What the Nose Knows: The Science of Scent in Everyday Life (7 page)

It is odd to think that a childhood’s worth of olfactory memories can be boiled down to a pocket chemistry set. Were the tomato fields of Davis and the cooking ketchup of the Hunt’s cannery—so powerfully evocative to me as a kid—just a particular shuffle of sixteen key odorants? Evidently so. Knowledge also brings insight. I now understand in molecular detail why my grade-school field trip to the Spreckels Sugar Company plant was such a stunning disappointment. As beets are processed into pure white refined sugar they first release geosmin (damp earth) and dimethyl disulfide (onion, cabbage, putrid). Later comes propionic acid (the pungent, rancid note in Swiss cheese and sweat), and finally hexenoic acid (musty, fatty). Those four notes were the heavy stew that oppressed my soul that day in third grade. Somehow, knowing that makes me feel better.

CHAPTER 3

Freaks, Geeks, and Prodigies

D
ON
G
IOVANNI
:
Zitto: mi pare sentir odor di femmina!

[Hush! I think I scent a woman!]

L
EPORELLO
:
Cospetto! Che odorato perfetto!

[My, my! What a nose!]

D
ON
G
IOVANNI
:
All’aria mi par bella.

[And a pretty one at that.]

—M
OZART,
Don Giovanni

T
AKE A FEW DOZEN PEOPLE AT RANDOM, AND YOU WILL
find a range of olfactory talent that stretches from
American Idol
–tryout bad to unbelievably excellent. There are people who cruise untroubled past the fetid plumes of garbage cans and subway vents, and others for whom the faintest milk fart escaping from an elderly relative is a nasal crisis. Olfactory sensitivity (technically, the lowest concentration at which someone detects a smell) is just one dimension of smell talent; other factors include an awareness of smells, and the ability to identify them and discriminate among them. Extreme variability is a hallmark of odor perception, and sensory scientists have identified many factors that contribute to it. It is now possible to answer a fundamental question: Who has a good nose and who doesn’t?

T
HE FIRST THING
to note is that people are not accurate judges of their own ability. When we asked people taking the National Geographic Smell Survey to rate their own sense of smell, we found a Lake Wobegon effect: most people were above average. The only way to assess someone’s ability impartially is with a smell test. These come in two types: identification tests and threshold detection tests. The former ask you to put names to odors, the latter ask you to detect progressively lower concentrations of a smell. Smell tests have been commercially available for years, but were formally recognized as a medical device by the FDA only in 2006; this may explain why they are an underutilized part of the physician’s diagnostic arsenal. The tests range from one-shot sniff tests, appropriate for quick screening during an office exam, to elaborate, hours-long testing with scores of odors that takes place in a research lab. Normal smelling is generally defined as a certain proportion of correctly identified odor samples, or a specific, very dilute concentration at which an odor should be smellable. An odd feature of smell tests is that the best one can score on them is “normal” there is no test that rates levels of excellence, no equivalent to a 150 IQ. In fact, there is not even an official medical term for smell genius.

Because smell tests are designed to identify people with dysfunctional noses, they are finely calibrated for degrees of underachievement. At the lowest end of the scale are people unable to smell anything at all; they suffer from anosmia, the technical term for complete smell loss. One notch up the scale are people with hyposmia, which is the olfactory equivalent of being hard of hearing; like deafness, it can be mild or severe. It has been estimated that 1 to 2 percent of the U.S. population suffers from anosmia or hyposmia. In both cases, the most common cause by far is infectious disease. Severe colds, flu, and sinus infections inflame the tissues lining the nose and kill off sensory nerve cells. In severe cases, or after a lifetime of accumulated damage, areas that were once rich with nerve cells are replaced with nonsensory mucus membrane, and the tissue takes on a moth-eaten appearance.

Head injury is the second leading cause of smell loss. A blow to the head can sever some of the olfactory nerve fibers that travel to the brain through tiny holes in the base of the skull, at a location between the ears and behind the eyes. There’s an old (and possibly true) story about a waiter carrying a tray of food at head height. As he exits the kitchen, the swinging door slams the tray into his forehead. Being a professional, he maintains his balance and proceeds into the dining room. As he serves the dishes, he realizes he can’t smell a single one. The speed with which the waiter discovers his loss might be unusual—most people don’t notice for days or weeks—but the mildness of the damaging blow is not. It takes very little force to cause smell loss. I cringe when I see kids heading the ball in a soccer game. I wouldn’t bet on them becoming chefs or perfumers.

