"The closer you get to the plant, the higher the levels of the mercury," said Suzanne Wisdom, who lives near the plant and works for the environmental group Oceana, which organizes protests against Olin.

Beyond chlorine's well-known use as a disinfectant, it is also a key ingredient for hundreds of products - from plastic to bulletproof vests to pharmaceuticals and vinyl.

The Olin plant in Tennessee is one of a handful in the nation still yoked to the century-old method that uses mercury to make chlorine and alkali.

These older plants use tons of mercury. In 2003, chlorine plants bought about 450,000 pounds of it. In 2006, as the number of chlor-alkali plants declined, mercury purchases fell to 58,000 pounds.

They also lose tons of the hazardous element each year. The industry and the EPA say the element never reaches the environment because it disappears in plant equipment.

Still, when these plants were more common, the EPA studied them and said the average plant could not account for tons of mercury each year. In 2003, the EPA said in a public notice: "The fate of all the mercury consumed at mercury cell chlor-alkali plants remains somewhat of an enigma."

Olin is a major benefactor and employer in Charleston and in Cleveland, the county seat. Gary Farlow, who runs the local chamber of commerce, said Olin is well-respected in the community. "They're a very good corporate citizen," he said.

In the area this spring, however, four local brothers circulated a petition at their public high school supporting a bill sponsored by then-U.S. Sen. Barack Obama. Noting that chlorine producers could not account for more than 130 tons of mercury between 2000 and 2004, Obama unsuccessfully sought to ban mercury use in chlorine plants by 2012.

"The Senate bill might eventually pass, but it'll be too late to prevent a lot of damage," said Dillon Yost, 16, who pushed the petition along with his brothers, Grant, 14, and twins Ross and Mason, 18.

Leonard Scott, Olin's director of manufacturing technology, said converting the Tennessee plant to mercury-free technology would cost "hundreds of millions of dollars" and threaten nearly 1,000 jobs.

Before joining Performance Track, Scott said, Olin spent $2.6 million to upgrade its emissions equipment, including roughly $1 million on state-of-the-art mercury monitors. Since December 2006, he said, this has reduced mercury emissions by 63 percent - down from around 1,084 pounds per year.

"Mercury is a very important issue to us," Scott said. "We live here, too."

Olin spokeswoman Elaine Patterson said, "The plant's use of mercury is always careful and controlled. . . . Because of the hard work of the people in our Charleston plant, we continue to meet the established requirements for Performance Track."

Nevertheless, the plant says it emits 420 pounds of mercury into the air and water per year. That's below the EPA's threshold - a special limit created for the few plants that still use mercury. By that standard, Olin has been in compliance on mercury for the last five years.

But Olin has been cited for other environmental violations. Since 2006, the year it joined Performance Track, Olin has been cited for two "significant violations," according to the EPA, although Olin has disputed one. Performance Track members who get three such violations are supposed to be expelled from the program.

When asked why Olin was invited into Performance Track given its track record in Charleston, its outdated technology, and its impact on the Hiwassee, EPA officials said the plant met its criteria.

Performance Track's national manager, Daniel Fiorino, said few industrial factories had perfect EPA compliance records. He said Olin's record on mercury emissions was irrelevant to its continued membership in the program.

"When you look at our requirements and what they are doing, they meet our criteria," Fiorino said. "Does that make them a green company? That's a larger issue, I think. . . . At some point, we would hope that they would move to the new technology."

Jackie Savitz, a senior director at Oceana, which alerted Olin's neighbors about the mercury pollution, said it was troubling that the EPA ducked an opportunity to help a major polluter clean up its act.

"If these are the leaders, I'd hate to see the laggards," she said.