CHARLOTTE - State regulators, responding to public opposition, say they will re-evaluate how much toxic mercury a proposed Duke Energy power plant expansion may emit.

The utility needs only an air permit before beginning work on a $1.8 billion addition to its Cliffside plant near Charlotte in Rutherford County.

The state Division of Air Quality had agreed with Duke that an analysis of "best available" mercury controls isn't needed because the plant's design meets federal standards.

Critics said that decision could mean the plant won't be as clean as technology can make it.

The air agency, which would issue the plant's permit, says it will now ask Duke to conduct a more in-depth analysis.

Coal-fired power plants are increasingly reviled because they are leading sources of carbon dioxide, the greenhouse gas linked to global warming. The expanded Cliffside is expected to more than double its emissions to 9.7 million tons a year when it goes online in 2012.

Coal plants have also come under scrutiny because they emit 40 percent of the mercury released into the U.S. environment. A highly toxic form of mercury has tainted fish across Eastern North Carolina. When pregnant women eat these fish, it can permanently impair their babies.

The question at Cliffside is whether the pollution controls Duke says it will install on its new boiler are the best available.

Cliffside released 140 pounds of mercury in 2005, according to state records. Duke says it expects the updated plant to cut that amount by about half, but it has also reported a worst-case estimate of 294 pounds a year.

The air agency will take a second look, said state environmental engineer Ed Martin, because of public comments and the state's mercury-control rules. The rules went into effect in January, about a year after Duke applied for its permit.

"We probably should have done a little better job of looking at that the first time," Martin said.

Duke says it expects total mercury releases from the modernized plant -- once four old boilers have been mothballed -- of 70 to 80 pounds a year. A much higher estimate that it made to the state, the company says, was based on the unlikely prospect that the plant would burn a type of coal mined in the West.

Two other Duke coal-fired plants in Gaston County released a total of 315 pounds of mercury in 2005.

Beth Henry, a retired Charlotte lawyer who has worked against the plant expansion because of her concerns about climate change, noted that mercury is only one of the toxic pollutants that come from coal plants.

About two dozen other toxic substances, in amounts from a few pounds to tons, will also come from the new Cliffside boiler, Duke's filings show. The amount of many of those pollutants may grow as the plant nearly doubles its generating capacity.

"I'm glad they're working on mercury," Henry said, "but honestly, that's just the tip of the iceberg. I am concerned about huge potential increases in numerous other heavy metals, many of them toxic, many of them known carcinogens."

The new Cliffside boiler will feature technology that is designed to catch pollutants that form ozone, haze and fine particles, but it is also expected to reduce mercury by 90 percent, Duke says.

Because of that technology, the state air agency had agreed with Duke that a separate analysis of controls for mercury wasn't needed under North Carolina law.

That was a mistake, said lawyer Gudrun Thompson of the Southern Environmental Law Center. If the analysis had been done, she contends, the state would have ordered Duke to further reduce Cliffside's mercury emissions.

Mercury, a natural element found in coal, is released into the air by coal-burning power plants. When mercury falls from the atmosphere into water, it can assume a highly toxic form that accumulates in fish.

Eating too much tainted fish can hurt the central nervous system and leave babies with permanent learning problems.

State health officials estimate that at least 13,000 North Carolina infants a year have elevated levels of methylmercury, the toxic form.

North Carolina advises pregnant women and children under 15 to avoid eating four freshwater fish species caught anywhere east of Interstate 85 (Eastern North Carolina waters more readily form methylmercury) and largemouth bass caught anywhere in the state.