Mercury in the environment
Mercury is a naturally occurring element found in oceans, mineral rocks, fossil fuels and soil. It can become airborne and redistributed throughout the environment when minerals are processed, fossil fuels are burned, rocks erode, volcanoes erupt and by sun and rain acting on soils. Mercury has been extracted from minerals and used by humans since Roman times.
Mercury is used in small amounts in household and commercial products, such as fluorescent lights, thermostats and thermometers, as well as in industrial processes. As a result, some manufacturing plants, hospitals, dental offices, schools and even homes inadvertently release mercury. In addition, when mercury-containing products are disposed, mercury can be released by incinerators that burn these discarded items.
Trace amounts of mercury are present in coal and oil. Consequently, when these fuels are used to generate electricity, some of this mercury is released into the air. Power plant emissions account for about one-third of the mercury emitted into the air by all sources in the U.S., but according to U.S. Environmental Protection Agency (EPA), accounts for only about 1 percent of the mercury emitted by global human activities. Researchers are trying to determine how much mercury from power plants actually enters aquatic environments, as well as how much of it actually enters the aquatic food chain. This is difficult because mercury has so many sources.
Mercury levels in ecosystems
Mercury levels in Midwest lakes, including Lake Michigan and Lake Superior, appear to be declining because mercury use is decreasing. For example, since 1980, We Energies has voluntarily reduced the use of mercury-containing equipment in power plants by about 10,000 pounds. This decline continues despite increasing coal use during the past several decades.
- Mercury in the air and water
Like many pollutants, mercury and certain of its many compounds can be toxic to humans at certain levels. Mercury is present in air and water but at concentrations not considered a health threat. When certain forms of airborne mercury settle in aquatic systems, mercury can be converted into a form (methyl-mercury) that accumulates in fish, which then can be absorbed by humans who eat these fish.
- Mercury in fish
Health experts are trying to determine a safe level for mercury consumption given that mercury and its toxic forms are naturally occurring substances, and that mercury, in varying amounts is present in all fish. Further, even people who consume large amounts of certain marine fish or shellfish may not be at an increased risk of health effects according to studies recently completed by U.S. EPA. In addition, a study of more than 700 mother/child pairs conducted in the Seychelles Islands (a group of islands in the western Indian Ocean) showed no health effects even though this population eats ocean fish nearly every day.
Routine mercury exposure via fish consumption among the general population in North America is considered to be of little health concern by U.S. EPA. Humans, other mammals and birds have evolved defense mechanisms to rid the body of small amounts of mercury. However, too much mercury can lead to many health problems, including a form of palsy, which can develop in an infant fetus when the mother has been exposed to very high mercury levels. Mercury poisoning can be fatal in extreme cases of very high exposure. The severe instances of mercury poisoning that are frequently cited have occurred when individuals have been directly exposed to high levels of mercury as a result of consuming contaminated grains (mercury has been used as a fungicide) or fish from waters where large discharges (tons) of mercury compounds have occurred.
- Protection measures
State and federal agencies' fish advisories provide guidance for fish and seafood consumption, particularly for pregnant women. The U.S. EPA and the U.S. Food and Drug Administration believe that eating one to four meals per week of most commercial fish is not likely to pose risks of adverse health effects for any individuals, including pregnant women and their young children.
Mercury controls for power plants
- Power plant emissions
In the 2000s, measurements at more than 100 U.S. coal-fueled power plants suggested that control devices for particulates, nitrogen and sulfur oxide emissions captured approximately 40 percent of the mercury present in coal. However, the amount captured at any one plant varied from less than 10 percent to greater than 90 percent. This substantial variation in mercury capture was due to the type of coal being burned and the type of emission controls installed at the plant. We now know that the type of coal being burned influences the chemical form of mercury released when the coal is combusted and this chemical form dictates capture by existing emission control equipment. For our plants, the approximate amounts currently being captured vary between 4 percent and 90 percent, with our coal fleet average being approximately 60 percent.
- Controlling emissions
We are actively involved in efforts to advance mercury control technologies for power plants and to develop workable state and federal mercury control rules. We conducted several research projects at our Pleasant Prairie Power Plant near Kenosha, Wis., and completed a major mercury control technology demonstration project at our Presque Isle Power Plant in Marquette, Mich. The work at both project sites provides valuable information on mercury controls for the entire electric power industry.
As part of our integrated air quality strategy, we have adopted solutions that combine various control technologies to achieve reductions for multiple emissions, including sulfur dioxide, nitrogen oxides and mercury. In addition to projects at existing plants, mercury emissions from the new units in Oak Creek, Wis., control mercury by more than 90 percent.