The project will last six to 10 years and involve burning about 6,200 tons of liquid blister agent. The disposal is complicated by the presence of an estimated 800 pounds of toxic mercury.

The mustard gas is the last phase of the program to destroy munitions stored at the Deseret Chemical Depot in Tooele County, which at one time housed more of the nation's chemical weapons than any other storage site. The destruction of the weapons by the Tooele Chemical Agency Disposal Facility, a sophisticated burn project, began in 1996 and will help the United States comply with the international Chemical Weapons Convention.

Workers at the Depot are sampling some of the 1-ton containers holding the mustard agent to figure out which of them have low concentrations of mercury. Workers will drain and incinerate those first, said spokeswoman Alaine Southworth.

At the same time, workers will design and install special sulfur-impregnated carbon filters that will scrub the mercury from the exhaust when the higher-concentration containers are later disposed of, Southworth said.

But a conservation organization questioned whether depot officials really know the technology will work, and argued the Army should instead employ a mustard gas disposal method used at Aberdeen Proving Ground in Maryland.

Vanessa Pierce, director of Healthy Environment Alliance of Utah, said the mustard agent could be neutralized with plain hot water without releasing burn emissions.

"It still has to be treated as a hazardous waste, but there is no risk of hazardous contaminants getting into our air," she said.

Southworth acknowledged the technology but said Aberdeen had only about a third as much gas to dispose of. Besides, the depot in Tooele already has an incinerator that has safely disposed of 7,400 tons of other nerve agents, she said.

The Army "feels both technologies are equal," but it is more effective to incinerate here, she said.

According to the Council on Foreign Relations, mustard gas (actually a liquid at room temperature) is one of the most potent chemical weapons. First used during World War I, the gas causes severe blisters, and external and internal bleeding, and strips the mucus membrane from airways. The effects take hours to develop, which means victims can suffer tissue damage before they even realize they need treatment. Mustard gas also attacks a cell's DNA, increasing the risk of cancer and birth defects, and remains active in the soil for weeks.

In 2004, the Army announced it would spend $50 million to change the way it burns mustard agent to avoid mercury emissions.

Though it's believed there are 800 pounds of mercury in the liquid chemical agent, Southworth estimated no more than one or two pounds of the deadly neurotoxin would be released into the atmosphere during incineration.

Pierce rejected the estimate.

"They don't know how much mercury is in all the containers," she said. In any event, she added, ''mercury contamination is a huge problem here. . . . It's better to take extra steps to ensure maximum protection for workers and the public."

The highly toxic element occurs naturally in the environment but also has been introduced through human activity, including burning coal for electricity and processing gold ore.

Mercury evaporates easily. When snow and rain redeposits it on land and in water bodies, certain chemical activity converts it to its organic form, known as methylmercury.

Methylmercury, which accumulates in the bodies of animals and humans, affects the human nervous system and is most harmful to fetuses and young children because it can cause developmental and neurological problems.

In February 2005, federal scientists reported finding in the Great Salt Lake some of the highest levels of methlymercury ever recorded in the United States. Though fish don't live in the lake, wildfowl feed on its brine shrimp, algae and plants.

After the study, Utah environmental quality and health agencies stepped up investigations into methylmercury contamination and issued five mercury warnings concerning fish and wildfowl.

Poison cleanup

* The U.S. Army's Deseret Chemical Depot in Tooele County, 60 miles from Salt Lake City, has begun incinerating 6,149 tons of liquid mustard agent.

* Depot officials estimate the mustard gas also contains about 800 pounds of elemental mercury. Emissions resulting from incineration will be controlled by a $50 million filtration system.

* This is the largest chemical agent destruction campaign the Army will conduct to meet requirements of the Chemical Weapons Convention of 1997.

Destroying the deadly arsenal

* The Deseret Chemical Depot has been operating for 10 years, and has destroyed 7,409 tons of chemical agent and more than a million munitions.

* The mustard gas incineration campaign is scheduled to be completed by 2016 but may end as early as 2012.

* The Deseret Chemical Depot is one of eight Army installations in the U.S. that currently stores chemical weapons. The Tooele Chemical Agent Disposal Facility was built for the sole purpose of destroying its chemical weapons stockpile.

* Operations began Aug. 22, 1996. The last of more than 53,000 artillery projectiles filled with deadly VX nerve agent was destroyed two years ago. Previously, the $1 billion incinerator at the depot destroyed about 6,000 tons of GB agent, also known as sarin.

- Source: U.S. Army Chemical Materials Agency