Mercury Pollution Background

Mercury pollution poses a serious threat to the environment and human health, even at low levels. Among other things, it can permanently affect the brain development of growing children and babies, especially if they are exposed in the womb.

Mercury pollution can come from many sources. The single biggest source is coal power plant emissions, although products containing mercury and some industrial processes can also contribute large amounts.

Mercury pollution can take many forms, but the most toxic is the methylmercury, which is created by microorganisms in or around water from inorganic mercury, like that from power plants.

For the latest guidelines and information on fish consumption advisories to avoid methylmercury exposure, see the Wisconsin Department of Health Services webpage “Eating Safe Fish.”

Key Points

  • One in 6 women of childbearing age may have mercury levels in their blood that is unsafe for a developing fetus.[1]
  • Coal-burning power plants are the largest source of mercury emissions to the air in the United States. The EPA estimates coal-fired power plants account for over 50 percent of all domestic human-caused mercury emissions.[2]
  • Over 234,000 pounds of mercury were present in products sold in the United States in 2004.[3]
Clean Wisconsin's Work

Clean Wisconsin works to reduce mercury pollution from all sources.  In the last few years, we have had significant victories in restricting mercury pollution from coal-fired power plants in the state with the passage of Wisconsin’s Mercury Rule in 2008. A year later in 2009, the state passed two bills restricting the sale of mercury-containing products and providing for recycling of electronics, many of which contain mercury.  All these achievements have significantly contributed to reducing mercury pollution in our state. Clean Wisconsin continues working to eliminate harmful mercury pollution and promote the health and safety of our lakes, our fish and our people.

Questions and Answers

What is mercury?

Mercury is a naturally occurring heavy metal. It is shown on the periodic table of elements as Hg and is in the same group as zinc and cadmium. It is not normally present in nature in its pure, liquid form, but it can be part of naturally occurring compounds with other elements. However, it exists in many forms (including its elemental form) in low levels in the air, water and soil. Human activities like burning coal are responsible for releasing large amounts of mercury in various forms and can lead to exposure levels that are toxic to humans and animals. In addition, mercury is used in many products, which can also lead to exposure.

In what forms does mercury occur?

Elemental, or metallic, mercury is a silvery heavy metal that is liquid and evaporates slowly at room temperature into a colorless, odorless gas. This is the form of mercury that was used, for example, in mercury thermometers, old thermostats and old batteries.

Besides this elemental form, mercury is also present in the environment in inorganic and organic forms. Inorganic mercury is a form of mercury where it combines with other elements in non-carbon-based compounds. These combinations, such as mercury salts, have been used in products like fungicides, antiseptics, disinfectants, pharmaceuticals and most commonly, florescent light bulbs. Organic mercury compounds are formed when elemental mercury is present in carbon-based molecules. The most common of these is the highly toxic methylmercury (MeHg), which consists of a single mercury molecule attached to a methyl group of one carbon and three hydrogen molecules. Methylmercury is formed from inorganic mercury that is converted by microorganisms in lakes and streams.[4][5]

Additionally, when mercury is first emitted from sources like power plants, it is generally either in its gaseous elemental form, bound in existing particles, or in an ionic or oxidized form that is highly reactive but has yet to combine to bind into longer-term compounds.

What are the sources of Mercury Pollution?

While low levels of mercury occur naturally, human activities since the Industrial Revolution have caused a significant increase in mercury levels traveling through the environment.[6][7]  In many places, this activity has brought mercury levels above those that are safe for human and animal life.

Today, the largest manmade source of mercury pollution is emissions from power plants. Coal-burning power plants account for 50% of manmade mercury pollution in the U.S.[2] (see our Mercury Emissions page). Another major source is the improper disposal of mercury-containing products such as light bulbs, thermometers, switches and electronics (see our Mercury in Products page). Other processes, such as the burning of medical and municipal waste, gold mining, chlorine production, laboratory use and leaching from landfills, also contribute to mercury pollution.[2][8]

How does Mercury Pollution contaminate the environment?

