CHARLOTTE - A coalition of environmental groups urged state regulators Wednesday to re-examine the air quality permit granting Duke Energy permission to build a coal-fired power generator in Western North Carolina.

But if the state Division of Air Quality ignores their request, the groups will appeal the ruling and could take legal action to stop construction of the $2.4 billion unit, said John Suttles, attorney with the Southern Environmental Law Center.

Citing a recent court ruling, Suttles said the permit allows Duke to discharge too much mercury, a "public health threat."

Though opponents have until the end of the month to appeal the air quality permit, the coalition Wednesday filed a letter with the agency to give regulators time to "do the right thing," Suttles said. His group filed the letter on behalf of 19 environmental organizations, including the National Parks Conservation Association and Sierra Club.

But he warned, "If they ignore us, we will take action. The permit clearly violates the Clean Air Act. This is about protecting the public's health."

Duke is building an 800-megawatt unit at its Cliffside Steam Station, about 50 miles west of Charlotte. Work began in January, a day after the agency approved the final air quality permit. When the new unit opens in 2012, the Charlotte utility company will close four of its five older coal-fired units at Cliffside. The five plants now emit about 157 pounds of mercury a year.

Mercury is a powerful neurotoxin that can damage developing brains of fetuses and very young children.

The U.S. Court of Appeals for the District of Columbia Circuit last month found that the U.S. Environmental Protection Agency violated the Clean Air Act when it scrapped a policy that required utilities to install the best available technology to capture mercury.

Duke said it is using equipment that will reduce mercury by 90 percent at its new facility. But opponents argue that technology exists that can reduce emissions by up to 98 percent.

"The real injustice here is that Duke could easily adopt readily available and affordable technologies to significantly reduce their mercury emissions," said Scott Edwards, legal director for New York-based environmental group Waterkeeper Alliance. "No one should support Duke's willingness to poison North Carolina's waterways and citizens with mercury simply to save pennies on the dollar."

Division of Air Quality spokeswoman Diana Kees said her agency is reviewing the coalition's letter and the court ruling. No decision has been made to reopen the permit. "We're taking it under consideration," she said.

Duke Energy spokeswoman Marilyn Lineberger said that the letter was no surprise and that Duke would continue with construction. "The groups have always said their main objective was to try to delay the project," she said. "This plant is good for our customers and it's good for North Carolina." She also said the federal court ruling doesn't apply to the new unit.

Nonsense, Suttles said.

The court ruling makes it clear that under the Clean Air Act, utilities have to identify and use maximum pollution control devices available for mercury -- and that wasn't done with Cliffside, he said. He said the law extends to other pollutants that power plants discharge. If the permit is re-examined, Duke would have to identify those pollutants and prove it was using the best and cleanest technology, he said.