The people who operate the federal toxic waste incinerator in Oak Ridge don't know exactly what comes out of its stack-or what effects those emissions might have on the health of its neighbors.

They do know the emissions are little more than water vapor. But concern is focused on the tiny particles-some 100 times smaller than the width of a human hair-that slip through.

"Nobody has 100 percent faith that they know what is coming out," says Jacqueline Okoreeh-Baah, manager of combustion permits for Tennessee's Department of Environment and Conservation.

Dozens of people who work nearby and have unexplained illnesses suspect the incinerator may be partly to blame. The Tennessee legislature will begin hearings this week to investigate how new out-of-state waste began arriving at the incinerator without state approval, and to hear from the people who are sick.

Last year, the churning, red-hot incinerator consumed 1,500 tons of toxic Cold War remnants.

In it were thousands of pounds of such exotic metals as antimony, barium, cadmium and uranium, as well as such commonly known poisons as lead, mercury and arsenic.

The people who run the incinerator know that much. And they know metals and radioactive particles cannot be burned.

But state regulators have never made operators prove how much metal the incinerator releases.

Since the incinerator began firing six years ago-and last year it increased its metals intake bysevenfold-it has only once been required to do a limited test on what it releases into the air.

That 1995 test showed air emissions for three metals and four other substances were far below what Tennessee allows. For instance, the state says the incinerator can safely release 72 pounds of particles a day. The incinerator put out just over 3 pounds.

Ann Walzer is not impressed.

"They're burning a mix of waste no one else in the country can burn," says Walzer, an environmental toxicologist who the Labor Department recently ruled had been laid off because she voiced concerns about health and safety at the Oak Ridge complex.

"Fifty-some people are sick and management is sitting there saying, 'We're in compliance.' "

Walzer and others believe the incinerator is somehow related to their illnesses. More than 50 people employed there filed health complaints of similar, unexplained symptoms with the Department of Energy in 1995 and last year.

They have no proof linking their illnesses to the incinerator. They simply note their problems started or flared shortly after the incinerator began burning DOE waste tainted with radioactivity, toxic metals and chemicals, including PCBs.

Rigorous procedures

Those who run the incinerator-Energy Department officials and federal contractor Lockheed Martin Energy Systems-say it is safe.

"There is no doubt in my mind-and I've visited a lot of incinerators-about the kind of rigor with which we operate," says Vince Adams, the Energy Department's manager there. "We operate with over 400 procedures. If someone goes out to close a valve, another person comes out to check it."

But the process is not foolproof.

Last week, Lockheed Martin officials discovered they had accidentally dumped hazardous ashes from the incinerator in a landfill that was lined to prevent leaking but not meant for even slightly radioactive hazardous waste.

And at this time last year, an emergency vent on the incinerator kept mysteriously popping open, four times within three months, releasing unfiltered gases into the air.

Workers fixed the problem, which they still can't explain, last July.

Charles Sedman, a chemical engineer who studies air pollution control at the Environmental Protection Agency's research laboratory in North Carolina, questioned this emergency vent opening straight from a burning chamber into the air.

"Some states prohibit that," he said. "If you're going to have an emergency bypass of the air pollution control, at least you could put it into the stack so it would go up in the air and away from people. If it went down on the ground, it would be an occupational hazard.

"We don't know exactly what would happen to the people. Unfortunately, these are things that we only find out down the road."

And there are additional unknowns that worry observers:

To burn properly, the ingredients of the incinerator's toxic soup must be known. Otherwise, too much metal or some other compound may be fed through at once, sending a spike of unburned particles into the air.

Technicians test the waste for 21 metals and dozens of other chemicals and radioactive particles before putting it into the incinerator. But they say they can't test for everything. And in some cases, the waste coming to the DOE incinerator may be more than 40 years old.

"It's hard to characterize that kind of waste," says Bill Linak, a combustion research scientist for the EPA. "You have to have a good handle on what you're putting in. It must be a nightmare to analyze some of this older stuff."

