July 2001

From New Scientist

Stench warfare

Can you imagine the most terrifying smell in the world? Stephanie Pain goes on the trail of a stink bomb so vile it'll blow your mind

GOTTA get out of here. Heart's pounding. Can't think. Can't speak. Daren't breathe. Just run. As the stench rolls down the street, panic spreads. Everyone's on the run now. They don't know what the evil-smelling odour is but their noses tell them it's dangerous, and within seconds their stomachs sound the general alarm. In two minutes the streets are empty. All that's left is a terrible stink.

This hasn't happened yet, but it could if the US Army succeeds in its effort to create the mother of all stink bombs. Their aim is to have a weapon that doesn't kill or injure anyone, but instead triggers fear, panic and an overwhelming urge to run away. The mixture of malodorous molecules has to add up to a pong so repulsive it's truly terrifying.

The search for the perfect stink bomb is part of the Pentagon's Nonlethal Weapons Program. The US Army wants a stink to drive away enemy troops or hostile crowds and to enforce no-go zones around sensitive military installations. It could also help peacekeeping forces keep warring factions apart by creating stench-filled exclusion zones. Police forces would have plenty of uses for a stink bomb, too. It would be ideal for ending a siege without firing a shot, or for dispersing rioters or even marking the ringleaders so they can't escape into the crowd.

"It would give us an offensive capability against large and unruly groups of people, if they are unwilling to move or are openly hostile," says Pentagon spokesman Lieutenant Colonel George Rhynedance. "And it would minimise the risk to our own people and to the antagonists." The Army has been down this route before, with a singular lack of success. During the Second World War, the Office of Strategic Services conjured up a secret weapon known affectionately as "Who Me?" It was a noxious fluid intended for use by the French Resistance. The aim was to humiliate German officers by making them smell foul. "Imagine the worst garbage dumpster left in the street for a long time in the middle of the hottest summer ever-and that gives you a taste of the Who Me? quality," says Pam Dalton, a cognitive psychologist at the Monell Chemical Senses Center in Philadelphia.

Who Me? was certainly loathsome enough for the job, but it had one big drawback. The mixture was so volatile that it was impossible to "bomb" the target without contaminating everything in the area, stink bomber included. "Suffice it to say it wasn't one of the more effective strategies," says Dalton, who is leading the search for a better stink bomb.

Could any stench really be so repulsive that it strikes genuine fear into people's hearts and causes them to flee? Smells certainly have the power to alter our behaviour. The aroma of fresh bread lures us towards the bakery, while the pungent pong of a filthy toilet sends us reeling back to the door. Some smells are already used as deterrents. In the US, signs warn that some roadside firs are sprayed with a potent mix of molecules, including fox urine, to protect them from Christmas-tree thieves. The smell is barely noticeable outdoors, but in the warmth of the home the foxy stench is overpowering. And one company has even considered marketing a vomit-scented fridge "freshener" to deter dieters from snacking.

Unpleasant is one thing; frightening is something else. Yet smells can trigger intense emotions-including terror. When odour molecules dissolve in the mucous membranes of the nostrils, they set off signals that take two separate routes into the brain. One path leads to the thalamus and cortex, where the signals are translated into conscious awareness of the smell. The other leads to the limbic region of the brain, the unconscious core where emotions are generated (see Diagram, opposite).

This is true for all kinds of smells, nice and nasty. However, a sniff of something nasty activates a particular part of the limbic system, the amygdalae-a pair of small, almond-shaped pieces of tissue deep within the brain. Studies in animals suggest that the left amygdala is acutely sensitive to the sight, sound or smell of anything dangerous and plays an important role in awakening fear. "Fear and odours are very closely linked," says Josˇ Pardo, who runs the Cognitive Neuroimaging Unit at the Veterans Affairs Medical Center in Minneapolis, Minnesota. "The amygdala has a key role in fear conditioning." And that goes for humans just as much as other animals. The sense of smell is evolutionarily older than sight or hearing-the other long-distance senses-and is designed to warn of dangers such as spoilt food or nearby predators.

