Investors showed immediate enthusiasm for Pittsburgh-based Calgon Carbon Corp., an industry leader in mercury-removal technology. Stocks were up the day of the ruling and continued to rise in the following days.

"We think that long-term it is going to be quite positive for us," said Calgon spokesperson Gail Gerono.

In its Feb. 8 ruling, the U.S. Court of Appeals for the District of Columbia Circuit struck down the Environmental Protection Agency's Clean Air Mercury Rule (CAMR).

The court agreed with opponents that the EPA, in adopting the CAMR for the control of mercury, had violated the Clean Air Act.

Mercury is a neurotoxin, a powerful poison that causes nerve and brain damage. It enters waterways and accumulates in fish and is especially harmful to children.

The 2005 CAMR aimed to cap U.S. power plant mercury emissions, which stand at about 48 tons a year. It would have reduced them by about 20 percent by 2010 and 70 percent by 2018.

Environmental groups and 17 states sued the agency, arguing that the Clean Air Act requires cuts as steep as 90 percent because existing mercury-removal technology can achieve those levels.

Last week's court action leaves the nation with no regulatory control over power plant mercury emissions while the EPA establishes a new rule based on available technology.

"It's sort of ironic," said American Electric Power spokesman Pat D. Hemlepp. "The environmentalists' challenge actually eliminates some of the reductions that would have taken place by 2010."

Some reductions will take place even without regulation.

"The majority of the mercury emissions reductions that we would achieve by 2010 will happen even without that rule (as) a co-benefit of other equipment that we are installing for sulfur dioxide and nitrogen oxides control," Hemlepp explained.

But, he added, "We would have had to also put some activated carbon systems in place on some plants to do some mercury capture--and those are the systems that we will likely end up delaying until we get some clarity on what's going to be required."

Activated carbon, the current industry standard for mercury removal, is injected in powdered form into the flue of a coal-fired plant.

"The activated carbon comes into contact with the mercury and (bonds to it by chemical attraction)," Calgon's Gerono explained. "The activated carbon that holds the mercury is taken to a landfill."

The technology can remove 90 percent of the mercury from the flue stream, she said.