The eyes get teary and the stomach weak. The gag reflex chokes the throat. Is that raw sewage? A rotting squirrel? The brain is too distracted to answer.
Pamela Dalton has uncorked the foulest smell on earth.
It comes from one of the vials that Dalton keeps under a ventilated hood in her laboratory, where the bottles carry impish labels: Burned Hair. Bathroom Malodor. And worst of all, Stench Soup, an odor so reeking of ripe Porta Potties -- or is it dead possum? -- that it fills the mind with white noise.
"That one takes over every aspect of your consciousness," Dalton says proudly of her creation, made in search of the world's most offensive odor.
Dalton is an expert in how the body perceives smells, a field shedding light on the workings of the nervous system, memory and emotion. But in recent years, her specialty has drawn interest from a less academic source: the Department of Defense.
Foul odors, it turns out, may be one of the most sensible ways for military commanders to disperse hostile crowds, empty buildings and keep intruders away from "no-go" areas. They are one of the most promising in the new crop of nonlethal weapons, intended to incapacitate people or equipment while minimizing the risk of death.
Nonlethal weapons are only a small part of the Pentagon's research and development program, but they received a major endorsement last week as a panel of the National Research Council called for a big boost in experimentation and spending. At the top of the panel's to-do list was increased study of foul smells, or "malodorants."
At the same time, some researchers are calling for renewed investment in nonlethal measures as a result of last month's hostage standoff in Russia. Authorities there used an opiate gas, intended to be nonlethal, to subdue armed Chechen guerrillas who had taken over a Moscow theater. More than 630 hostages were saved, but the gas killed at least 119 people.
"The Russians ended up having a lot of deaths because they didn't have adequate technology to act in a nonlethal way," said Larry Erickson of Kansas State University, which has researched the environmental effect of malodorants for the Navy. "It's appropriate for some of these studies to go on to find nonlethal ways to manage a variety of situations."
But nonlethal weapons could be counterproductive, some critics say. While seemingly humane, they might prompt an enemy to escalate the level of violence.
"It's laudable for the military to explore less lethal means of applying force, but they have to stay out of the chemical and biological area -- including malodorants," said Edward Hammond of the Sunshine Project, which opposes chemical weapons.
If the United States pursues odor weapons or even nonlethal chemicals, other nations would follow suit. Those nonlethal programs, Hammond said, could easily mask efforts to develop far more deadly chemical weapons.
The interest in "more humane" weapons dates to at least the 1960s. But military planners began to take them more seriously in recent years, as troops increasingly faced hostile civilians on peacekeeping or humanitarian missions in places such as Haiti, Somalia and Bosnia-Herzegovina. With TV cameras ready to broadcast civilian deaths worldwide, soldiers often had to navigate rock-throwing crowds with only two choices: the bullhorn or the bullet.
A critical boost came in 1995, when Marine Lt. Gen. Anthony Zinni asked for an infusion of nonlethal weapons, such as rubber bullets, to help him manage the final withdrawal of United Nations forces from Somalia. The next year, the Pentagon created an office to focus on nonlethal weapons.
Now researchers are developing slippery sprays that cause vehicles to lose traction. Underwater rope barriers could soon protect U.S. vessels, an attempt to head off disasters like the attack two years ago on the U.S. destroyer Cole in Yemen by a small boat laden with explosives.
Lasers and other energy beams are being studied for their ability to cast a confusing glare, immobilize vehicles or disperse crowds with a harmless but burning sensation in the skin.
These efforts are tiny by Pentagon standards, with between $20 million and $34 million spent each year, but now the Pentagon is being urged to do more.
Rubber bullets and similar devices are in the field, but these target individuals or small groups. "Demand is growing for more capable systems with wider-ranging effects," said the National Research Council panel, which advises the government on scientific matters.
It singled out odor weapons as among the most promising nonlethals.
Foul smells -- and what makes them offensive -- have been a longtime interest of Pamela Dalton, a vivacious woman with a broad smile.
As a 19-year-old undergraduate, Dalton was mugged while visiting a friend in New York. During the struggle, the assailant put his hand over Dalton's mouth.
A month later, Dalton was in a friend's home while some children were gluing a model car. "All of a sudden, I began to feel my heart racing, and I felt flushed and had to sit down," she recalled. "I was either having a heart attack or feeling intense anxiety."
Another attack came a few weeks later while Dalton was at the New Jersey shore, near some people using glues to repair a boat. "This time, I had more of a feeling that the anxiety had been triggered by something," Dalton said. She realized that glue odors reminded her of the mugger.
"My attacker must have been a glue-sniffer," she said. "It wasn't the odor itself that was making me sick. It was the odor in association with my life being threatened."
Feeling doubly violated, Dalton set out to re-encode how her body reacted to the smell of glue. Using her experience with meditation, she put herself into a relaxed state and then opened a tube of glue, replacing fearful associations with calm ones.
