If all our genius lies in our nostrils, as Nietzsche remarked, the nose is an untrained genius, brilliant but erratic. The human nose can detect a dizzying array of smells, with a theoretical upper limit of one trillion smells—yet many of us are incapable of describing these smells in words more precise than smelly and fragrant.
Our auditory and visual receptors offer little mystery—they were mapped and explained by scientists many decades ago—but human olfactory receptors were discovered only in 1991. This might be an indication of the massive complexity of smelling: the human body has only four visual receptors, compared with more than four hundred olfactory receptors. Or perhaps it’s a matter of cultural priority: smells are so often thought of as unwanted intrusions.
Smell begins when odorous molecules—often called “odorants”—are whisked through the air into the nose. The air bumps through the nasal passages, where it is warmed and filtered, and arrives at the olfactory epithelium, a mucous-lined layer of the nasal cavity where olfactory neurons nestle like carrots in earth. These neurons detect the smells, but it’s the olfactory receptor proteins that actually bind to odorants. These receptors, in turn, fire an electrical signal to the olfactory bulbs, two buds that hang from a bundle of nerves connecting to the brain, and which are located at the bridge of the nose, roughly where your eyeglasses would rest.
The olfactory bulbs are thought to be the brain’s primary processing center for smell . The bulbs take in information from the olfactory receptors, encode it into a unique odor signal, and then pass this signal to the olfactory center in the brain’s cortex. Olfactory neurons regenerate every four to eight weeks; over time, they respond to whatever smells they encounter most often. That means you can train your nose to smell more effectively just by practicing.
How a receptor detects a smell remains a deep riddle to scientists. The odorant’s shape appears to determine which olfactory receptors it binds to; beyond that, we have no idea why molecules smell as they do. Take the example of benzaldehyde, which smells like bitter almonds, and which can be found in maraschino cherries and marzipan. Add a double-bond of oxygen to its tail and the smell shifts to cinnamon. But throw on another five-carbon atom chain and the smell shifts again to a generic floral. There’s no pattern at the molecular level that scientists can discern—yet.
Olfactory receptors are concentrated in your nose but exist throughout the body. Your kidneys, for example, have olfactory receptors—they can “smell” signals from your gut bacteria after meals and moderate your blood pressure in response. (Like taste, smell is a chemical sense; what the kidney receptors are really doing is detecting chemical changes.) Sperm, too, are guided by smell: swimming blindly in a silent void, they are drawn by the egg. Your lungs, blood vessels, muscles: they’re all constantly smelling.
Smells can be detected by technology. Electronic noses monitor food factories for spoilage and nuclear reactors for leaks. But we won’t transmit smells on the internet anytime soon—an electronic exchange of smells is impossible. Attempts to do so—like Cyrano, the 2016 “digital smell speaker,” or its 2014 predecessor by the same inventor, oNotes —are basically cons. What’s transmitted is not the smell itself but an electronic signal that releases a prepared vial of scent on the receiving end. Smells can be digitized and recorded, but they can’t be mediated by telecommunications. You are the smeller, and you can smell things only in person.
- I say “thought to be” because this basic relay of signals has recently been thrown into doubt by a controversial study of certain left-handed women who apparently lack olfactory bulbs but can still smell everything normally.
- Notes allowed an iPhone user to snap a photo and tag it with smell descriptors in a companion app called oSnap. The user could then send this message—the oNote—to another oSnap user whose phone was connected to a receiving device, the oPhone DUO. This device would then emit a preloaded smell that best matched the transmitted descriptors. (When oPhone DUO couldn’t find a match in its preloaded scents, which was often, the receiving phone’s mobile app would just offer a vivid description of the smell.)
In the Indian city of Kannauj, perfumers have been making a scent called mitti attar for centuries. From April through May, workers pry blocks of parched clay from the ground. The blocks are baked into disks in ovens, which are then warmed over water distillers. When the clay reaches the right temperature, steam slowly releases the earth’s smell. That steam is captured and distilled into vats of sandalwood oil, the perfume’s base. The smell that’s released is petrichor, the scent of dry earth after rain, a bright mineral tang edged with a hint of sourness. In Kannauj, this is the scent of the precise day when months of dry heat give way to monsoons.