The main way that the environment is contaminated with mercury starts with power plants and similar sources emitting it into the air. This air pollution then contaminates the land and water through a process called deposition, which removes mercury from the air and brings it down to earth. The main form of this deposition is mercury being drawn out of the air by precipitation (rain or snow), which happens more rapidly with ionic mercury.[6]

Direct contamination of land and water can also occur through processes such as runoff, leaching from landfills or production and improper disposal of mercury-containing products.

What happens to mercury that is deposited in lakes and streams?

Mercury pollution from human sources has higher concentrations of ionic mercury, which is the only form of mercury that is both water-soluble and can form organic compounds.[7] This mercury, once it reaches lakes and streams, is taken up by microorganisms that convert it to methylmercury and incorporate it into their systems. Since it isn’t quickly or easily removed, this methylmercury bioaccumulates in individual organisms. Additionally, as it travels up the food chain, it can also biomagnify when being passed from one organism to another, increasing the concentration of mercury in the organism. As a result of this biomagnification, fish and seafood in many locations across the U.S. have such high concentrations of mercury that consumption is hazardous to the health of animals and humans alike.

What are bioaccumulation and biomagnification?

Bioaccumulation is when a substance like mercury enters an organism faster than it can leave, increasing the concentration of the substance in the organism. Methylmercury bioaccumulates in animals because it can be absorbed into fat, where it is not quickly removed from the body. Biomagnification happens when animals higher up on the food chain eat animals polluted with bioaccumulating substances.  When a predator eats many polluted animals that have accumulated the substance, animals higher on the food chain get larger and larger doses of it.

What does biomagnification look like in the real world?

As an example of biomagnification, start by imagining small amounts of methylmercury (MeHg) in a body of water. This MeHg is absorbed by tiny organisms like krill, which slowly accumulate it in their bodies. These krill are food for fish like salmon. While each krill may not contain much MeHg, a salmon will eat a lot of those krill, and the small amount of MeHg from each one will accumulate in the salmon.

Next, these fish become food for larger fish like tuna. When a tuna eats a lot of polluted smaller fish, and it will then accumulate all the MeHg those fish ate. When a still larger fish, like a shark, eats a lot of those tuna, they are exposed to and accumulate a large amount of MeHg from each one. When a person eats shark, they then get a large dose of MeHg, since it was in effect collected from a very, very large amount of contaminated krill.

What are the ecological effects of mercury pollution?

Those members of the ecosystem most harmed by mercury pollution are mammals and waterfowl that eat large quantities of contaminated fish.  Mercury can weaken bird and fish eggs, cause birth deformities, and cause behavioral changes that can affect hunting and survival abilities.[9] For example, studies have found negative effects of certain mercury exposure levels on common loon reproduction, hatching rates, immune function, behavior, oxidative stress and neural histology because of their fish-heavy diet.

How can I be exposed to toxic mercury?

The single most common source of hazardous exposure to mercury is through consumption of fish or other seafood that is contaminated with methylmercury. To minimize this exposure, we have fish consumption advisories, like those described on the Wisconsin Department of Health Service’s webpage, Eating Safe Fish.”

However, you may also be exposed to mercury as vapors in the air, for example, from broken fluorescent light bulbs or from contact with other mercury-containing products.  Additionally, some medical treatments (such as dental amalgam fillings) can also cause low levels of mercury exposure. Visit our Mercury in Products page for more information on these types of exposures.

What are the effects of mercury exposure on health?

Health effects of mercury exposure vary with the type of mercury and extent of the exposure. Most problematic is exposure of children to methylmercury, especially while in the womb.  These exposures can have significant developmental effects. Other symptoms from high levels of mercury exposure can include damage to various body systems. Severe exposure can also affect memory, attention and motor and visual spatial skills.[10] There is also indication of cardiovascular impacts, chromosomal damage, immunotoxic effects and reproductive and renal toxicity. Mercury is listed by the International Agency for Research on Cancer as a possible human carcinogen.[11]

How does mercury exposure effect child development?

Methylmercury is a neurotoxin that is particularly dangerous to developing fetuses and young children.  It has been shown to have significant impacts at very low levels on development, including neurodevelopment, such as impairment of memory functions and reduced performance on neurobehavioral and neuropsychological tests (e.g. tests of IQ). Unfortunately, biological processes result in accumulation of mercury in fetuses and breast milk at higher concentrations than those found in the mother.[12] This increases the potential for high levels of exposure to developing children.