With the exception of a stuffy nose, smell loss is a long-term condition. Smell may return after a flu or sinus infection, as the damaged sensory cells are gradually replaced by new ones, but recovery can take months and your abilities may never return to their original levels. The probability of recovery declines with age. In cases of head trauma, the prospects are bleak; the severed nerve fibers rarely reconnect. Consider the results of a typical study: a year after their initial visit to the doctor, 32 percent of postinfection patients showed improvement, compared with only 10 percent of the post-injury group. The realization that millions of Americans suffer from smell loss spurred the National Institutes of Health to underwrite basic research into odor perception. The ultimate goal of this work was to find ways to cure smell loss. Yet, despite decades of substantial funding, effective medical treatment remains elusive.

Sudden smell loss is psychologically devastating. By far the biggest impact is on eating: anosmia steals the pleasures of the table. Without its aroma, food in the mouth becomes a bland, chewy mass, and drinks become equally flavorless. Faced with dull food, some people lose appetite, eat less, and lose weight; others eat to feel full and end up gaining weight. Smell loss can alter mood—patients often show symptoms of depression, and psychological well-being, friendship, emotional stability, and leisure activities all take a hit. Some people find that their sex life suffers. In the wake of smell loss comes the anxiety of constant vigilance. Anosmics worry about gas leaks, undetected fire, spoiled food, and lapses in personal hygiene. They adopt coping strategies such as frequent bathing and laundering. Anosmics report smell-related hazardous events—burning a pot or eating spoiled food—more often than normal smellers, but there is little data to suggest a higher rate of actual injury.

In rare instances, people are born without a sense of smell. As it’s hard to miss what you’ve never known, people who are anosmic from birth tend to regard their condition with bemusement. A few even manage to find a silver lining. The ex-boyfriend of a young English anosmic told her, “You were the best girlfriend in the world. You let me bring curry home from the pub every night, and I could fart as much as I liked.” One newspaper reporter who is smell-blind from birth regularly covers smelly stories for a major U.S. daily. This is either a heartwarming story of a disability overcome, or journalistic malpractice of the first order. Perhaps, in a zany way, it is both.

Somewhere off the main continuum of normal to partial to complete smell loss lie the bizarre pathologies of odor perception. A person with phantosmia, for example, perceives a smell when none is there. These olfactory hallucinations can be vague (“a chemical smell”) or quite specific (one patient said, “It reminds me of a flower I smelled in Samoa”). Phantosmia is a tricky diagnosis for a doctor to make: the hallucinated smell comes and goes and may not occur in the course of an office visit. The physician must first rule out all possible organic sources for the weird smell, especially sinus or gum disease. The physical causes of phantosmia are diverse and include seizure, migraine, and brain tumor. When a real odor gives rise to a distorted perception, the condition is called parosmia. The distortions in such cases are almost always unpleasant; patients say things smell foul, rotten, or burned. Such was the case of a sixty-year old woman who awoke one morning to find that every odor smelled like burnt toast. Eleven years later, despite treatment with antibiotics, antivirals, vitamins, beta-blockers, anticonvulsants, and zinc sulfate, her condition was unchanged. Most parosmics can tell you which smells are distorted; the most common are gasoline, tobacco, coffee, perfumes, fruits (mainly citrus and melon), and chocolate. Parosmia almost always occurs after an upper-respiratory-tract infection or head trauma, where smell function is reduced but not completely gone. This leads researchers to speculate that parosmia is an “incorrect rewiring” of the connections among regenerating nerve cells following damage to the olfactory system. Among smell pathologies, the most appalling is cacosmia, in which everything smells like shit.

In Philip K. Dick’s sci-fi novel
The Simulacra
(1964), there is a character named Richard Kongrosian, a psionic pianist who plays the instrument telekinetically. He also has a history of mental instability. An annoying advertisement triggers in Kongrosian the delusion that he has a bad body odor. He becomes obsessed with BO and washes compulsively, but in vain; the smell lingers. His ability to play the piano from a distance notwithstanding, Kongrosian is a poster child for a real-life psychiatric disorder known as olfactory reference syndrome, which is characterized by persistent hallucinations of body malodor.

