CHICAGO - Those raindrops falling on your head are polluting Midwest lakes and rivers with mercury, several environmental groups said Monday.

Mercury is showing up in Chicago rainfall at levels 42 times greater than is considered safe by federal standards, according to Andrew Buchsbaum of the National Wildlife Federation. Mercury levels in rain are even higher in Detroit and Duluth, Minn., he said.

The group has no comparable data for Ohio but would like to begin gathering it, said Michael Murray, a staff scientist with the federation in Ann Arbor, Mich. He said past studies indicate that more than 16,000 pounds of mercury are released into Ohio's air each year, mostly from coal-fired power plants and incinerators.

The data was collected from government and university studies with the help of the U.S. Environmental Protection Agency, Buchsbaum said.

Most of the mercury in rain is coming from coal-fired electric plants and municipal and medical waste incinerators, he said.

"Unfortunately, the largest contributor to the problem, the electric utility industry, continues to get a free ride on its mercury pollution," said Peter Morman of the Environmental Law and Policy Center. "While other sources are reducing emissions, no such requirements exist for coal-fired power plants."

Morman said mercury pollution by Midwestern utilities likely will increase because deregulation will prompt them to generate higher levels of electricity.

Scott Miller, Commonwealth Edison's air quality engineer, said ComEd is cooperating with the EPA studies on mercury emissions. He said the utility has no plans to convert its coal plants to gas.

A naturally occurring metal, mercury accumulates in fish and becomes more concentrated as it moves up the food chain. In humans, the toxin can slow fetal and child development and cause brain damage.

Buchsbaum said data collected by the University of Michigan Air Quality Laboratory found that rain falling on Chicago's South Side had mercury levels ranging from 5.4 parts per trillion to 74.5 parts per trillion.

The EPA considers mercury levels in the Great Lakes to be safe for wildlife at 1.3 parts per trillion. For humans it is 1.8 parts per trillion.

The Agency for Toxic Substance and Disease Registry in the Department of Health and Human Services says that based on the latest studies, people can consume as much as 0.3 micrograms of mercury per kilogram of their body weight without health risks. Its previous standard was 0.1 microgram.