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Smell is intimately linked to emotion, memory, and our individual experience of the world. How does this primal ability work? Scientists are still sniffing out the answer.

By Annie Monjar 

Photography by Jared Castaldi


I find something calming about the smell of gasoline. When I was a kid, I used to loiter at the door of my dad’s home office on Sunday nights, begging him to take me for a drive. The stop at the gas station, fragrant with fuel, signaled that I could put my childhood anxieties aside, leaving behind untouched homework and the prospect of learning long division. Today, whenever I’m at a gas station I feel a small pang of longing—explainable only when I stop and think about how that specific scent affects me.

But like most people, I rarely do. I began life with a big breath of air, and since then I’ve taken just about every gulp, all those particles and sen-sory stimuli, for granted. 

When I consider my senses, smell comes last. I’ve never been able to tell the difference between rosemary, thyme, and marjoram. I don’t garden. I eat too fast. All tea smells like burnt, damp grass to me. And I suspect I’m not alone. The saying “stop and smell the roses,” after all, exists because we don’t often do just that. It’s no secret that, in our sensory family, smell is often overlooked. We rarely discuss it, and it gets little notice—unless it forces our attention.

In high school, I was on the cross-country team. One fall afternoon, while we were out stomping the sidewalks, someone sniffed something unusual. Was it a skunk? Fertilizer? One of my friends shrugged and smiled: “I can’t smell.” It seemed like such an afterthought. I hardly even heard her. 

A couple years into our friendship, I began to understand that it wasn’t just that she couldn’t smell that skunk, or that fresh pile of dog doo, or whatever it was; she couldn’t smell anything. She had congenital anosmia, having been born without a sense of smell. When I finally realized the full extent of her condition, it seemed either too unusual or too irrelevant to spend much time mulling over. In my olfactory oblivion, I certainly didn’t take that early opportunity to consider what smells add to our lives. 

The real value of scent has always been a mystery. Freud thought that our sense of smell was vestigial. He had a point: It predates vision and hearing—single-cell organisms use chemical sensing as a way to detect nutrients and hazards. He also believed that our repression of smell was a sign that we had evolved from our primordial days. Those of us not concocting perfumes or nosing wines for a living certainly do live in a smell-suppressed, sanitized world. We have FDA regulations and modern plumbing; our noses aren’t bombarded with aggressive odors the way they would have been when horses filled the streets and refrigeration ran on ice. When we get nostalgic about Victorian times, we understandably skip the stinky parts. If you were forced to give up one of your senses, who would blame you for choosing smell?

And yet, when I think of my old friend, I wonder what it would be like to not fully experience the air around me. Would my memory of drives with Dad be quite as visceral? Would a vase of flowers mean as much? Would coffee provide the same morning jolt before I drank it? Would life be quite as rich? 

In The Man Who Mistook His Wife For A Hat, Oliver Sacks, the famous writer and neurologist, recounts the story of a young medical student who goes on a drug bender one night (dabbling in PCP, cocaine, and speed) and wakes up to find his sense of smell heightened, in “a world of pure perception, rich, alive, self-sufficient, and full.” He sniffs the world “like a dog,” rejoicing in the pleasures of our most overlooked, “primitive” sense. He eventually makes a full recovery, but, while acknowledging his gratitude for having regained his composure, he speaks of a tremendous loss: “I see now what we give up in being civilized and human.” 

I’m starting to see it, too. When I pay more attention to smells—pressing my face closer to things, loitering in produce aisles a little longer—I realize that I’ve spent most of my life ignoring the hold that smell has over my every inhalation. But I suspected there was more to smell than meets the untrained nose. So I went to visit a place where teams of people give this forgotten sense the attention it deserves.


Olfactory Code-Breakers

The Monell Chemical Senses Center occupies an industrial building in West Philadelphia, indistinguishable amidst blocks of research buildings owned by the University of Pennsylvania except for one feature: a large bronze sculpture of a partial face. (“Look for the nose,” says a security guard who gives me directions.) 

Inside the small lobby I meet Casey Trimmer, a petite, cheerful postdoc with long brown hair. She takes me to the elevator and leads me down to the molecular biology lab, where she does research on how genetic differences influence odor perception—why, for instance, variation in our genes makes cilantro smell fresh and herbal to some but soapy and chemical-like to others. The lab, for its part, smells fruity and inviting.

Trimmer explains olfaction to me in this way: Whenever we take a whiff of a flower—or of a basement biology lab—small molecules called odorants travel up our nostrils into our nasal cavity, where they make contact with the mucus-covered tissue of the olfactory epithelium. Here, millions of nerve cells pick up those fragrant molecules with antenna-like cilia covered in olfactory receptors (aka odor sensors) and send an electric charge to the brain. Together, the receptors act as a kind of switchboard. When the smell of, say, coffee makes contact with the receptors, some of them activate, lighting up a unique pattern that signals “dark roast” to the brain. 

Odor is the only sensory information that’s sent directly to the limbic system, the part of the brain responsible for emotion and memory. All other such data is mediated by another part of the brain—and only after getting filtered through a variety of other bodily mechanisms. (Sound, for instance, funnels into the ear canal, rattles the eardrum with vibrations, travels to the cochlea, and creates neural signals that only then get picked up by the auditory nerve.) In terms of our perception, you could say that smell is the world at its most raw.  



