What the Nose Knows: The Science of Scent in Everyday Life (4 page)

I
F HISTORY IS
littered with the wrecks of Universal Classifications of Smell, we can still learn something from surveying the ruins. What they have in common is a surprisingly limited number of elementary categories: either 4, 6, 7, or 9, depending on who you like. The mind-boggling variety of smells in the world is reducible to a manageable handful of nameable odor classes, just as the brain carves the range of visible light into a handful of focal colors. Suppose one adopted the standard perfumery categories as an approximation of the pleasant sectors of smell space; this amounts to one or two dozen classes (woody, floral, fruity, citrus, etc.). What more would one need to encompass the stinks and stenches of the world? The fecal category would cover a lot of territory—from benign horse manure to the intolerable air in a rock concert privy. A category for urinous could include the sour smells in a nursing home and the heavier fourth-quarter reek of urinals at an NFL game. We’d have to add a class for retch-inducing smells—vomit and really stinky feet—and another for fishiness in all its gradations. Skunk, sulfur, and burning rubber could constitute yet another class. Finally, the putrid stench of rotting meat probably deserves its own banner. These six classes would capture most of the bad smells abroad in the world. Which is more amazing—the huge number of possible odors, or the tiny number of odor types?

C
AN SUCH A
stripped-down system of classification handle the olfactory complexities of the real world? It turns out that the human brain already does a pretty good job of reducing that complexity. The Australian psychologist David Laing was the first to tackle the relevant question: How many smells can we pick out of a complex mixture by nose alone? He began with a set of distinctive odors such as spearmint, almond, and clove, each easily identifiable on its own. He created mixtures—beginning with combinations of two odors at a time—and asked people to identify as many components as they could. The more odors he added to the mixture, the more difficult it became to identify even a single ingredient within it. The degree of difficulty was surprising. For example, in a mixture of three or more odors, fewer than 15 percent of people could identify even one component. Laing made the test easier: he gave people a target odor and asked them whether they could smell it in the mixture. Even then, they could rarely find the target in a mix of more than three odors. Could the problem be lack of skill? Laing tested perfumers and flavorists. The professionals were better than amateurs at identifying two and three items in a mixture, but even with their training and experience, they failed to pick more than three odors from the mix. Laing reasoned that mixtures of simple, single-chemical smells are somehow unnatural and hard to pick apart. So he repeated the experiments using as mixture components such complex odors as cheese and chocolate. The results were the same: no one could bust the four-odor limit. Were the individual smells not distinctive enough? Some odors, such as orange, almond, and cinnamon, blend together easily; perhaps those that blend poorly, such as mushroom, cut grass, and mandarin, are easier to pick out of a mixture. Laing found this was true to a point, yet the four-item barrier held firm.

Why are we so feeble at smelling our way through a bouquet? Our ability to gather olfactory information is formidable: the human nose detects single smells at extraordinarily low concentrations. We do a better job of collecting smells than we do of tracking them in a complex mixture. The Laing Limit suggests that the problem is not in the nose but in the brain. We have limited ability to think about smells analytically.

In the end, the question “How many smells are there?” may not be as relevant as “How many odor categories do we need to make sense of the world?” The answer to that question will reveal much more about how the brain handles the information that the nose provides.

CHAPTER 2

The Molecules That Matter

You cannot suppose that atoms of the same shape are entering our nostrils when stinking corpses are roasting as when the stage is freshly sprinkled with saffron of Cilicia and a nearby altar exhales the perfumes of the Orient.

—L
UCRETIUS

S
TRICTLY SPEAKING, SMELLS EXIST ONLY IN OUR HEADS.
Molecules exist in the air, but we can only register some of them as “smells.” Odors are perceptions, not things in the world. The fact that a molecule of phenylethyl alcohol smells like rose is a function of our brain, not a property of the molecule. A tree burning in the forest does not smell if no one is there to smell it. The planet Mars has no atmosphere and is too cold for human life, yet the chemical composition of its surface suggests that if we could sniff it, it would reek of sulfur. Perhaps someday we will have the opportunity. Apollo moon-mission astronauts noticed that the lunar dust they tracked back into their craft smelled like wet ashes in a fireplace, or burned powder from a shotgun shell. Humans flying back from Mars may need to hang a little pine tree in the cockpit window.

