The science of stink

We all have our favorite smells, which remind us of our favorite things. The smell of baking bread may make you think of Grandma’s house. The scent of lilacs may remind you of warm summer evenings.

Then there our are less-favorite smells, like the smell of an outhouse on a hot day, or the smell of sneakers after a 20-mile hike.

Bad smells are a hot topic of research right now at the Monell Chemical Senses Center in Philadelphia because the U.S. Department of Defense officials has asked the center to formulate a universally offensive odor the military could use for crowd control.

The “odor bomb” they’ve come up with so far is said to smell like rotting garbage, human waste and burning hair, and is so potent it makes people want to flee in disgust, as well as causing shallow breathing, an increased heart rate and nausea.

Here’s how the sense of smell works: air passes over an area in the nose called the olfactory epithelium, which is about four or five square centimetres in size and covered by mucus. Molecules from “smelly” substances must penetrate the mucus in order to reach the sensitive receptor cells beneath. These cells–about 10 million of them–are equipped with hair-like cilia, studded with proteins. Scent molecules interact with these proteins, causing the cells to send a message to the brain.

Smell has a powerful ability to provoke emotion and memory. Each half of the brain contains an almond-shaped clump of cells called an amygdala, which plays an important role in emotional reactions. There are direct links between the amygdala and the parts of the brain that process the sense of smell. Nasty smells (such as the Monell Centre’s “odor bomb”) evoke a powerful response from both of the amygdala, whereas pleasant smells evoked only a weak response in the right amygdala. This is probably why we react so violently to really bad smells, and why people who are constantly exposed to unpleasant smells tend to feel bad emotionally.

Most things that really stink do so because of the action of bacteria. Excrement and flatus (to use the scientific terms) smell bad because as bacteria in the large intestine break down food, they produce smelly, sulfur- or nitrogen-rich organic compounds such as indole, skatole, mercaptans (skunk smell) and hydrogen sulphide (rotten-egg smell).

Garbage, too, stinks because the bacteria breaking it down release smelly gases.

Similarly, feet and underarms stink because of the excretions of sweat-eating bacteria.

At the Monell Centre, chemist George Preti has spent 30 years investigating such unpleasant human smells as underarm odor, bad breath and “fish odor syndrome,” a genetic disease called trimethylaminuria which makes the sufferer smell like fish, no matter how many times they take a bath or change clothes. Another researcher, Charles Wysocki, is working with Preti and the Pennsylvania Department of Agriculture to make farms smell better, not so much for the sake of the farmers as for the sake of the urban types whose bedroom communities are springing up in once-isolated agricultural areas.

It’s a quest that would have baffled our ancestors, who might even have been impervious to the “odor bomb.” Through most of history, after all, humans have lived with very strong smells (what with the lack of bathing, open sewers in the streets, and the reliance on horses). It’s even safe to say that our sense of smell has been crucial to our survival as a species, since it warns us of bad food, bad water and even bad animals with very long teeth lurking in the forest.

It’s only recently that we’ve tried so hard to eliminate unpleasant odors entirely–probably an impossible goal, considering that ideas of what smells bad and what smells good vary so much from culture to culture.

There’s a famous story of a group of American G.I.s who, during the invasion of Normandy, passed the open mouth of cave from which poured the most horrendous smell. Convinced the cave must be full of rotting corpses, they incinerated its contents with a flamethrower.

Turned out the cave actually belonged to the Roquefort cooperative, and was full of cheeses, ripening and molding to perfection.

What stinks and what doesn’t, it seems, is very much in the nose of the sniffer–which may bode ill for the success of the “odor bomb.”

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