Pardo and his colleague David Zald found they could send the left amygdala into overdrive with nothing more than a whiff of a ghastly smell. They wafted a cocktail of sulphide gases-a sort of synthetic fart-past the noses of volunteers, while scanning their brains to see which parts were doing what. The captive sniffers tensed their muscles and reported feelings of revulsion, disgust and fear. The smell activated both amygdalae, but the more revolted and fearful people felt, the more active their left amygdala became.

How we perceive and respond to a smell depends on whether we recognise it and what sort of associations it has. Humans have an extraordinary memory for smells, and just a whiff of something can bring past experiences rushing back. A smell can be enough to revive old terrors. "For some Vietnam veterans the smell of mould, the sort you get on tent fabric, will trigger a fear flashback," says Pardo. "The smell of jet fuel or burning flesh can produce terrible fear." For people who haven't lived through such terrifying experiences, unfamiliar smells are more likely to prompt panic than even the nastiest odours they've smelled before. "Any odour has the potential to strike fear into someone's heart," says Dalton. "With a new smell you have little to go on. If you can't categorise it you don't know if it's dangerous. There's little you can do but run away." That's exactly the reaction Dalton wants to provoke. The perfect stink would trigger an emotional response-preferably one that sets you running-before the reasoning part of the brain can work out what's going on. In theory, it's possible. "The pathways are certainly there," says Pardo. The hard part is finding one smell that works on everyone. There's little evidence that humans are born with preferences for certain odours, the way they are with tastes. But even if people are, these preferences can easily be overcome by experience. "People who live around horses like the smell of manure. Some people even like the smell of skunk," says Dalton. People's likes and dislikes also vary enormously between cultures. To people in South-East Asia, for example, the fetid smell of the durian fruit holds the promise of something delicious. To almost everyone else, it's stomach-churning. "For a novice it's almost impossible to get it past the nose and into the mouth," says Dalton.

At Monell, Dalton and her team have been searching for a stench that transcends any cultural differences. They tested a range of horrible smells on five groups of volunteers of different ethnic origin-whites, African-Americans, Asians and Hispanics from the Philadelphia area, and a group from the township of Grahamstown in South Africa's Eastern Cape province.

Dalton's team quizzed the volunteers on the repulsiveness of each stench, how it made them feel and whether they thought it was harmful. And as the volunteers breathed in the foul odours, the researchers logged their bodies' physiological responses. They found that with the vilest smells, people take shallower breaths, their hearts beat faster, and their stomachs churn more vigorously. Most of the bad odours they tested proved disappointing. Everyone hated the smell of butyric acid-a cross between rancid butter and sweaty feet-but few people thought it was harmful. Burnt hair, supposed to mimic burnt flesh, turned out to be surprisingly inoffensive. Even the smell of vomit-an off-the-shelf mix called Proprietary Vomit Odor-was only mildly objectionable.

The response to one of Monell's special mixtures revealed a major cultural difference. This odour was developed to mimic the smell around sewage treatment plants and is reminiscent of gently decaying rubbish with a hint of faecal matter. Everyone found it disgusting, but the people from Grahamstown were also afraid of it. "To us in the US, it's a nuisance, but we can usually walk away from it," says Dalton. "In the townships, where there's little modern sanitation, it's a health hazard and people are very afraid of it. They think breathing it would be harmful."

No luck with these, then. But Dalton did find two loathsome odours that transcend culture. One is a truly repugnant mixture called US Government Standard Bathroom Malodor, a stink concocted to test the efficiency of deodorant cleaning products. "It's very pungent," says Dalton. "More precisely, it smells like shit, but much, much stronger. It fills your head. It gets to you in ways that are unimaginable. It's not something you are likely to come across in the real world."

The smell is so awful that some volunteers began to scream and curse after just a few seconds' exposure. Even though the smell is quite harmless, almost everyone thought it would damage their health. Dalton wasn't surprised. "If anything transcends culture it should be something like this," she says. "There aren't many cultures that embrace human waste and this is far worse than any regular human waste."

