It was a momentous victory for area environmentalists when Illinois' largest coal-fired power company agreed to curb toxic emissions from all six of its plants, including two in Chicago.

Under a landmark deal with Gov. Rod Blagojevich in December 2006, the power company Midwest Generation pledged to cut mercury emissions 84 percent by 2009. That promise is well above federal standards, which call for cuts of 70 percent by 2018.

"It's pretty darn ambitious," said Brian Urbaszewski, director of environmental health problems at the Respiratory Health Association of Chicago.

But Chicagoans at risk for mercury pollution -- specifically children and women of child-bearing age who consume fish caught in area waters -- have more than Illinois coal plants to worry about.

In February, a Washington, D.C., appeals court struck down the federal law governing mercury emissions. The ruling means that coal plants across the border in Indiana, which has no state mercury law, can emit as much of the chemical as they want.

"The court decision surprised us," said Robert Elstro, air quality spokesman for the Indiana Department of Environmental Management. "We are concerned about how it will impact our work to protect Hoosiers and our environment."

The law that was struck down, known as the cap-and-trade mercury law, allowed each utility company to trade a specific number of mercury emission permits among plants. It was criticized by environmentalists and at least a dozen states, including Illinois, who sued, claiming it failed to prevent mercury hot spots.

Hot spots occur in cities located near coal plants that have either traded for, or purchased, more mercury permits. Because of the emissions that result when plants trade for more permits, people in those areas are at risk of being exposed to higher levels of the chemical.

Striking down the law, however, removed all federal-level restrictions on mercury emissions.

Mercury is a neurotoxin known to cause learning disabilities in children and neurological problems in adults who consume fish from contaminated waters.

Emission particles are known to travel as far as 50 miles through the air before settling in bodies of water. Once there, they can be absorbed into the muscle tissue of large fish that are caught and then eaten by humans. It is one reason some say a strong federal law is so important.

"It's really scandalous how much foot-dragging there has been about regulating mercury," said Raymond Pierrehumbert, professor of geophysics at the University of Chicago.

The State Line Generating plant, built on landfill in Indiana near where 103rd Street meets Lake Michigan, is so close to the state border that its billowing smoke casts shadows on Chicago's East Side residents.

According to an EPA database that tracks toxic chemical releases, State Line emitted more mercury in 2006 than two plants in Chicago.

Indiana ranks fourth nationwide in mercury emissions, most of which come from coal plants, according to the state's Sierra Club chapter. State Line had the fifth-highest rate of mercury emissions in the Midwest, according to a July 2007 report organized by a former federal EPA official.

Dominion Generation, the Richmond, Va.-based company that owns State Line, said it is waiting to see the new EPA rules before making changes. Meanwhile, company spokesman Dan Genest said plans to curb mercury emissions are up in the air.