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Stink bombs have long been the weapon of choice of teenage boys everywhere. Now the U.S. Department of Defense is looking to harness the power of foul odor to beef up its arsenal.

It's easy to see why an overpowering stench might be an appealing weapon: Despite the moans and groans (and, in some cases, screams) of those unfortunate enough to catch a whiff, no physiological harm results from malodorants, as the foul-smelling concoctions are called. Unlike many other nonlethal weapons—tear gas and pepper spray, for example, which can cause bronchial irritation—malodorants are essentially harmless.

Experimental psychologist Pam Dalton has been working on the development of a globally unpleasant odor since 1998 in what she calls a "fortuitous collaboration" with the Department of Defense. Dalton and her colleagues at the Monell Chemical Senses Center in Philadelphia have chemically mimicked some of the foulest naturally occurring odors—body odor, burnt hair, vomit, fecal matter and decaying carcasses among others—and presented them to volunteers to rank. Volunteers were recruited from a range of cultures and ethnicities in an effort to determine the most universally despised odor.

Dalton says her research focused on a set of basic theoretical questions: To what extent is odor perception a learned behavior? Is there some commonality across cultures? Evidence for ethnic differences in smell perception is not new, she says, but what's interesting are other factors affecting how smells are perceived. Her research suggests that odor perception is surprisingly context-dependent. Butyric acid, a common ingredient in vomit and foot sweat, for example, evokes an unpleasant reaction from volunteers—unless they "have been clued to think about food," Dalton says. Butyric acid is also present in certain strong cheeses, and when volunteers developed expectations of food, they typically found the odor more bearable, she says.

As for the least tolerable smells, the team's research suggests that the smell of "organic decomposition"—rotting flesh and organic decay—is the most universally reviled.

The research has recently attracted some controversy. In February, the Sunshine Project, an international organization devoted to building a global consensus against biological weapons, published a news release asserting that U.S. defense officials are developing nonlethal weapons—and malodorants, in particular—targeted at specific ethnicities. The release noted a "disturbing preoccupation with ethnicity" in documents related to projects sponsored by U.S. defense programs.

Dalton says this claim "couldn't be further from the truth."

Her government-funded project isn't the first time malodorants have been considered as potential weapons. In World War II, the original formulation of one of Dalton's concoctions—a sulfurous, putrid-smelling compound called "Who Me?"—was shipped to Paris, intended for dispersal by the French Resistance among German soldiers in order to humiliate them. It remains unclear whether a "Who Me?" attack was ever successfully staged; "Who Me?" is a difficult blend of chemicals to keep contained, Dalton says. It has a high concentration of sulfur, a light molecule, and often escapes, much to the dismay of those trying to contain it.

Since the research first received notice last summer, Dalton has received calls of all kinds: people wondering how they can use malodorants for their own designs. For example, can the smell be used to keep skiers on trails and off protected land? Can it be bottled and used for self-protection? In most cases, malodorants could effectively be used in many creative ways, Dalton says. The chemical formulations of the most unpleasant smells her team could produce have been passed along to defense officials for further evaluation.

Technological developments often build one from another, and so countermeasures are most likely in the works. In a December 2001 Science article, Steven Munger, a chemosensory scientist at the University of Maryland, and his colleagues identify CNGA4, a protein subunit they believe to be important in regulating the speed of adaptation to an odorant signal. In animals lacking functional CNGA4 subunits, the speed of adaptation slows nearly 70-fold, says Munger. Dubbed the brain's "nose plug," CNGA4 might be able to provide protection from foul-smelling weapons.

Perhaps by hyperactivating the biochemical pathway, desensitization could become almost immediate—although, Munger warns, there would be no selectivity in the desensitization process. An increase in CNGA4 would generally "ratchet everything down," affecting sense of smell as a whole, he says.—Rebecca Sloan Slotnick