Semantics aside, an odor perception is usually caused by a physical substance—molecules light enough to evaporate and be carried on air currents to our nose. (There are strange exceptions: some observers of the early aboveground nuclear bomb tests experienced a metallic smell within moments of the blast, and in the rare condition known as phantosmia, patients perceive a smell in the absence of any external stimulus.) The sensory cells in our nose convert a chemical signal (the molecule) into an electrical signal (a nerve impulse) that travels up the olfactory nerves to the brain for interpretation. Since airborne molecules trigger odor perceptions, we should, in principle, be able to match a molecule to every odor. Hydrogen sulfide smells like rotten eggs and amyl acetate smells like banana—how hard can it be to complete the list? Very hard, it turns out. Most aromas in nature are elaborate bouquets, mixtures of dozens if not hundreds of different molecules.

Prior to 1955, complete chemical analysis of the aroma from a cup of coffee was beyond the reach of routine science. It would have been taken years to extract, isolate, and purify the scores of volatile molecules found in it. The invention of gas chromatography in the mid-1950s made possible the rapid analysis of aromatic mixtures and revolutionized the science of smell. Despite its importance, the gas chromatograph (or GC) remains little known to an otherwise technology-savvy public. Take a smelly substance—an apple or an oyster, it doesn’t matter—put it in a blender, then run it through a GC, and you will get a visual record of its volatile components.

At the heart of the GC is a Slinky-esque coil of very thin tubing that would stretch ten to thirty meters if unwound. As a first step, the smell sample is injected into the coil, where it is absorbed into a polymer that coats the inside of the tube. The Slinky sits in a little oven, which heats up in preprogrammed steps over the course of two minutes to two hours, depending on the setup. A stream of helium gas enters one end of the coil and exits the other. As the temperature rises, odor molecules are driven out of the polymer and into the gas stream. The process is orderly: each type of molecule evaporates and enters the stream at a specific temperature, depending on its molecular weight, and emerges from the end of the coil in a burst roughly two seconds long. The amount of material in each burst shows up as a peak on a timeline. The more molecules, the bigger the peak. A pure sample of a single chemical, say phenylethyl alcohol, yields a single peak. A complex mixture like rose oil produces a series of peaks, varying in height, representing the more and less plentiful components in the mixture.

Because it is highly detailed and unique to each sample, the visual profile created by the GC is often likened to a fingerprint. The difference is that a fingerprint is static—a direct physical impression—while the GC is dynamic: it takes a complex smell and pulls it apart in time. Perfumers liken a smell to a musical chord; if this is the case, then the GC plays it as an arpeggio.

As individual odors emerge from the GC, they can be fed into another device called a mass spectrometer, which provides a definitive identification of the molecule. By the mid-1970s the GC/MS linkage had been automated and labs around the world were churning out detailed chemical analyses of natural products. This was a mixed blessing for smell scientists. Run orange pulp through a GC/MS and you get a laundry list of volatile components. Do they all smell? Do they all contribute to the total orange aroma? How can we tell?

Since the early days of GC, chemists have sniffed at the exiting gas stream to see if they could recognize the emerging components by nose. Some volatiles, such as carbon monoxide, are entirely odorless to the human nose; otherwise each GC peak corresponds to a distinct smell. The size of the peak is not a reliable index of odor power. A big peak may deliver very little odor (which means the molecule is not very smelly) and a tiny peak may pack a punch (the molecule is a potent odorant). Cornell University chemist Terry Acree pioneered what is known as gas chromatography-olfactory or GC-O, which is essentially a formalized way of sniffing the GC vent to correlate smells with specific molecules. Acree devised a way to express numerically the relative odor potency of each chemical within a complex sample. He divides a chemical’s concentration in the sample by the minimum concentration needed to smell it on its own. Molecules with an odor impact index hovering around 1.0 are just at the level of detectability. Molecules with high multiples contribute more to the overall odor, while those with multiples less than 1.0 are seldom detectible; at best they lend a grace note to the overall composition.

