Of all the ways in which we sense the world, touch and smell are the only two in which we come into direct contact with the things around us. This is obvious in the case of touch: you must run your fingers over a brick wall to feel its roughness, or sit on a stone bench to feel its hardness, or step into a bath to feel the warm water on your skin. But it’s the same with smell. When you walk along a street in New York City and smell roasted pea- nuts, rotting garbage, grilled beef, and cooking oil, all mingled with diesel exhaust, you’re coming into contact with tiny bits of those things. Or if you sit still upon a hillside on a breezy day, and smell a sequence of fresh-mown hay, manure, water from the lake, wet mud from the ditch, and pine trees mixed with honeysuckle, it is because each one has come to you, riding on the wind. In both cases, the air has brought you a few molecules of each substance, to sample with your nose.
The first thing that happens when you take an ordinary breath is that air enters your nostrils. As it heads toward the back of your throat, it flows past three fleshy shelves in the interior of your nose. Some of it also passes over a structure deep inside, hidden behind the topmost shelf, against the base of the skull.
This is theolfactory organ,which detects odors. Its nerve cells, buried in a mucous membrane, send fibers up through a perfo- rated patch of skullbone so thin that it is easily fractured. If the bone breaks, meningitis can set in, because the brain at that point is very close to the germ-filled outside world. But the
patch of bone needs to be thin and perforated, so that the long fibers of the olfactory nerve cells have access to the brain.
When you sniff a flower, you take in more air than you do when breathing normally. The air flows and eddies past the ol- factory organ. In mammals, a single sniff lasts in the range of 100–250 thousandths of a second, just enough time for the brain’s olfactory centers to recognize and interpret the scent.
Rats are better at this than people—they can accurately identify an odor in less than 100 thousandths of a second. But although humans are slower, they are remarkably good at detecting and distinguishing more than ten thousand odors, in vanishingly small amounts.
When you touch something, you feel its texture. This gives you a clue to what it might be, but it doesn’t tell you what it is made of. When you smell something, you are identifying its chemical structure. The olfactory organ is an exquisitely sensi- tive chemical detector that can identify a substance on the basis of just a few molecules dissolved in air. Did you ever wonder why the air smells different on a warm summer evening after it has rained? The reason is that the drops of water coating every- thing—leaves, pavement, earth, grass—dissolve a little bit of these things, along with whatever is on their surface. As the wa- ter evaporates, molecules are released into the air in complex mixtures and you sense them through your olfactory organ. At various temperatures, different combinations of molecules dis- solve in the water and are released into the air. So the air smells different on a spring day, in the heat of summer, and in midwin- ter. The air’s scent can tell you the seasons, the time of day, and your location—countryside or city street, ocean beach or moun- tain lake. Gardenia, honeysuckle, orange blossom, and jasmine are all more pungent in the evening in summer. Fresh-mown grass and hay are summer, too. Wood burning in fireplaces is fall and winter.
Even if you have never been in southern Florida or along It- aly’s Amalfi Coast on a spring evening, you would immediately recognize the odors of methyl anthranilate and geosmin. And even if you had never received a dozen fragrant blooms on Val- entine’s Day, you would know two things about geranyl acetate:
that it is both related to and different from methyl anthranilate.
You would know that both scents are floral, and that they origi- nate from two different kinds of flowers. Geranyl acetate is the chemical that gives the rose its scent, and methyl anthranilate comes from orange blossoms. Geosmin (literally, “earth smell”) is a tertiary alcohol that comes from algae and that gives wet mud its scent. In 200 thousandths of a second, your nose and brain can detect chemical differences and similarities among these odors. The olfactory system registers smells much the same way the visual system registers images and the auditory system regis- ters sounds. First, the component features of the smell are de- tected; then the brain puts them together to form an olfactory image of your surroundings.