What can I do to protect my health and the health of the planet?

Protect your health by following safe fish and seafood consumptions guidelines.[13][14] For the latest guidelines and information on fish consumption advisories to avoid methylmercury exposure, see the Wisconsin Department of Health Services webpage Eating Safe Fish.”

You should also choose alternatives to mercury-containing products when possible and properly dispose of products that do contain mercury. Cleaning up any spills that do occur in a safe and effective manner will help protect yourself and the environment.

Quick Facts
  • Coal-burning power plants are the largest source of mercury emissions to the air in the United States. The EPA estimates coal-fired power plants account for over 50% of all domestic, human-caused mercury emissions.[2]
  • An estimated 234,268 pounds of mercury were present in products sold in the United States in 2001.[3]
  • Mercury pollution can take many forms, but the most dangerous is methylmercury (MeHg, or CH3Hg) that has been converted by microbes in the water and on land to a form that is more easily absorbed by plants, animals and people.
  • Methylmercury exposure can have significant impacts at low levels, especially for children while in the womb. These impacts include impaired brain development, leading to lower IQ scores.[9]
  • It is estimated that one in 6 women of childbearing age may have mercury levels in their blood that are unsafe for a developing fetus,[1] leading to over 410,000 children born each year in the United States being exposed in the womb to methylmercury (MeHg) levels that are associated with impaired neurological development.[15]
  • Studies have linked elevated mercury in the blood or tissue of fish, birds and mammals with negative effects such as reduced reproductive success, hormonal changes and motor skill impairment.[15]
  • Because of biomagnification, the biggest source of human exposure to methylmercury is through eating polluted fish.
  • Fish consumption advisories for methylmercury now account for more than three quarters of all fish advisories in the United States, and Wisconsin has significantly more advisories than the next closest state.[9]
  • Minnesota researchers determined that mercury concentrations in fish may have increased by a factor of 10 over the last century, based on comparing modern fish to fish preserved in the 1930s.[16] Northern Wisconsin lakes have had a quadrupling of mercury since the late 19th century, when industry began.[7]
  • Other large sources are industrial boilers (about 7% of U.S. mercury emissions), burning hazardous waste (about 4%), and electric arc furnaces used in steel-making (also about 7%). Burning municipal waste and medical waste was once a larger source of emissions.[17]
  • While Wisconsin does not currently have any large iron mining operations or processing facilities, facilities that process iron are the second-largest source of mercury pollution in Minnesota.[18]
Wisconsin Data, Trends, and Legislation

Mercury pollution is a very important issue in Wisconsin. We produce most of our electricity through the combustion of coal, which releases mercury into the atmosphere. The bioaccumulation of this toxic mercury in ecosystems, especially aquatic systems, has adverse effects on human and animal life. This is especially problematic in Wisconsin where much of our recreation, tourism and diet depend on fishing.[7]

  • The EPA estimates that over 10,000 children born in Wisconsin every year are prenatally exposed to elevated levels of mercury, an exposure that puts them at risk of having lower IQs and reduced memory.[19]
  • It is not uncommon to find concentrations of 3 ng/L to 5 ng/L in Wisconsin lakes or rivers, significantly above the 1.3 ng/L water quality criterion.[20]
  • Wisconsin Department of Natural Resources and Department of Health now lists every inland body of water in the state under a fish consumption advisory as a result of mercury pollution.[21]
  • Sediment studies of the bottom of northern Wisconsin lakes showed a fourfold increase in mercury levels since the late 19th century, when industry first came to our state.[7]

Current Law

There is a collection of significant federal and state legislation related to mercury pollution both in the air and in products.  There even exists a Binational Toxics Strategy between the United States and Canada centered around eliminating toxic pollutants from the Great Lakes basin.[22]


Nationally mercury pollution in the air is regulated by the Clean Air Act.  Recently, the EPA finalized specific Mercury and Air Toxics Standards (MATS) within the Clean Air Act that regulate mercury pollution emitted from coal and oil-fired power plants. For more information on these standards see our Mercury Emissions page.  Another significant federal legislation is the National Emissions Standards for Hazardous air Pollutants (NESHAP).