For years, scientists assumed that humans could detect some 10,000 different scents. A recent study from Rockefeller University, however, suggests that our noses are far more sensitive than we ever thought. By creating odor mixtures in which some smells overlapped, then asking volunteers to pick out which ones didn’t, researchers determined that we can actually detect more than 1 trillion smells. 

Monell scientists work tirelessly to crack the code of these scents. Trimmer and her colleagues are trying to figure out what odors light up which combinations of receptors and how genes influence that process. She says they know the receptor combos for at least 40 odors. Less than a trillion to go.

In the hallway outside Monell’s molecular biology lab, industrial-size refrigerators are packed with tiny tubes containing the DNA of different types of olfactory receptors. Inside the lab itself, wooden shelves and cabinets bear fragrant chemical concoctions meant to stimulate the cells. Trimmer unscrews one of the bottles and holds it up to me. It’s androstenone, a steroid hormone secreted by the testes of male pigs. She explains that the smell of androstenone is remarkably polarizing: Half of humanity gets knocked back by its putrid aroma, while the other half either reports the faint smell of vanilla or detects nothing at all. I get vanilla; one of Trimmer’s labmates, sitting in the chair next to me, cringes when the bottle is opened. Androstenone has been a topic of discussion over at the EU, where bans on pig castration, currently under consideration, could have unappetizing consequences. “It’s a problem if half the population thinks their pork smells like sweaty urine,” Trimmer says.

As she puts the bottle away, I ask why unlocking the receptor patterns of smells matters so much. What is the end goal?

She takes a breath. “The end goal is to get a better picture of how the olfactory system does this. How does it encode that information for you?” In other words, what smells are we programmed to respond to, and why? The genetic differences in how we perceive odors could have evolutionary significance: “Until we know what the receptors are binding to, it’s hard to draw conclusions about things that have evolutionary importance. Now, we’re still in the ‘let’s figure out what the system is doing’ stage.”

The lab now smells faintly like possibility.


A Scent So Personal It’s Scary

After a quick visit to one of the chemistry labs a couple floors up, where researchers are analyzing earwax for clues about personal data like ethnicity and stress levels, Trimmer takes me to a conference room. There I meet Pamela Dalton, a scientist at Monell who specializes in olfaction. She’s in her late 50s, with short strawberry-blonde hair and a clear, authoritative tone that bespeaks how used to smell-advocacy she is. One particularly intriguing facet of olfaction, she tells me, is how it elicits such strong emotional responses in people. Scientists term the phenomenon “olfactory-evoked recall,” and while the neurological underpinnings of scent-linked nostalgia remain unclear, its existence is well-documented and could have medical significance: Research has found that a diminished sense of smell can be an early biomarker for neurodegenerative diseases like Alzheimer’s and Parkinson’s. 

Dalton herself is no stranger to the power—and pain—of smell-induced memories. While an undergrad at Connecticut College, Dalton went to New York City to visit a friend. After going out to dinner, they returned to the friend’s apartment building. Dalton left briefly, alone, to pick up milk for the next morning’s  coffee. In the elevator on the way back, a man grabbed her from behind, put his hand over her mouth, and demanded her money and jewelry. 



Six months after the mugging, panic attacks began. For reasons that she couldn’t identify, her heart would pound, her palms would get clammy, and her head would swirl. One time it happened at a friend’s house. Another time, she was on a boat. “It happened enough that I said, ‘This is crazy, what is the common element?’” 

The robbery was the only trauma she had experienced, so she reluctantly made herself recall that night and all its frightening details. Contained within the adrenaline-soaked memory was the link: Her mugger was a glue-sniffer. He had reeked of it. She realized that when she had broken down at the house, her friend’s son had been working on a model car with glue. On the boat, some deck grommets were being refinished with glue. The potent, industrial aroma brought her back to that fearful moment in an elevator, triggering a surge of deeply rooted emotions. “The idea that an odor could retrieve not only a memory but an emotional state was just incredibly intriguing to me,” she says. Similarly, “PTSD victims, when they smell odors from war, even before they can say what it is or why it’s happening, have that immediate anxiety and panic. It’s really profound how well it can bring back an emotional state.”

Years went by, and Dalton left a job in international finance to study experimental psychology at NYU. Working on a study in which subjects viewed images of faces and then put their recognition abilities to the test, she decided to use odor as a variable. To avoid smells that would already have meaning or memories associated with them—like fresh cookies or Chanel No. 5—she bought room sprays from Indian shops. She found that the test subjects were better able to recall faces when they were reexposed to the scent from their study environment. Under those familiar conditions, they were also more confident in their answers. Yet most had no idea the room was scented. “I was seeing that odor was playing a role in their ability  to retrieve a memory without conscious awareness,” Dalton says. “It was binding to memory somehow, but it was just in the background.” 