When the molecules of odor hit the mucous membrane of the olfactory organ, they dissolve in the mucous fluid around the nerve cells and are carried along by capillary action accord- ing to their size. This is exactly how chemists assess the relative size of molecules via chromatography, which operates on the fact that chemicals of different weights and electrical charges travel through water at different speeds. The larger, least soluble ones—the ones with low electrical charges—travel slowest, and the smaller, highly charged particles travel fastest.
When an odor molecule arrives at the olfactory organ, it comes into contact with the olfactory nerve cells. These cells have tinyciliaor hair-like structures which contain proteins of different shapes. The proteins are the receptors for the odor molecules. How can we possibly distinguish ten thousand odors?
How can we identify a scent in a flash of a second? Smell was the
last of the senses that scientists figured out. It seemed impossible that a single organ could possibly detect so many different odors so accurately, sensitively, and quickly.
The answer came in 1991, when Linda Buck and Richard Axel discovered a “super-family” of more than one thousand genes that determine our ability to detect and distinguish odors.
In 2004 they received the Nobel Prize for their discovery. Most of these genes are active in rodents and other animals. Only about three hundred fifty are active in humans, but they suffice, with all of their permutations and combinations, to enable us to detect the full range of odors in our environment.
This large family of closely related genes produces a group of closely related proteins. Each protein folds into a slightly differ- ent configuration, and in the middle of its folds contains a tiny pocket with a unique shape. It can detect the presence of a single molecule of water, or a certain type of carbon atom, or a ben- zene ring with a particular orientation. It works because each scent molecule has precisely the right shape to fit the cleft.
Once the properly shaped odor molecule lands on one of these proteins, it binds tightly in its pocket, like a key within a lock. A molecule that doesn’t fit will not bind—or if it fits im- perfectly, it binds less tightly. Once it clicks into its spot, the molecule changes the shape of the receptor protein on which it sits, thus starting a cascade of biochemical events inside the cell: a channel forms in the cell surface, allowing electrically charged sodium and calcium atoms (ions) to enter; as they do so, the charge of the cell membrane changes, triggering an elec- trical impulse that travels to the nerve ending. This is the begin- ning of the transformation of a scent into electrical impulses in the brain.
The nose can detect not only the type of chemical it smells, but also the chemical’s concentration. The lower the concentra- tion—that is, the fewer the number of molecules dissolved in a
given volume of air—the weaker the smell. The higher the con- centration, the stronger the smell. The closer you are to the source of the smell, the more pungent it will be, because there are more molecules concentrated around its source. As the dis- tance increases between you and the smell, the concentration gradientgoes from high to low: the number of molecules de- creases and the odor dissipates. Watch the puff of exhaust from a bus and you will see a dense cloud that gradually thins out as it floats away. You are watching the concentration gradient of the exhaust fumes. You can see these fumes because they consist of large particles, but the same happens with tiny molecules. The difference is that, with small molecules, you are perceiving the gradient with your nose instead of with your eyes.
When a dog follows a scent, it will follow the trail from weak to strong. The same is true for the wild pigs that sniff out truffles—those tasty French fungi that sell for hundreds of dol- lars per ounce. When the pigs pick up the scent, they will dig un- til they get to the source. Lobsters “sniff” the water with their antennae and can detect dissolved molecules. By comparing the concentrations detected by the two antennae, they can tell the direction the molecules are coming from and can trace them upstream until the concentration peaks. Our nostrils work the same way.
People—like dogs, rodents, pigs, and lobsters—smell in ste- reo. Each of our nostrils registers a slightly different concentra- tion of an odor, and our brain can detect this infinitesimal differ- ence. We can thus tell the direction the smell is coming from.
People are much less skilled at this than dogs, which can easily pick up a scent and follow a trail. But scientists have shown that humans can be trained to do this, too.