The Administrative Code, NR 445 Control of Hazardous Pollutants, is Wisconsin’s Air Toxics Rule. This rule originated in 1988, was revised in 2004, and republished in August 2008. The general purpose of NR 445 is to regulate emissions of hazardous air pollutants (HAPs) by stationary sources so that they do not harm human health or plant and animal life.[23][24]

Other smaller or more specific legislation in Wisconsin deal with bans on mercury in schools, disposal of electronic waste that may contain mercury or other toxins, and more.[25] [26] However, Wisconsin is still lagging behind a number of other states with broader-reaching “extended producer responsibility” legislation, which requires manufacturers to pay for the safe recycling or disposal of mercury-containing products.[27]

Cited Resources
  1. K Mahaffey, R Cliffner, and C Bodurow (2004). “Blood Organic Mercury and Dietary Mercury Intake: National Health and Nutrition Examination Survey, 1999 and 2000.” Environmental Health Perspectives, 112(5): 562-570, April 2004
  2. Basic Information on Mercury (United States Environmental Protection Agency)
  3. Trends in Mercury Use in Products (Northeast Waste Management Officials Association, June 2008)
  4. Mercury in Our Environment (Wisconsin Department of Health Services)
  5. Frequently Asked Questions – What is Mercury? (United States Environmental Protection Agency) 
  6. Chapter 2 – Mercury: Forms, Fate & Effects (Massachusetts Department of Environmental Protection)
  7. Troubled Waters: Mercury in Wisconsin’s Lakes and Fish (Wisconsin’s Environmental Decade)
  8. Mercury in the Environment and Water Supply (University of Wisconsin Eau-Claire)
  9. Mercury in The Environment Fact Sheet 146-00 (United States Geological Survey, October 2000)
  10. Mercury Health Effects (United States Environmental Protection Agency)
  11. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Volume 58 (World Health Organization, International Agency for Research on Cancer)
  12. How Heavy Metals Affect Neurotransmitters Production and Balance (Manuela Malaguti Boyle and Geoff Beaty, September 2010)
  13. Eating Your Catch – Making Healthy Choices (Wisconsin Department of Natural Resources)
  14. Eating Safe Fish (Wisconsin Department of Health Services)
  15. Mercury Contamination in Forest and Freshwater Ecosystems in the Northern United States (Driscoll et al. 2007)
  16. Dirty Energy’s Assault on Our Health: Mercury (Environment America, January 2011)
  17. Frequently Asked Questions – biggest sources in the U.S. (United States Environmental Protection Agency)
  18. Estimated Mercury Emissions in Minnesota for 2005-2018 (Minnesota Pollution Control Agency)
  19. Regulatory Impact Analysis of the Proposed Toxics Rule (United States Environmental Protection Agency March 2011)
  20. Wisconsin’s Mercury Strategy and Monitoring (Wisconsin Department of Natural Resources, April 2012)
  21. Wisconsin Department of Natural Resources (nd). “Mercury Rule Media Kit.” Web. Retrieved 2013 from
  22. Great Lakes Binational Toxics Strategy (United States Environmental Protection Agency)
  23. Wisconsin Air Toxics Rule (NR 445) Air Program Fact Sheet (September 2012)
  24. Note: Wisc. NR 445.03. General limitations. No person may cause, allow or permit emissions into the ambient air of any hazardous substance in a quantity or concentration or for a duration that is injurious to human health, plant or animal life unless the purpose of that emission is for the control of plant or animal life. Hazardous substances include but are not limited to the hazardous air contaminants listed in Tables A to C of s. NR 445.07.
  25. 2009 Senate Bill 107, 2009 Wisconsin Act 50 (State of Wisconsin, October 2009)
  26. 2009 Wisconsin Act 44 (State of Wisconsin, October 2009)
  27. Extended Producer Responsibility State Laws as of January 2013 (Product Stewardship Institute)


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Please cite this resource as: Clean Wisconsin, Inc. “Mercury Pollution Background.” Clean Wisconsin Enviropedia. Retrieved from