Hey Beavis, Pull My Finger

One might expect the chemistry of certain bathroom malodors to be well understood. What other stinks are experienced on so personal a basis? For years, medical students were taught that the main ingredients of fecal odor were skatole and indole, nasty-smelling molecules created by the breakdown of meat protein during digestion. This claim persisted in textbooks despite never having been confirmed by direct chemical analysis. The shit finally hit the gas chromatograph in 1984 when researchers in Salt Lake City ran some poop through a GC and sniffed the results. Skatole and indole, although present in the sample, contributed relatively little to the typical fecal odor. The key actors turned out to be sulfur-containing compounds such as methyl mercaptan, dimethyl disulfide, and dimethyl trisulfide. Despite this dramatic reversal of conventional medical wisdom, the gastroenterological community remained unmoved. Finally, in 1998, investigators at the Veterans Administration Hospital in Minneapolis took the next step and performed an exacting chemical and olfactory analysis of farts. Their experimental methods were straightforward: “To ensure flatus output, the diet of the subjects was usually supplemented with 200 g pinto beans on the night before and the morning of the study.” Gas capture was simplicity itself, though the details are squirm-inducing: “Flatus was collected via a rectal tube…connected to a gas impermeable bag.” When the bags of ass-gas were analyzed, the main contributors were once again sulfur-containing molecules: hydrogen sulfide, methyl mercaptan, and dimethyl sulfide.

By comparing bean-powered samples from men and women, the intrepid Minnesotans were able to settle a long-running dispute between the sexes. The data proved (as men have claimed for centuries) that the farts of women are stinkier, on a volume-for-volume basis, than those of men. Since men produce a greater volume than women, however, the overall gag factor remains about even. As part of their research, the team tested a device called the Toot Trapper, a fabric-covered foam cushion coated with activated charcoal. The cushion is worn inside one’s pants and, according to the manufacturer, absorbs the offensive odor of intestinal gas. The Minneapolis team tailored a pair of fart-proof pants from Mylar sheets and duct tape. When volunteers wore the pants along with a Toot Trapper, the captured gas was indeed less smelly. (“Toot Trapper” strikes me as a lame brand name for this useful product. If I were the marketing consultant, I’d go with something more robust, like “Blast Master 3000.”)

Lyrical accounts of child-rearing dwell on the wonderful smell of a baby’s head. Less sentimental observers note that infants are prodigious gas-factories. In 2001 a group of pediatricians found that diet affects the chemical composition of baby farts (technically, they analyzed the gas produced by poop samples stored at body temperature for four hours). The gas from breast-fed babies was heavy on (odorless) hydrogen and very low on stinky methyl mercaptan. Babies fed milk-based formula had intermediate levels on every gas measured. Infants fed soy-based formula produced a lot of hydrogen sulfide (rotten-egg smell) and also the most methane. The good news is that methane is odorless; the bad news is that it contributes to global warming.

Another cherished belief is that one’s own little bundle of joy produces better-smelling poop than the other kids. Remarkably, this belief holds up under strict scientific scrutiny. Mothers of fourteen-month-old babies contributed dirty diapers, which were sniffed from cardboard buckets. Each mother compared a diaper load from her kid to that of an anonymous sixteen-month-old who provided the reference sample. The other baby was stinkier when the dueling buckets were unlabeled; labeling the buckets (e.g., “Jason” versus “Other Baby”) didn’t increase the effect, nor did switching the labels reduce the effect; this means the mothers were not letting maternal pride interfere with their odor judgments—they really do find other children stinkier. This study also proves that some sensory psychologists have way too much time on their hands.