The Greek word petrichor, meaning “blood of stones,” was coined in 1964 by two Australian scientists, Isabel “Joy” Bear and Richard Thomas, who were researching this substance and its purpose. As part of their work, the scientists extracted a golden oil from a variety of soil types: sand, dirt, clay. They learned that plants secrete these fatty acids, mostly palmitic and stearic acids, into the soil, and that those secretions become concentrated between rainfalls. After a drought ends, plants often have a surge of growth, and so Bear and Thomas suspected that petrichor was a fertilizer. It turns out, however, that it’s a defense. Plants secrete the fatty acids to slow the growth of nearby plants, reducing competition when water is scarce.
Petrichor emanates from millions of raindrops falling all at once, giving this smell a stereoscopic quality. In 2015, a team of MIT scientists determined how petrichor reaches our noses. Using high-speed cameras, they observed that when a raindrop hits a porous surface, it traps tiny air bubbles at the point of contact. In a slow-motion video, you can see a raindrop hitting a surface. The drop briefly assumes a doughnut shape, then flattens into a disk. Infinitesimal droplets rise from the disk like fireflies buzzing over a lake. Those droplets lift petrichor from the soil, infusing the air.
Petrichor reminds you of the idea that smell is what happens when a substance becomes airborne. In the case of petrichor, that substance is a golden oil, secreted into the soil by rivalrous plants. Smells ride on air and impart personality to it.
Petrichor also reminds us that air is three-dimensional. The air’s stillness after a rain shower is monumental, pungent, and temporary. (Picture petrichor as a golden cube of air above the ground, trembling like amber-scented Jell-O.) As you smell, you’re observing a change happening here and now. With petrichor, it’s particularly noticeable. Each smell pins a moment in space and time, with you, the smeller, as its witness.
Body odor exists on three levels. At the uppermost level is what’s known as BO, which is battled (or not) with deodorants, showers, and fragrances. The midlevel smell is determined by cultural factors like diet, occupation, and hygiene. Beneath these scented layers of sweat, lotions, and last night’s meal, you’ll find a person’s baseline smell. This note is subtle, enveloping, and unchanging beneath the daily fluctuations. Unlike synthetic fragrances, which are designed to be noticed, this baseline scent is amplified only by body heat. To observe it at all requires drawing close.
Among other tangled human reasons, we choose mates whose bodies smell good to us, partners whose major histocompatibility complex (MHC) genes differ robustly from our own.
Mating by smell is complex. For example, smell preferences can get scrambled in women who take oral contraceptives. As their bodies are chemically tricked into believing they’re pregnant, these women prefer the smell of partners—male or female—whose MHC profiles resemble their own. (Imagine the chaos of going off the pill, and off your partner’s scent: How much is smell implicated in divorce?) Mating isn’t always centered on baby-making, either. When asked to sniff T-shirts worn by gay or straight men, gay men can identify—and prefer—the smell of other gay men.
Many diseases announce themselves with a shift in bodily odor. Typhus makes the body smell of freshly baked brown bread, tuberculosis of stale beer, yellow fever of the butcher’s shop, plague of overripe apples. Diagnosis by smell is both quaintly outmoded—how recognizable is measles today by its smell of plucked feathers?—and newly relevant. Trained doctors and dogs can detect the smells of Parkinson’s disease, malaria, multiple sclerosis, COVID-19, and cancers of many kinds.
Your baseline smell is unique to you, more so than a thumbprint. The digital chemical sensors used at TSA checkpoints could theoretically be updated to differentiate you from your identical twin or match you to a database of individual smells. (You can’t willfully stop emitting your baseline smell.) Bodily smell also reveals our thoughts, or at least the heated pendulum of our moods. Apocrine glands work overtime during stressful situations, giving emotional sweating a more pungent tang than heat-reduction sweating. Humans can detect joy, fear, frustration, and sadness by smell alone. Each person’s smell forms a nimbus or halo around their body.
I’ve been inhaling my partner’s scent for a quarter century, but if asked, I would have trouble describing its particulars. It’s warm, well balanced. However, I can describe what it registers inside me. Burying my face in his neck is an action I’ve repeated thousands—maybe millions—of times. His smell is a particular and deep kind of home. I feel still, encircled, known. My blood pressure plummets and my stress hormones evaporate. (Your lover smells this way to you too.) To me this smells like relief, like a concentrated jet of grace.
Pencil wood used to be much more fragrant. Pencils in the early nineteenth century were made of eastern red cedar from Florida, Georgia, and Tennessee. These pencils smelled of spicy black pepper and cinnamon and their shavings were colored pinkish-red. As recently as 1890, red cedar was so abundant that Southerners built barns and fences with it—until the production of millions of pencils thinned the wood supply and jacked up prices.
