National Geographic Today
The task: to develop an odor so universally repulsive it would be
considered unbearable by people from all cultures. That's the challenge
the U.S. Department of Defense has presented to scientists at the Monell
Chemical Senses Center in Philadelphia, Pennsylvania.
The military wants to use the odor to control crowds and deter people from restricted areas, according to psychologist Pamela Dalton, who studies the development of odor preferences at Monell.
The advantage of using stink bombs, she explained, is that odors, unlike pepper spray and tear gas, are not dangerous. And they are effective at low concentrations.
Little research on cross-cultural odor preferences has been done, said Dalton, but she and her colleagues have avoided food and personal products as candidate odors because they tend to be culture-specific.
Instead, the scientists are focusing on odors with biological originsvomit, human waste, body odors, burnt hair, and rotting garbage. Confronted with these stenches, people from most cultures try to avoid the sources, said Dalton.
Dalton and her colleagues created a mixture of these odors and tested on volunteers, who wore hoods while the offensive odors were slowly infused.
"People pretty much thought it was the worst odor they had ever smelled," said Dalton. "First, people's heads would jerk back, their face would contort with revulsion, and then they would hold their breath for as long as possible." That was followed by shallow breathing, as the volunteers tried to inhale as little as possible.
The odors also induced increased heart rate and stomach uneasinessearly symptoms of nausea.
Creating these vile cocktails does not require use of original source materialall the odors were chemically synthesized.
To create the smell of decaying flesh, for example, Dalton collected a dead mouse from a trap and placed the animal in a plastic bag to "age." After some time, she used a syringe to collect particles from the air that surrounded the decaying specimen and analyzed the chemical components. Once the natural chemical makeup of the rotting mouse odor has been determined, the stench could be reproduced directly from chemical agents.
Rotting organic material produces many volatile compounds rich in sulfur, such as mercaptoacetic acid. Compounds called fatty acids produce a rancid, sour smell. And the stench of human waste is primarily the result of a compound called skatole.
A mixture of these various odors provides a complete assault against the senses. "People can get acclimatized to individual smells, but a mixture of odors lasts longer and provides more of a punch," said Dalton.
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