It's becoming increasingly harder for the poor to breathe due to nearby coal plants, a study released this week states.

Low-income communities are disproportionately affected by health-threatening pollution from coal-fired power plants in Illinois and other Midwestern states, a report by the NAACP says.

People living within 3 miles of a coal plant are more likely to inhale pollutants that cause respiratory problems such as asthma, researchers said. They also said people living within 3 miles of a coal plant are disproportionately low-income and minorities.

"It's important not just to be shutting down coal plants but to be shutting down coal plants that impact low-income communities and communities with people of color," said Adrian Wilson, the report's lead researcher and a doctoral student at the University of Massachusetts at Amherst.

"All too often the burden of this health threatening pollution falls disproportionately on people who live in poorer communities," said Jack Darin, director of the Sierra Club, Illinois chapter, who wasn't involved in the study.

The NAACP -- along with the Indigenous Environmental Network and Little Village Environmental Justice Organization -- points the finger at several of the largest energy companies, including Edison International, Dominion and PSEG, as the worst offenders of pollution that causes health problems.

"For our people, this is a life-or-death issue. This is environmental racism," said Tom Goldtooth, executive director of the Indigenous Environmental Network.

The report, entitled "Coal Blooded: Putting Profits Before People," assigned letter grades to more than 350 coal-fired power plants in the United States. Researchers ranked the plants according to how they affected low-income communities and people of color by examining sulfur dioxide and mono-nitrogen oxide emission levels from the Environmental Protection Agency's Clean Air Markets Program from 2007-10.

It also calls the idea of "clean coal," which Republican presidential candidate Mitt Romney raised during debates in October, into question. "Clean coal" is coal that has been processed to remove carbon dioxide.

In the report, however, researchers write that "clean coal" doesn't "address the continued effects of SO2, NOx, mercury and other pollutants on the local communities where coal plants are located."

Researchers deemed 75 plants as "failing" by Environmental Justice Standards, claiming the plants produced 8 percent of electricity in the United States in 2005 but accounted for 14 percent of SO2 and 13 percent of NOx emissions from U.S. power plants.

Within 3 miles of these "failing plants" live 4 million people, with an average per capita income of $17,500. More than half -- 53 percent -- of those are people of color.

Indiana and Michigan both have five "failing plants," and Wisconsin has three. The most, nine of the 75, are in Illinois. The two worst on the scorecard were the Crawford and Fisk Generating Stations in Chicago.

Naming Crawford and Fisk, however, casts doubt on the validity of the report, which some power companies called "outdated."

"We closed Crawford and Fisk in September in a collaborative process between the mayor [Rahm Emanuel], us and leading organizations in the city," said Susan Olavarria, director of communications and governmental affairs at Midwest Generation, an Edison subsidiary.

Olavarria went on to defend Midwest Generation's record, saying it was among the first in the nation to set mercury emissions standards.

Members of the coal industry contend that the plants create economic opportunities in low-income areas.

Lisa Camooso Miller of the American Coalition for Clean Coal Electricity said: "Shutting down coal plants would increase energy costs and destroy jobs and this would be especially devastating to family budgets. We know that energy costs have skyrocketed in recent years and have disproportionately hurt black and Hispanic households."

(Additional reporting by David Tonyan, Medill News Service.)