Reefer Madness

One complex botanical smell has had an outsized cultural impact on the nation. Rod Blagojevich captured it well when, during his campaign for governor of Illinois, he admitted that he had smoked marijuana, saying “it was a smell that we all, in our generation, are familiar with.” He added, “I didn’t like the smell of it.” In contrast, Andy Warhol allegedly said, “I think pot should be legal. I don’t smoke it, but I like the smell of it.”

I once received a phone call from a graphic artist when I worked for the fragrance company Givaudan Roure. He was designing the booklet for a solo CD by a member of a well-known rock band and wanted to print it with ink that smelled like marijuana. Could my company supply such a smell? His request put me in an awkward spot. Technical hurdles were not the issue; our perfume chemist assured me that he could work up a good pot smell. (He also hinted broadly that the project would go faster if he had a high-quality sample to work from.) The decisive factor was financial—sales are measured in pounds of fragrance oil sold and by the price markup on the raw materials. In this case, the expected sales volume was minuscule and not worth the time perfumers would spend on it. Still, the project held a certain allure.

The more I thought about it, the more complications came to mind. Can one replicate the smell of pot without using delta 9-tetrahydrocannibinol (or THC), the psychoactive ingredient? If so, could it still get you busted by a drug-sniffing dog or your homeroom teacher? Would my company be legally liable for the consequences?

THC, and its chemical cousins, are not volatile and are therefore odorless. If a chemist stripped the THC from pot, the result would be genuine-smelling but buzz-less, the psychedelic equivalent of decaffeinated coffee.

When I reach him on the phone, I find Dr. W. James Woodford to be a genial fellow with a Southern accent. He is a fragrance and flavor chemist and the man who invented the first drug pseudoscent. Early in his career, during a stint as a guest researcher at England’s New Scotland Yard, he encountered large samples of contraband cocaine. Woodford knew that the pure cocaine alkaloid was odorless, but when he sniffed it in the evidence room he noticed a distinct aroma. When exposed to air and moisture, cocaine chemically degrades and yields a sweet, prunelike odor. Woodford’s scientific curiosity was piqued, and he traced the scent to a molecule called methyl benzoate. Methyl benzoate is found in flower scents; there’s lots of it in snapdragons and petunias, and some in tuberose and ylang. Perfumers use it all the time, especially in fragrances of the
Peau d’Espagne
type.

Cocaine is illegal, as are its direct chemical precursors and metabolites. Woodford managed to replicate the scent of cocaine with methyl benzoate and a few other ingredients, all of which are chemically unrelated to cocaine and therefore perfectly legal. Woodford patented his drug pseudoscent formula in 1981, and the government was soon using it to train dogs and drug enforcement personnel. Woodford let the government use it for free. “I didn’t make any money off of it,” he says. Others were not so charitable, and soon an entire industry blossomed. The Sigma-Aldrich chemical supply company, for example, carries Sigma Pseudo™ Narcotic Scent Cocaine formulation, priced at $37.20 for 100 grams. They also sell an LSD formulation and another that mimics the scent of pot. Forensic chemists at Florida International University have created a fake Ecstasy aroma.

Drug dogs trained to find cocaine are, in fact, recognizing the scent of methyl benzoate rather than the cocaine molecule itself. This displacement effect is true for other major targets of drug dogs. Ecstasy gives itself away through the cherry-pie scent of piperonal, and methamphetamine has a characteristic cherry-almond scent from benzaldehyde. So yes, dogs find the drugs, but they should really be called drug-associated-odor-sniffing dogs.

Fragrance clients are nervous about having too real a pot smell for fear of alerting drug dogs and police. What if drug traffickers used these for their own ends? They could flood an airport with pseudoscents and sneak their contraband through while dogs and cops are chasing false leads. It hasn’t happened yet, but Woodford recognizes the danger. “There’s potential for mischief,” he says.