The experience set Dalton off on her career of studying responses to odor. Now 46, she is helping the Army understand how diesel fuel or other smells may touch off anxiety in Persian Gulf War veterans.
The mugging also taught Dalton a lesson that is important for military research. Odor weapons, she says, would be more like psychological warfare than chemical warfare.
When inhaled, some substances can burn tissues or send the heart racing. But these are irritants and drugs, not pure odors, Dalton says. Odors can cause physical effects, but these come about only because people have memories or associations with the odor.
The smell of Christmas cookies may bring comfort, while the smell of sewage might send the heart pounding, thanks probably to a deep-seated impulse to avoid disease-laden substances.
People can also panic when they don't understand a smell. In one of many similar episodes after last year's letter-borne anthrax attacks, two workers in Maine were hospitalized with headaches after opening a box with an unexpected liquid and a strange smell. Testing proved it to be tartar sauce.
While Dalton has worked on trying to calm the panic response to odors, the military has tried to exploit it.
During World War II, military researchers developed a repulsive, fecal-smelling mixture and packed it in a squirt tube. It was given the humorous name of Who Me?
The goal was to put Who Me? in the hands of French Resistance fighters, who would spray the material on German officers to humiliate them.
Whether anyone ever employed it is unclear.
In the 1960s, one company created a mixture called Go Home, which in theory would thin the less-motivated people from a rioting crowd.
In 1997, researchers at the Army's Edgewood Chemical Biological Center asked Dalton to revisit the possibilities of foul odors. They quickly realized that the challenge was to find odors that were disliked by people of all cultures and ethnicities.
"People like the odors they grew up with, and they don't necessarily like odors they are unfamiliar with," Dalton said. For example, durian fruit is a treat in many parts of Asia, while the smell would chase most Europeans from the room. "To me, it smells like rotting flesh and garbage," Dalton said.
Through newspaper advertisements, Dalton found whites, blacks, Latinos and Asian Americans who agreed to sample odors. She also traveled to South Africa to work with volunteers.
Last week, Dalton offered a small replay of the odor tests in her laboratory at the Monell Chemical Senses Center, an independent research facility here devoted to smell and taste.
First up was a bottle that smelled vaguely like blue cheese. "It's actually a vomit smell," Dalton said.
"Vomit is a very ambiguous odor, because it has elements of food in it. There's a lot of variation in how people respond to it."
Then Dalton offered a whiff of the storied Who Me? It smelled like the repellent odor added to natural gas, with a heavy dose of spoiled mushrooms.
After a pleasant sniff of vanilla and orange -- a palate cleanser, so to speak -- Dalton pulled out a bottle marked Bathroom Malodor, mixed according to a government formula. The odor was invented so that makers of bathroom cleansers could test their products.
Pee-ew! That was bad.
Bathroom Malodor had a strongly fecal smell, with sharp notes of spoiled eggs and an undertone of rotting rodent. After a few whiffs, the stomach churned and an odd feeling gathered in the throat.
In the tests with various ethnic groups, Bathroom Malodor was the hands-down winner as the most widely reviled smell. "We got cursed in a lot of different languages," Dalton said. "It was: Get that out of here!"
Unfortunately, Dalton had not created Stench Soup by the time of the test. Combining the worst of Bathroom Malodor and Who Me?, that odor is so distasteful that it chases all thoughts out of the mind. In informal tests, Dalton's colleagues have found it to be the most obnoxious odor of all.
Still, it is unclear whether odors could be "weaponized."
First, no one has tested whether the smells are so bad that they would disperse rioters or cause people to abandon plans to storm an embassy.
Second, the military would need ways to store and disperse the malodorant. One possible solution is to encapsulate odor molecules in small casings, much like cold capsules. The University of New Hampshire has received Pentagon grants to study the idea.
The military also is considering a nonlethal mortar shell, made from strips of lightweight carbon fiber, that would not produce dangerous shrapnel.
Third, an odor weapon would have to meet ethical and legal tests. The National Research Council panel wrote that it doesn't consider malodorants to be "chemical weapons," which are outlawed by international treaty.
But others disagree. "A malodorant is a chemical compound which exerts a temporary incapacitating effect on people, and in my view it's a toxic chemical under the Chemical Weapons Convention," said Mark Wheelis, a microbiologist at UC Davis who has written widely on chemical weaponry.
Dalton still is studying foul odors, but she is no longer working for the Pentagon. Still, she may have some pointers for the military. Occasionally people send her e-mails and letters, eager to share their ideas for revolting brews.
In one recent message, a man wrote about how he tried to eliminate a dead raccoon from a roadway by setting it on fire.
"The smell of the smoke made me so sick, I could taste it for days," he wrote, helpfully adding: "I just thought I would let you know. Maybe it will give you some ideas or something to go on."