Mercury is a naturally occurring metal that exists in many forms. Forms of mercury can be organized into three groups: metallic mercury aka elemental mercury, inorganic mercury and organic mercury.

Metallic mercury is a shiny, silvery-white metal commonly seen in thermometers. It is liquid at room temperature; as temperatures increase, however, some metallic mercury will evaporate into mercury vapors. Other uses of metallic mercury include production of chlorine gas and caustic soda. It is also used in dental fillings and batteries.

Inorganic mercury compounds are formed when mercury combines with other elements, such as chlorine, sulfur or oxygen and are often called mercury salts. Mercury salts are sometimes used in skin lightening creams and antiseptic creams and ointments. Inorganic mercury is also used in some fungicides.

Organic mercury combines with carbon and forms organic mercury. There are number of organic mercury compounds. The most common in the environment, however, is methylmercury. Methylmercury is of particular concern because it is produced by microorganisms in water and soil and can build up in certain types of fish and accumulate up the food chain. Plants have very low levels of mercury, even if they grow in soil contaminated with mercury. Mushrooms, however, can accumulate high levels if grown in soil contaminated with mercury.


Most of the mercury found in the environment is in the form of metallic mercury and inorganic mercury compounds. Metallic and inorganic mercury enters the air from mining deposits of ores that contain mercury, from the emissions of coal-fired power plants, from burning municipal and medical waste, from the production of cement, and from uncontrolled releases in factories that use mercury. People are also exposed to mercury by consumption of fish or shellfish contaminated with methylmercury, release of mercury from dental work and medical treatments, and practicing rituals that include mercury.

Thimerosal in Vaccines

Thimerosal is a mercury-based preservative that has been used for decades in the United States in multi-dose vials (vials containing more than one dose) of medicines and vaccines. There is no 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.

Previous research reports that thimerosal was linked to autism and similar neurological developmental conditions has been shown to be criminally negligent and of no scientific merit.

Federal Guidelines

Biomonitoring Screening Values

Blood, urine and hair tests can be used to measure mercury levels in your body.

The 4CSBC uses the National Health and Nutrition Examination Survey (NHANES) 95th percentile level as a screening threshold. Only five percent of the NHANES participants (representing the American population) have levels that exceed the 95th percentile level. Everyone else is below that level. Thus, that level is considered elevated. It may or may not be harmful. The 95th percentile screening threshold for mercury is 2.0 μg/L

Health Effects

Exposure to mercury is very dangerous. The nervous system is sensitive to all forms of mercury, especially methylmercury and metallic mercury. Exposure to all three forms of mercury can cause permanent brain damage, kidney damage and harm a developing fetus. Further effects on the brain include irritability, shyness, tremors, changes in vision or hearing and memory problems. Short-term exposure to mercury can cause lung damage, nausea, vomiting, diarrhea, increase in blood pressure or heart rate, skin rashes and eye irritation.

Mercury exposure is particularly dangerous for children and the developing fetus. Children are more sensitive to mercury than adults and it can be passed during breast feeding from mother to baby. Mercury can also pass from mother to fetus. Possible health effects on the child or fetus include mental retardation, incoordination, blindness, seizures and inability to speak. Pregnant women should avoid areas where liquid mercury has been used, and avoid eating fish that contain high levels of methylmercury, including swordfish, tilefish, king mackerel, and shark.

Further research is necessary to collect human cancer data for all forms of mercury. The EPA, however, determined that mercuric chloride and methylmercury are possible human carcinogens.

Sources of additional information

Agency for Toxic Substances and Disease Registry (ATSDR)

Centers for Disease Control and Prevention (CDC)

U.S. Environmental Protection Agency (EPA)

Last Update: May 9, 2016