A team comprising researchers from the University of Cali- fornia at Berkeley, Pennsylvania State University, and the Weiz- mann Institute in Israel dribbled essential oil of chocolate
for ten meters through a grassy field, and tested whether people could follow the trail by scent. The thirty-two participants were not allowed to use any visual, auditory, or tactile cues—they were swathed in clothing, their eyes were masked, and their ears were covered. Crawling on hands and knees, twenty-one of the volunteers, nine women and twelve men, succeeded in tracking the scent on their first try. With nostrils blocked, they failed. The researchers then had two men and two women track the scent three times per day, three days per week, for two weeks. The participants improved dramatically in speed and accuracy with the training. They followed the same method that dogs use when tracking the scent of meat dragged over the ground: they sniffed repeatedly and zigzagged through the field. Though hu- mans’ nostrils are only about two centimeters apart, this is suf- ficient for people to detect slight differences in the concentra- tion of a scent cloud, and thus provides information about the scent’s location and source.
So as you go about your day, you encounter patches of scent that differ in shape and size and composition. Your perception of the intensity of each scent will vary not only with its concen- tration but also with the speed of your encounter—how quickly you move through the cloud. The information is coded in pack- ets of nerve-cell firings that fit the duration of each sniff you take. The speed of electrical firing is also determined by the concentration of the substance in the air: the more concen- trated the smell, the earlier and faster the nerve cells will fire. As a result, you gain a sense of both the chemical composition of your surroundings and the spatial distribution of each chemical in the air. Because the brain sorts these chemicals by individual features as well as by categories, you also know something about the family of fragrances that you encounter—whether they are floral, grassy, leafy, earthy, or briny; fishy or meaty; delicate or pungent; acrid or sweet. And in this way you form a 3-D image
of the chemical composition of the world around you. Besides informing you about the chemical and physical landscape, your sense of smell also tells you about your social landscape—who is nearby, whether you find them attractive, and what dangers might be lurking beyond the range of your eyes and ears.
In downtown Philadelphia, a few blocks from the 30th Street Train Station, the urban landscape offers an amazing sight: an enormous golden statue of a nose! This ten-foot-high gilded sculpture includes a pair of full lips and part of a face, but has no eyes or ears. It looks like something one might find in an Egyp- tian tomb. Created in the 1980s by sculptor Arlene Love, it is a perfect symbol for the organization over which it presides: the Monell Chemical Senses Center, a research institute founded in 1968 and affiliated with the University of Pennsylvania. The center is entirely devoted to the study of smell and taste.
In the 1980s, Lewis Thomas—physician, cancer researcher, immunologist, poet, and philosopher of science—became the institute’s board chairman. He had always had a particular inter- est in the phenomenon of smell. His father had been a country doctor. When Thomas was young, he had sometimes accompa- nied his father on visits to patients and had observed that smell was a very important part of the doctor’s diagnostic toolkit.
Prior to the early 1900s, doctors had only their noses as a means of detecting abnormal substances in body fluids. To test for dia- betes, they would smell or even taste the patient’s urine, to see if it was sweet. They would sniff the patient’s breath; an acrid smell indicated excess lactic acid, a symptom of diabetic coma.
The breath of a patient with pneumonia gave off the sickly sweet odor of certain kinds of bacteria. Even today, we use breath- alyzers to detect excess alcohol. For early twentieth-century doc- tors, their noses were their breathalyzers.
Thomas was intrigued by the fact that dogs had such keen
powers of odor detection—that, for instance, police dogs could find a person from a single sniff of a shred of clothing. One thing particularly struck him, as an immunologist: a dog can tell any two people apart, unless they’re identical twins. It’s the same with the immune system, which makes no distinction between people who have the same genetic makeup. Identical twins can receive transplants of organs and tissues from each other without becoming ill. All the rest of us require extensive tests to find a compatible donor. The molecules that determine such a match are calledmajor histocompatibility(MHC) mole- cules, and they are expressed on the surface of every cell. They are what give each individual, or each set of twins, a unique im- munological identity. In his book The Lives of a Cell (1975), Thomas proposed that histocompatibility antigens might also be involved in our sense of smell. This theory turned out to be cor- rect. Some fifteen years later, the researchers who proved it won the Nobel Prize.
