Of the 50 "dirtiest" power plants with the highest carbon dioxide emissions in the country, all are coal-fired. Texas accounts for five on the list, and Indiana and Pennsylvania each have four, the Environmental Integrity Project annual study found.

US power plant CO2 emissions actually fell 2 percent in 2006 from 2005, but the report focused on the fact that a new wave of coal-fired plants -- about 150 nationwide -- could increase CO2 pollution by 34 percent by 2030, the study said.

"The power industry is racing to build more coal-fired power plants," said the report's principal author, Ilan Levin, an attorney for EIP.

"Once utility companies secure their air pollution permits, we can expect them to argue that these new plants should be 'grandfathered,' or exempt from any pending limits on greenhouse gases."

Coal-burning power plants make half the electricity used in the United States. And the United States in the latest United Nations report -- for 2003 -- was at 23 percent the top national producer of CO2 emissions, with China second at 16.5 percent.

Energy company advocate Scott Segal of law firm Bricewell & Giuliani LLP said the EIP study is flawed.

"This is a summer ritual for the environmentalists," Segal said by telephone. "They release essentially the same report that tries to draw conclusions from generic monitoring data about specific plants.

"The real record for the power sector, as well as manufacturing, is that there have been significant gains in air quality."

The EIP study was based on data from the US Environmental Protection Agency and the US Department of Energy.

States with three plants on the list were Alabama, Georgia, North Carolina, Ohio and West Virginia, while Florida, Kentucky, New Mexico and Wyoming each had two plants on the Top 50 list.

New England and the Pacific Coast do not have any power plants on among the Top 50 list for CO2 polluters. But coal-fired plants in New Mexico, Arizona and Utah that serve California are among the top polluters, the study found.

The EIP study also ranked the top producers of three other pollutants - sulfur dioxide, nitrogen oxide, and mercury.

The study found that sulfur dioxide emissions are improving due to federal regulations, nitrogen oxide emissions fell in 2006 and are expected to continue falling, and while mercury emissions were steady, they are expected to fall mainly as a "co-benefit" of sulfur dioxide controls.

The EIP study said US power plants make 40 percent of CO2 emissions, about two-thirds of sulfur dioxide emissions, 22 percent of nitrogen oxides emissions, and roughly a third of all mercury emissions.