit EPA's What You Need to Know about Mercury in Fish and Shellfish.

How can mercury affect children?
Very young children are more sensitive to mercury than adults. Mercury in the mother's body passes to the fetus and can pass to a nursing infant through breast milk. However, the benefits of breastfeeding may be greater than the possible adverse effects of mercury in breast milk.

If a pregnant woman ingests mercury at high levels, harmful effects that may be passed from the mother to the developing fetus include brain damage, mental retardation, lack of coordination, blindness, seizures, and an inability to speak. Children poisoned by mercury may develop nervous and digestive system problems and kidney damage.

What is thimerosal?
Thimerosal is a very effective preservative that has been used in some vaccines and other products since the 1930s. Thimerosal contains approximately 49% ethylmercury. There is no convincing scientific evidence of harm caused by the low doses of thimerosal in vaccines, except for minor reactions like redness and swelling at the injection site. However, in July 1999 the Public Health Service agencies, the American Academy of Pediatrics, and vaccine manufacturers agreed that thimerosal should be reduced or eliminated in vaccines as a precautionary measure.

Today, according to the U.S. Food and Drug Administration, the only vaccines routinely recommended for children 6 years of age and younger that contain thimerosal are: one vaccine for DTaP, and three vaccines for influenza (flu). These four vaccines contain only trace amounts of thimerosal.

Is the mercury in thimerosal the same as the kind found in certain fish?
No. Thimerosal contains a form of mercury called ethylmercury. In contrast, methylmercury is an environmental contaminant. In March 2004, the FDA and the EPA released a joint advisory for the consumption of fish for women who may become pregnant, pregnant women, nursing mothers, and young children. The advisory is based on an EPA standard of 0.1 mcg/kg per day of methylmercury, which is not a component of thimerosal.


Please see References for a list of data sources for this information.

Page last reviewed: September 24, 2008
Page last updated: October 22, 2007
Content source: Immunization Safety Office, Office of the Chief Science Officer
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