While emotions and memories tied to smells can be disturbing, like Dalton’s mugging experience, the psychological effect of a smell can be centering as well. Dalton notes that certain odors—particularly the scent of a maternal figure—can promote focus and functionality in children with autism and ADHD. An anchoring scent like peppermint can sometimes curb out-of-hand cravings or other unpleasant, runaway thoughts. And the scant scientific evidence that exists on aromatherapy candles suggests that they are calming simply because they remind us of peaceful moments in our past, like the end of a yoga class or a massage. 

And if nothing else, a notable scent—a wind-borne waft of blossoming trees on Philadelphia’s otherwise swampy Schuylkill River, for instance, where I often run—connects us, quite literally, with where we are. It gives us pause. As babies, we put things in our mouths to get their full sensory texture; as adults, the best we can do is take a deep breath. When I run along the river, I am plugged into my headphones, thinking about the emails I need to send and the dry cleaning I keep forgetting to pick up. But when I remember to breathe—when a smell reminds me to do so—I’m simply outside, next to the river, part of the scene.


A Sense Beyond Words

Freud didn’t much care whether we got the most out of blossom-filled afternoons. He believed that smelling was a backward way of navigating the world around us. Yet the hazards of not being able to smell are more acute than those of us with fully functional olfactory systems might realize. Sharon Lysinger, a mother of three who lives outside Philadelphia, was 56 when she lost her sense of smell. After a three-week bout of an upper respiratory virus left her with a hacking cough, she noticed she was burning a lot of the dishes that she had cooked for her family for years. One day, while working at her electric sewing machine, she backed away and saw smoke rising up around it. The pedal had caught fire. She hadn’t smelled it. 

Lysinger’s doctor assured her that she would regain her sense of smell once the infection had worked its way out of her system. Four years later, she doubts it will ever return. The effects of the loss range from bothersome—she has to pay closer attention to expiration dates—to dangerous, to just plain sad. She can’t tell when to change her grandson’s diaper and has to be more attentive to the gauges in her car; her husband recently found that it had been leaking transmission fluid.

Americans with Lysinger’s condition may number in the millions; an estimated 1 to 2 percent of the population suffers from anosmia. Monell recently announced a three-year, $1.5 million research project on causes and potential treatments. 

Restoring smell would do more than bring odors back to anosmics. It would bring them renewed taste, too. According to Marcia Pelchat, a Monell researcher who studies food preferences and the effects of aging on chemical senses (aka smell and taste), our chief experience of food comes not from taste alone but from flavor: a combination of sensory stimuli, most prominently taste and the small puffs of aromatic molecules traveling from the back of our mouths up the nasal cavity. Olfaction turns nondescript green leaves into basil and sweet, icy cream into a strawberry milkshake. Congenital anosmics, who have spent their lives without smell, can enjoy the taste and texture of food, unaware that they lack full flavor. But for people like Lysinger, who lose their ability to smell abruptly, the loss can be devastating. She can pick up on very sweet or salty tastes, but the feeling of food in her mouth mostly just serves to remind her of what it used to be like. 

And that’s just eating. She especially misses the fragrance of the lilacs and roses in her garden, as well as other scents of nature. “In the summer, we go to the Jersey Shore for the day, usually Ocean City,” she says. “And I’m not saying that it’s not enjoyable; it’s just that there’s a piece of it that’s missing.” She still enjoys the sights, the sounds, and the feeling of sand between her toes. “But I miss the smell of the ocean.” 

Anosmics find it hard to describe their plight to others, largely because we lack the vocabulary to do so. While a car horn can be “beep,” an apple can be “tart,” and a flower, “pink,” a smell can only be described as its own shadow. Even perfumers use words that just swipe at the perimeter of smell, detecting only subtle differences in their
mixtures or comparing smells to other entities: “Floral.” “Freshly mowed grass.” “Romantic.” In A Natural History of the Senses, author Diane Ackerman discusses this problem: “When we try to describe a smell, words fail us like the fabrications they are,” she writes. “Smells are our dearest kin, but we cannot remember their names. Instead we tend to describe how they make us feel. Something smells ‘disgusting,’ ‘intoxicating,’ ‘sickening,’ ‘pleasurable,’ ‘delightful,’ ‘pulse-revving,’ ‘hypnotic,’ or ‘revolting.’” 

This lack of vocabulary tends to make our smell experiences private. Our taxonomy for the taste of wine, the chords of classical music, and the colors of modernist paintings is boundless: People make a living finding words to describe these. But with our smells, we are alone—they are more personal than any other sensory experience. Smell is the synapse between the world and the brain, and it’s our most visceral memory cue.

But maybe that’s the most satisfying thing about it. I share a lot these days: photos of glittering foliage, my most recent stab at a lattice pie, my Friday-afternoon playlists. I tweet about how awful it is getting out of bed on a freezing morning. But the way the first real day of summer drowns my nose in humidity with its steaming grass and hot pavement or the way the smell of tobacco transports me to the den where my long-dead grandfather kept his pipe collection belongs only to me. These are brief, specific glimpses of the past that only I can access. 

This is, perhaps, the strongest argument for the power of scent: It turns each breath into a puff of the world that’s ours alone. 


Annie Monjar is the managing editor of Philadelphia Magazine.

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