Some odors are more heinous than others. Then there’s Stench Soup.
What is the most universally disgusting smell to humans?
— Lizzie in Austin, Texas
Smells can be pretty subjective.
In 1998, Pamela Dalton, a cognitive psychologist at the Monell Chemical Senses Center, was tasked with developing a stink bomb for the Department of Defense. Her experiments found that people from different backgrounds and different parts of the world, who grew up smelling and eating different things, often completely disagreed about which smells were good or bad.
The best candidate Dr. Dalton found for a universally distasteful smell was something called “U.S. Government Standard Bathroom Malodor,” a substance that was designed to mimic the scent of military field latrines, in order to test cleaning products. She chose the aromatic liquid as the base of her stink-bomb recipe. The resulting formula, which she called “Stench Soup,” may well be the worst smell ever created.
Mary Roach, a science writer, is one of the few humans who’s tried inhaling Stench Soup. In her 2016 book “Grunt: The Curious Science of Humans at War,” she described the aroma as “Satan on a throne of rotting onions.”
Is Stench Soup really the worst smell in the world? That’s hard to say, in part because researching bad smells can be difficult. Chemists tell cautionary tales about a substance called thioacetone, which in 1889 was the subject of experiments in a lab in Frieberg, Germany. One thioacetone reaction produced a smell so bad that it spilled out of the lab and swept through the city, causing widespread panic and evacuation, along with a lot of people being sick in the streets.
Derek Lowe, an industrial chemist who’s written about thioacetone, said it was hard to know for sure what chemicals produced the smell in the 1889 incident. The thioacetone would have been converted into another chemical — he suspected one called gem-dimercaptan — which might have undergone further reactions to create yet more compounds. No one seems eager to repeat the experiments to figure out precisely what molecules are produced.
“Pretty much all the compounds you could draw out of thioacetone are going to reek,” Dr. Lowe told me. But exactly how much it would reek is uncertain: “Not a lot of us have smelled a gem-dimercaptan. It could well be intense. I am not minded to find out.”
The thioacetone story raises a question: How far can a bad smell spread? The odor from the lab in 1889 reached about half a mile in all directions before it dispersed. Could a smell be so potent that it spread all the way around the world, making the entire planet stink?
The strength of aromas is measured by their “odor detection threshold,” which is the amount of the stuff you need to add to the air before average people can smell it. Gasoline has an odor detection threshold of about 100 micrograms per cubic meter. If a gallon of it evaporated into the air from a high place, it would produce enough vapor to make the air smell like gasoline for 600 feet in all directions.
There are substances smellier than gasoline. Ethyl mercaptan, the stuff added to natural gas to make gas leaks easier to detect, has an odor detection threshold of just 1 or 2 micrograms per cubic meter. A few pools of ethyl mercaptan the size of the reservoir in the middle of Central Park could, if spread evenly throughout the atmosphere, make the entire planet smell like a gas leak. Methyl mercaptan, which is even smellier, might only take one reservoir to stink up the globe.
But not all strong smells are bad. One of the substances with the lowest odor detection threshold is vanillin, the main component of vanilla extract. Estimates vary, but its odor detection threshold is probably around 0.1 or 0.2 micrograms per cubic meter, significantly lower than ethyl or methyl mercaptan. That means that one or two oil tankers full of vanillin could conceivably be used as an air freshener powerful enough to give the entire Earth a slight scent of vanilla.
Dr. Lowe said that the worst thing he’d ever smelled in his career as a chemist arose when he inadvertently combined dimethyl sulfide (think farts) with some silicon he was putting through a reaction called a Peterson olefination. Neither odor would smell great on its own, but combined they produced something transcendently foul. “It smelled like what you’d imagine the exhaust of a U.F.O. to smell like,” he said. “It was spectacularly weird and horrible.” In the infinite universe of chemistry, who knows what smells are waiting to be discovered?