Another candidate for the title of the world's worst smell is an updated version of that old wartime weapon, Who Me? This classic has a bouquet rich with foul-smelling molecules, dominated by a sulphurous pong. "If I had to predict one class of odours we had a predisposition to react negatively to, it would be the sulphur compounds," says Dalton. "It's important to detect food spoiling or carcasses rotting. It must have significance in terms of survival."

Neither of these smells is likely to make the perfect stink bomb by itself. But together they just might, says Dalton. "You get a bigger bang for your buck with a mixture," she says. A combination of two of the world's worst smells should affect everyone-even those who might be "smell-blind" to one of its components-and should create something so far removed from anyone's experience that the fear factor kicks in.

But before the troops roll out armed with stink bombs, there are practical problems to solve. You have to deliver the stench without getting it everywhere, as happened with the original Who Me? And the mix must disperse easily enough to be effective, yet not disappear so fast that people just hold their breath and wait for the pong to pass. "Once we've got the sensory properties right, the chemists can tweak the mix to make it move in a particular way," says Dalton. "They can add chemicals to make it hang around near ground level or move higher in the air."

Dalton also has to discover what proportions of Bathroom Malodor and Who Me? produce the most evil-smelling mix. "There are no clear physical or chemical principles to determine how we perceive the end result," she says. "It's trial and error." And that poses a risk that makes her hesitate. "If I take that step, I'm just not sure I could keep anyone here working with me."

The human stink bomb
All debt collectors have ways of making people pay up. Andy Smulion, a man employed by a London magazine to collect unpaid bills, had an almost infallible technique. There was nothing illegal about it and no one got hurt. Smulion would simply turn up at the defaulter's office wearing his most vile-smelling clothes and hang around until they could stand the stink no longer. His "victims" described the stench as part skunk and part sewage, with a whiff of rotten eggs. It was a case of cough up or throw up. According to Parade magazine, which reported Smulion's tactics in 1979, he got results. And no one ever tried to beat him up: they couldn't bear to get close enough.

Bad day in Salt Lake City
On 18 March 1999, the Chevron oil refinery in Salt Lake City belched out a small, smoky cloud that drifted along near the ground towards the city centre. As it reached the Utah Symphony Hall, the air conditioning intake sucked some of it into the building, where two thousand children sat. At the first sniff of the strange smell, a couple of children complained of feeling ill. Suddenly everyone felt sick. Once outside the hall, children lay around on the grass gasping for breath, and dozens were taken to hospital.

As the cloud travelled over the city, panic spread. Chevron admitted at the outset that its plant was responsible for the cloud but pointed out that the vapours contained nothing harmful-just a mixture of hydrocarbons. Despite the reassurances, the "odor hotlines" didn't stop ringing for the next two weeks: people were so convinced that there was something harmful in the air that they took fright at the first whiff of something unusual.

Something rotten in the Senate
On Monday 16 August 1999, a little after 9 am, staff at the Dirksen Senate Office Building in Washington DC were settling down to work. Then someone in the cafeteria smelt something unfamiliar. It wasn't very strong but it was definitely a bit "off". No one could see where it was coming from and no one could identify the odour. That made it suspicious. Was this a terrorist gas attack? Someone raised the alarm. The building was quickly evacuated and nine staff from the cafeteria were rushed to hospital. People fled, leaving police, a hazardous-materials team, an advanced life-support unit, doctors, four teams of paramedics and the local fire chief to investigate. They didn't find any chemical weapons. But they did find a bag of rotting onions-they had been peeled and sliced for the salad bar and then forgotten.

The unfamiliar smell had wafted through the air ducts, spreading fear as it went. If people had been able to identify the smell, they wouldn't have panicked. "But when people don't recognise a smell they assume it's a hazard," says psychologist Pam Dalton.

New Scientist issue: 7th July 2001

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