The US Forest Service recommended incense cedar as a replacement wood for pencils: it was cheap and functionally ideal. But manufacturers worried that pencils made with incense cedar—the pencils we have now—would be too pale and too weakly scented to fly with consumers. So around the year 1920, manufacturers dyed and perfumed incense cedar to simulate red cedar. History doesn’t record when this practice stopped, but eventually, the smell of pencils dwindled.
As for what’s inside the pencils, it turns out that pure graphite has no smell. But pencil manufacturing has progressed a long way from pure graphite. In their original form, pencils were made of a lead alloy wrapped in paper or string; then pure graphite slats were encased in red cedar wood; then graphite powder was purified and mixed with clay or wax to form a slat encased in wood. These additions helped stretch the limited supply of graphite in a way that was truly industrial and scalable. And it gave pencils variety as a consumer product. A mix containing more clay and wax yields a sharper, paler line, whereas a pencil containing more graphite writes darker and softer. The mechanical pencil inserts of today smell bright, clean, forward, and uncomplicatedly metallic. That smell isn’t from the graphite, though—it’s from the clay and wax additives. And yet the smell of modern graphite is worth pausing over. In a real sense, it’s the smell of an early triumph of the Industrial Revolution.
The smell at the other end of a pencil, the eraser, isn’t usually detectable: it’s often too dried-out to smell like much of anything. But a block eraser’s smell, on the other hand, is an indicator of its quality. An odorless eraser is a cheap and useless one, while aromatic erasers contain more natural rubber, the best material for erasing. Natural rubber erasers smell cheerful, ugly, forthright. Erasers smell most when they’re being used. The act of erasing makes us hunch closer to the paper. To blow away the twisted rubber strings, you must first inhale and smell what you’re doing. This is smell in action—a pencil’s odors of incense cedar, clay, wax, and rubber emerge not while you are writing, but while you are sharpening and erasing, in the pauses between.
Camphor demands attention. It pierces the nose, spreading a frosty latticework over the face. After that first sharp hit, the smell recedes into a pleasing, wintergreen roundness. Sniffing camphor is rousing, like drifting through a current of cold water in a warm lake.
These intense qualities appealed to mathematician Francis Galton. His thought experiment in what he called “arithmetic by smell” was carried out with various smells, including camphor, which he documented in a quirky 1894 paper. “Arithmetic may be performed by the sole medium of imaginary smells, just as by imaginary figures or sounds,” Galton wrote. “I taught myself to associate two whiffs of peppermint with one whiff of camphor; three of peppermint with one of carbolic acid, and so on . . .” Galton declared the experiment successful, adding and subtracting scents with giddy abandon, although he “did not attempt multiplication by smell.”
Camphor activates what is called the trigeminal system of temperature, touch, and pain nerves in the face and nose. Like other trigeminal smells, such as eucalyptus, skunk spray, and habanero peppers, camphor provides an example of how our senses can blur, compound, amplify, and overlap each other. No sense operates in isolation from the others—if one is removed from the equation, it affects the whole. This explains why, for instance, it’s difficult to balance on one leg while blindfolded. Other sense interactions are even more curious. A group of scientists recently found neural evidence in mice of “smounds,” smells whose perception is directly affected by hearing a particular tone at the same time.
Camphor comes from the wood of Southeast Asian laurel trees. Its potency is by design: the smell repels insects and fights microbes. In humans, it reduces inflammation, numbs aches and pains, clears the nasal passageways, and calms a cough. The Chinese nickname for camphor, ping-pien, or “ice flakes,” suggests its sensory  effect, while its other nickname, lung-nao-hsiang, or “dragon’s brain perfume,”  evokes an otherworldly quality. Sanskrit poetry associates camphor with the moon. In the tenth-century poem “Saundaryalahari,” camphor flakes falling from the lips of the goddess Devi cool the searing heat of three smoldering cities.
In Hindu temples, burning camphor is used to activate the third eye, stoke intention for prayer, and purify the mind. (The etymology of the word smell in most languages relates to the word for smoke.) Smells like camphor can demarcate a sacred space and time for deep contemplation. In its fragile intensity, a smell can be a pop-up mosque or chapel: a spiritual state you dwell in briefly, before it wafts invisibly off.
