Mercury is a silvery, liquid metal at room temperature, but like water, mercury can evaporate and become airborne. Because it is an element, mercury does not break down into less toxic substances.
Mercury enters the environment from:
- Natural sources such as volcanoes and the weathering of rocks
- Our intentional uses of mercury
- Our unintentional releases of mercury from burning fossil fuels and smelting metals
Mercury is emitted into the air from activities such as burning coal and producing taconite. A number of other possible sources of mercury exist, including cement plants and gasoline combustion. Mercury can also escape to the environment when items containing mercury are broken or thrown away.
Once mercury escapes to the environment, it circulates in and out of the atmosphere until it ends up in the bottoms of lakes and oceans. Depending on its chemical form, mercury may travel long distances before it falls to earth with precipitation or dust.
Eventually this mercury ends up in lakes or oceans, where it can accumulate in fish. Once in the lake or stream, bacteria and chemical reactions change the mercury into a much more toxic form known as methylmercury. Fish become contaminated with methylmercury by eating food (plankton and smaller fish), which has absorbed methylmercury. Read more about mercury contamination in fish.
As long as the fish continue to be exposed to mercury, mercury continually builds up in their flesh. Fish that eat other fish become even more highly contaminated. Thus, the fish most desirable for many anglers — bass, walleye and northern pike — become the most affected, and larger fish tend to be the most contaminated.
When people eat the contaminated fish, the methylmercury remains in their bodies for a long time. If they eat fish containing methylmercury faster than their bodies can get rid of it, the methylmercury accumulates in their bodies and can be toxic. Many states, including Minnesota, have fish consumption advisories to inform people about how many meals of fish they can safely eat over a period of time.
Where the mercury in northeastern Minnesota's lakes comes from