Stench warfare
07 Jul 01
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).
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 JosÉ 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.
Stephanie Pain
From New Scientist magazine, vol 171 issue 2298, 07/07/2001, page
42
© Copyright New Scientist, RBI Limited 2001