Cool, Wet Summer in Illinois, Illinois State Water Survey

ISWS Press Release

For Immediate Release March 13, 2008
Mercury Levels in Illinois Rain and Snowfall Remain Unchanged
David Gay - (217) 244-0462,
Lisa Sheppard - (217) 244-7270,

Every state except for Alaska and Wyoming has issued an advisory warning pregnant women and children to limit fish consumption due to highly toxic methyl mercury that builds up in fish tissue. Data from the Mercury Deposition Network (MDN), which monitors mercury concentration and deposition in rain and snowfall, indicates that generally mercury levels in the atmosphere are falling, but not by much.

Precipitation deposits mercury from the atmosphere to Illinois waters, although mercury occurs naturally in Illinois soils, according to David Gay, Acting Coordinator of the National Atmospheric Deposition Program, which coordinates the MDN at the Illinois State Water Survey in Champaign, IL.

Illinois rainfall averages 10 nanograms of mercury per liter of precipitation, which is about average compared with other states. Peak concentrations occur in summer. Nationwide, the highest deposition of mercury from the atmosphere occurs along the Gulf of Mexico and in Florida.

Mercury is converted to methyl mercury in water and sediments, and accumulates from small organisms up the food chain to large predator fish species.

When humans consume mercury, it takes about 70 days for half of the mercury that was ingested to be removed. It is particularly dangerous for unborn and young children. Large doses of methyl mercury could cause damage to the heart, kidneys, and the central nervous system.

Mercury is emitted to the atmosphere and surface water from many natural sources, including volcanoes and wildfires. Man-made sources include coal combustion, medical waste incineration, and chlorine production, among others.

Mercury concentrations found in Illinois waters and in fish may not have initiated from Illinois industry.

"It's not necessarily a local problem," said Gay. "Mercury stays in the atmosphere for six months to two years. You can't say that the mercury came from a particular smoke stack. It could just as easily have come out of China."

Mercury concentrations in precipitation are measured weekly at 100 sites throughout the U.S., including six sites in Canada, and reported to the MDN.

Measurement data are available on the MDN web site,

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