- As opposed to sensuous. There’s a whole body of premodern literature in several cultures that considers camphor an anti-aphrodisiac.
- Also the title of a book by R. A. Donkin, Dragon’s Brain Perfume: An Historical Geography of Camphor.
Gunpowder recipes have changed over time, but its bouquet has remained consistent: eggy, sulfuric black powder as the base note. A tang of urine from saltpeter. The particulate density of charcoal.
Another way to say it is that cannon fire smells kinetic. It fills the empty air with mass, boom, taste, and grit. It roils and churns the air, remixing its smells. If a street scene pixelated into rubble before your eyes, it would look as unreal as a movie. But the smells and dust clogging your nostrils would be undeniably real.
Illness isn’t usually considered among the ills humans have fought with cannons. But cannon fire was once employed to ward off the plague. According to miasma theory, prevalent until 1880 , disease was caused by bad smells. Miasmas leaked from garbage heaps, privies, hospitals, and poorhouses, and also from cemeteries, swamps, caves, even cracks in dirt sidewalks. Some miasmas killed outright; others could be neutralized with aeration and by maintaining one’s personal odor balance. Bathing was cautioned against: better to keep your skin’s pores plugged against disease.
The prevalent strategies for fighting miasmas were wrong-headed in a way that only debunked science can be. Annick Le Guérer’s book Scent: The Mysterious and Essential Powers of Smell traces their false logic. Some scientific factions favored counteracting miasmas with pleasant smells like scent boxes, cigars, lozenges, and syrups; a wealthy person could carry a tiny, fragrant citrus tree or wear a perfumed sachet near the heart. One sixteenth-century text advises physicians to approach patients armed with a juniper branch and a pomander ball. (One reason for the advent of the stethoscope was that it enabled the examination of a patient at a distance.)
Other factions fought miasmic smells with even worse smells. Jean de Lampérière, in a 1622 text, suggested a protective body rub of dried peacock dung and goat urine. There may have been some merit to this strategy, however—the smell of goats (and cattle, sheep, and camels) does in fact repel the fleas and ticks that transmit the bubonic plague.
Enter the cannon. In seventeenth-century France, professional perfumers carried out plague fumigations, decontaminating households after victims were carted off. Perfumers lit bonfires in front of the victim’s house, shut all windows, and then set to work indoors. They burned perfumes in pans, gutted straw mattresses, put dirty linens in hot ovens to cook the bad odors out of them, and so on. The checklist was vigorous, extensive, and always smelly. Meticulous perfumers capped off their work by firing cannons in the streets to “dispel the infection that may linger in the woodwork or on the outer walls of buildings,” according to one contemporary report. (Of course, the cannon firing caused its own problems, rupturing building foundations, breaking windows, and attracting looters.)
The fight against nasty odors often does more than eliminate them. The battle against miasmas—and noxious urban smells, in the era of germ theory—shaped the urban landscape. Invisible smells produced real and visible effects. Decisive moments of reek spurred swift reforms. In the summer of 1858, London suffered a heat wave that left the Thames running low and revealed that the river, from which the city drew its drinking water, was basically an open sewer. For six weeks, later termed the Great Stink of London, the city was suffused with the revealed stench of excrement. One self-styled “Sufferer in Thames Street” described the smells emanating from the waterfront as “a thick, warm steam, surcharged with odeurs from every imaginable abomination penetrating into the apartment, and into you.” With blinding speed, Parliament green-lit a public works project that had previously been stuck in committee, creating a massive system of sewers, pumping stations, and water treatment plants. Similarly, Paris suffered its own Great Stink in 1880, resulting in similarly rapid and far-reaching urban planning reforms. To fight the smells, lawmakers on both sides of the English Channel paved sidewalks and whitewashed walls, engineered sewer systems, established health boards and zoning ordinances, introduced environmental reforms, widened streets, and planted public gardens to serve as “urban lungs.”  Disgusting odors are nothing if not intensely motivating.
As a smell, cannon fire has a long and surprisingly varied backstory. It’s the smell that used to waft over scenes of warfare and of plague. It has signified both destruction and protection. And now, in the modern era, you smell cannon fire almost nowhere at all, a poignant end for such a potent scent.
- Miasma theory dates as far back as Seneca.
- Melanie A. Kiechle wrote an entire book about this topic called Smell Detectives: An Olfactory History of Nineteenth-Century Urban America.