That's an amount that shocks environmentalists because, by contrast, the nation's 497 mercury-emitting power plants sent 49 tons of the toxin into the air that year, Oceana reports.
"Sixty-five tons of mercury is a lot," says Jon Devine, a lawyer for the Natural Resources Defense Council (NRDC). "If a significant amount of this stuff is going out into the atmosphere, then these plants could rival power plants in terms of their contribution to mercury emissions in the US."
Chlorine industry officials argue that most of the mercury probably stays on the factory site trapped inside equipment and factory pipes.
"The chlorine industry can account for 99 percent of our total mercury inventory - but we realize that's not good enough," Tracy Cullen, a spokeswoman for the Chlorine Institute in Arlington, Va., writes in an e-mail. "We're fully committed to 100 percent accountability. And we're now working with EPA on new, more advanced methods for monitoring fugitive air emissions."
The best information so far indicates that unaccounted-for mercury is not being leaked into the air, the EPA says. Also, the industry's mercury consumption has fallen from an annual average of about 160 tons in the early 1990s to 30 tons in 2002, an 81 percent drop.
Even so, what happened to the missing mercury is a mystery. "The fate of all the mercury consumed at mercury cell chlor-alkalai plants remains somewhat of an enigma," the EPA declared in 2003 in the Federal Register.
"We agreed to conduct a detailed study to help us account for the mercury," says Cynthia Bergman, an EPA spokeswoman in an e-mail. "The unaccounted-for mercury from mercury cell chlor-alkalai plants is a very important issue to the agency."