Under prodding from environmental groups, the Environmental Protection Agency is taking another look at a handful of U.S. chemical plants that cannot account for as much as 65 tons of mercury they may be releasing into the environment each year.

The nine chlorine manufacturers that utilize techniques dating to the 1950s have denied they release enough of the toxic substance to endanger public health. But some environmental groups and lawmakers contend the plants may be posing a more serious health threat than the nation's 11,000 coal-fired power plants that together emit 48 tons of mercury every year.

"There's been a tremendous amount of attention on power plants," said Linda Greer, a senior scientist with the National Resources Defense Council (NRDC). "But there's this sneaky other source that may be as big, if not bigger, that EPA hasn't regulated well at all."

Last December, the EPA issued a new rule to regulate the chemical industry's handling of mercury, but it conceded that the government and industry cannot account for 65 tons of the toxic substance that may be escaping into the environment annually.

Jeffrey Holmstead, an assistant EPA administrator in charge of air policy, confirmed this week that his agency would undertake the review. "Our engineers said we are quite confident that mercury is not going into the air," he said. "Exactly where it's going, we don't know. There's been enough concern about this, we want to go back and see where it's going."

Mercury is a powerful airborne neurotoxin that can penetrate the food chain and damage the brains and nervous systems of children and fetuses.

The process of using mercury to turn salt, or sodium chloride, into chlorine gas and caustic soda became popular after World War II. Ninety percent of the industry now uses modern technology that does not involve mercury, but plants in Alabama, Delaware, Georgia, Louisiana, Ohio, Tennessee, West Virginia and Wisconsin still use the old method.

Each year these plants buy massive amounts of mercury -- in 2002 it totaled more than 100 tons -- to replace the amount they lose in production. According to chlorine industry officials, most of the missing mercury goes into the pipes.

"We're just certain the mercury does not leave these plants in unreported quantities," said Art Dungan, vice president for environment, safety and health at the Chlorine Institute. "We have reduced the use of mercury throughout our industry for many years."

According to Dungan, EPA tests in 2000 showed that only a minimal amount of mercury -- 600 grams a day -- escaped into the atmosphere from individual plants, a figure that he said falls within federal requirements.

Industry officials estimate that consumption of mercury by the chemical plants has dropped significantly in recent years -- to 30 tons in 2002, for example -- even though mercury purchases may be on the rise. But environmentalists note that the industry cannot fully account for the difference between the amount of mercury purchased and the amount consumed by the plants.

But when it issued its new regulations last December, the EPA acknowledged that "the fate of mercury consumed" by these plants "remains somewhat of an enigma."

The administration said the new rules would cut known mercury air emissions from chlor-alkali plants by 94 percent. Dungan called the regulations "pretty darn stringent" -- adding that the industry has voluntarily reported its mercury use since 1996.

But the NRDC and the Sierra Club challenged the rules in a lawsuit, arguing that federal officials were ignorant about the disposition of large amounts of mercury each year.

They pointed to the case of HoltraChem, a chlorine manufacturer southeast of Bangor, Maine, that closed in September 2000. State officials concluded that mercury not only permeated the plant, but had seeped into a river. They began a huge cleanup that has lasted years and cost millions of dollars.

"There's certainly mercury in the groundwater . . . [and] in the soil," said Stacy Ladner of Maine's Department of Environmental Protection.

Despite an extensive cleanup, state officials have not recovered more than 33 tons of mercury that was unaccounted for in the plant's operation, even after draining the pipes.

Eighteen senators wrote to EPA Administrator Mike Leavitt last week demanding that the agency take more aggressive steps to monitor the chemical industry. "This is obviously a serious issue that cannot be ignored," said Sen. Olympia J. Snowe (R-Maine), who signed the letter. "The risk of not controlling mercury emissions cannot be overstated."

Jeffrey Holmstead, an EPA assistant administrator, said EPA engineers are confident mercury is not going into the air.