Fire may be one of the oldest forces known to humankind, but what it does to a mixture of hazardous chemicals and metals is, in many ways, still not completely understood. Metals and chemicals change forms and interact in fire. For instance, operators may feed one form of chromium into the incinerator, but it could change to a more toxic form in the flames.

Incineration can even create compounds that scientists have yet to identify.

"We just don't really understand it completely," says James D. Kilgroe, considered one of the top incineration experts at the EPA's research lab in North Carolina. "I'm not going to give incineration blanket approval, but I think it can have acceptable risk if it's done properly."

The technology doesn't yet exist to continuously monitor everything coming out of an incinerator stack. The EPA and private companies are working on such a monitor but have not yet created one.

"It's like the Holy Grail," Linak says. "And it's kind of like rocket science trying to hook onto this dirty old stack. We just haven't gotten it to work yet."

Monitors at the K-25 complex, where the incinerator is located, continuously check for radiation. But the only stack emissions continuously monitored are oxygen, carbon dioxide and carbon monoxide.

The theory is this: If those three simple measurements stay within a certain range, the fire must be doing its job. It is assumed that combustion is so complete virtually everything is being turned to water vapor or ash.

Even so, scientists know some portion of whatever goes in the incinerator also will slip out the stack.

"This system may be fine," Sedman says. "But my concern is that very, very fine particles might find their way through."

The incinerator operators say its filtering system removes 99.984% of the airborne particles.

Even if it could be known exactly what was carried in the steam curling out of a towering incinerator stack, very little is known about what happens to the human body when it's exposed to small amounts of several different toxic particles at the same time.

Scientists have good information on how some individual metals, such as lead and mercury, affect the body.

To calculate what's safe, scientists use the best analysis of what's being burned and extrapolate what's coming out of the stack. They then calculate what the effect of one pollutant might be on one person over a 70-year period.

But the best they can do is estimate the combined effect of different substances.

"The bottom line is, no one really knows or understands to any great degree of accuracy what one might expect with multiple toxic exposures," says Dan Paschal, supervising research chemist in the environmental health laboratory at the Centers for Disease Control in Atlanta. "And in real situations, exposures are almost always simultaneous and always low-level."

Those who study the effects of metals and other toxic substances are "very concerned" about these combined effects, Paschal says. "The problem is, to find the answers requires a lot of research and a lot of money."

Finding the answers seems most urgent now, say the sick workers. Their symptoms are mostly worsening, and more waste is on its way.

Can't stem the flow

The Energy Department last year began bringing in waste from DOE sites in Missouri and Colorado. While Gov. Don Sundquist's administration allowed those shipments, the governor now says he will fight DOE plans to bring additional wastes from sites in New Mexico, Nevada and Idaho.

However, the Energy Department says the state doesn't have the power to stop the truckloads of low-level radioactive and toxic waste rolling into the state to be burned at Oak Ridge.

In the meantime, sites in New York, California and Illinois also have asked to send waste, although they do not yet have DOE approval.

Legislative hearings begin Tuesday on the matter. In the coming weeks, lawmakers plan to hear from sick workers who have questions about the incinerator.

Adams and other managers say they believe the employees are indeed sick. Some have been granted disability payments. Others are out on sick leave.

"I'm sure they're feeling ill," Adams says. "But the origin or source, nobody knows."

Officials are about to get a better picture of what's coming out of the stack. Tennessee is requiring the operators to prove how much of certain metals they are emitting. This "trial burn" could take place later this year.

The test will actually measure only four metals-cadmium, chromium, arsenic and lead. For the remaining 17 metals going into the incinerator, operators will meet regulations by controlling how much of those metals goes in. They say they'll feed in amounts that would meet guidelines even if all of it came out the stack.

The trial burn is a special test for the incinerator's new hazardous waste permit. Tennessee only requires regular tests of air emissions every five years.

"I'm surprised the state doesn't make them redo it more often," said Richard S. McGee, executive director of the Center for Environmental Engineering and Science in Ann Arbor, Mich. He describes himself as "pro-incineration."