At the same time, and independently, two researchers at the Memorial Sloan-Kettering Cancer Center, while breeding histocompatible mice for immunology research, noticed that male mice preferred to mate with a female who carried a differ- ent histocompatibility type. This observation and the Monell Center research that sprang from it proved what Thomas had postulated: that mammals do detect immune molecules through their nose. It is these molecules, excreted in the urine, that dogs sniff in order to tell which other dogs have marked the territory.
Other molecules can also attract.
In his essay “A Fear of Pheromones,” Lewis Thomas dis- cussed the remarkable effect exerted by a female moth when it sprays the pheromone bombykol. Pheromones are small, fatty, odorless molecules that easily dissolve in air and are released from sweat glands near the hair follicles in animals. Pheromones are very powerful attractants to the opposite sex. It has been
“soberly calculated,” Lewis wrote, that if a female moth were to release “all the bombykol in her sac in a single spray, all at once, she could theoretically attract a trillion males in the instant.”
(“This is, of course,” he noted wryly, “not done.”) The organ that detects these vaporous compounds is thevomeronasal com- plex,which is located much closer to the nostrils than the olfac- tory organ and consists of two tiny pits on either side of the nasal septum with direct connections to the brain, including the parts of the brain that govern reproduction. In his essay, Thomas referred to a seminal paper that had been published the year be- fore by a young Radcliffe student named Martha McClintock.
McClintock had noticed that all the women in her college dorm seemed to get their period at the same time. She published an article inNature postulating that pheromones could be the cause of the synchronization of the menstrual cycle in women who room together. The phenomenon also occurs in female ro- dents housed in the same cage. McClintock went on to study this phenomenon in more detail, and in 1998 published another paper inNatureconfirming and extending the original findings.
She showed that in other species pheromones govern many behaviors, including mate preference, dominance relationships, and weaning.
Researchers at the Monell Center found that humans can de- tect the moods of those around them through their sense of smell. The study started out as a science project by a seventh- grader named Daniel McGuire, the son of one of the research- ers. It was later replicated at the Monell Center laboratories and written up in the journal Perceptual Motor Skillsby researchers Denise Chen and Jeannette Haviland-Jones. Twenty-five col- lege-age men and women wore gauze pads under their arms while watching a fourteen-minute video that was either scary or funny. Forty women and thirty-seven men were then asked to smell the pads and report whether they thought the people who
had provided the pads were afraid or happy. The participants identified the smell of pads worn during the frightening video as
“scent of fear” more often than would be predicted by chance alone. Women proved to be much better at this task than men:
they correctly identified both “fear odors” and “happy odors.”
Although much research remains to be done on the way odors and volatile compounds secreted by our bodies relate to our moods, it is clear that these invisible compounds form an important part of our landscape and influence many physiologi- cal functions. How does our perception of chemicals in the air affect our bodies? Do scents have the power to heal?
Since the Middle Ages, pilgrims have walked the thousand- kilometer trail from the Tour St. Jacques in Paris, through the Massif Central in the heart of France, along the northern coast of Spain to the cathedral at Santiago de Compostela. The pil- grims took many routes, starting in various cities throughout western Europe, such as Frankfurt and Rome. Most passed through France, often through the town of Lourdes in the foot- hills of the Pyrenees. Others passed through Chartres, a town just outside of Paris. But when they crossed the Pyrenees, all converged on a single route: the Camino Frances (“French Way”). And all ended up in the town of Compostela, near the western tip of mainland Europe, a windswept ocean cliff called Finisterre, meaning “where the land ends” or “end of the world,” as it surely seemed to be.
The pilgrims’ goal was the shrine of Sant Iago (Saint James), who was an apostle of Jesus and whose remains, according to legend, were buried in the cathedral. Once the pilgrims arrived, filthy, wet, cold, and stinking from their long trek, they sought shelter in the church. They huddled on the floor and waited for the Mass to start. But before a word of prayer was uttered, six monks would hoist an enormous silver botafumeiro,or censer,