I
T PROBABLY COMES
as no surprise that men and women differ in smell ability. This has been confirmed many times with a variety of test methods and in cultures around the world. Women rate themselves as having a better sense of smell, and the data back them up. Women detect odors at lower concentrations and are better able to identify them by name. A German psychologist found that men and women are equally good at remembering colors and musical tones, but women are better at remembering smells. Humorist Dave Barry’s wife would not be surprised:

At least five times per week, my wife and I have the same conversation. She says: “What’s that smell?” And I say, “What smell?” And she looks at me as though I am demented and says: “You can’t SMELL that?” The truth is, there could be a stack of truck tires burning in the living room, and I wouldn’t necessarily smell it. Whereas my wife can detect a lone spoiled grape two houses away.

Sex differences are based on group averages; there is much variability within each sex, and large overlap between them. But in general, women are better. Or, as Dave Barry put it, men suffer from Male Smelling Deficiency Syndrome.

What explains the female superiority? There is little evidence of sex differences in the nose. Dave Barry’s nose probably looks and operates much like his wife’s. The brain is a different story. Recent evidence suggests that brain structures related to odor perception differ in size and cellular architecture between men and women. Whether these anatomical variations explain Barry’s quip remains to be seen. We do know that some male-female differences in perception (the fact that women often rate smells more intense and unpleasant) are mirrored by differences in the underlying brain-wave response.

Female smell superiority is partly due to women having higher verbal fluency; verbal skills boost performance on tests of odor memory and odor identification. Another factor is hormones. A woman’s smell sensitivity varies across her menstrual cycle and is greatest around the time of ovulation. Hormone effects are not simple; they interact in complex ways with cognitive factors. This interaction produces one of the most dramatic olfactory sex differences ever observed in the lab. Sensory researchers Pam Dalton and Paul Breslin tested men and women for their sensitivity to a specific odor. With repeated testing over the course of thirty days, the women became much more sensitive to the odor, while men did not. The effect was confined to the tested smell; sensitivity to a different odor did not change for men or women. The enhanced sensitivity can’t be attributed to practice; the women weren’t getting better at threshold tests in general. They became more sensitive because they paid close attention to low levels of odor while being exposed to it multiple times. Most remarkably, Dalton and Breslin didn’t find enhanced sensitivity in prepubescent girls and postmenopausal women. The phenomenon is limited to women of reproductive age. This implies that female hormones are needed to make it happen, and in fact it can be observed in postmenopausal women who take hormone replacement therapy.

Sex differences are evident within days of birth: baby girls turn toward novel odors and spend more time smelling them than baby boys do. The anthropologist Lionel Tiger attributes the difference to evolution. In our long history as hunter-gatherers, he says, it was the females who gathered fruits and vegetables, and a good sense of smell was valuable in judging ripeness and safety. Tiger’s view—essentially a biologized version of “women spend more time cooking”—will not be received warmly in some quarters. Yet it’s hard to see how a cultural explanation can explain sex differences in two-week-old infants.

W
ITH AGE, OLFACTORY
performance begins to deteriorate. The first signs of decline are detectable in the early forties—at least under laboratory conditions—and the pace accelerates in the sixties and seventies. Interestingly, the rate of decline varies with the odor. Rose and banana, for example, are easily perceived until people are in their seventies, while mercaptans (the natural-gas warning odor) show a drop among people in their fifties. Some age-related smell loss can be traced to the nose itself—the accumulated wear and tear of infections and minor blows to the head. Some of the loss is traceable to the brain. For example, odor identification ability depends on how much short-term memory the test requires. Because short-term memory declines with age, elderly people score better when the odor test is presented in a simple yes/no format than in a multiple choice format that requires more memory capacity. In any case, decline is not inevitable; a given seventy-five-year-old may outperform a given twenty-five-year-old. Perfumers, in fact, usually get better with age. Experience and skill more than compensate for any dimming of acuity that comes with age. I know of no fragrance house with a mandatory retirement age for perfumers.