In some relationships, women are the heart, the vision, and the brains. In our marriage, my wife is the nose.
“What happened to the milk that was in the fridge?” she’ll ask.
“I drank it.”
“Didn’t you notice it went bad?”
“You were eating sardines again, weren’t you?” she’ll accuse.
“I had ’em for lunch 2 days ago!”
“There’s something dead in the house,” she’ll insist.
Sure enough, in the basement, behind the couch, is a tiny dead mouse.
“Women generally have a better sense of smell than men,” says Richard Doty, PhD, director of the Smell and Taste Center at the Hospital of the University of Pennsylvania. “And the disparity between the genders gets greater with age.”
The reason is not well understood, but it appears to be a cradle-to-grave advantage. “If you put breast pads [from nursing mothers] in a bassinet and observe the rooting or orienting behavior of babies, the females will generally be more responsive,” he says. And when researchers in Brazil autopsied the brains of men and women over 55 with jobs that could optimize their sense of smell (restaurant kitchens, for example), they found the women had 43% more cells in their olfactory bulbs on average than the men.
It’s just one of many fascinating facts being uncovered about our sense of smell, or olfaction. COVID-19’s adverse effect on olfaction brought it popular and scientific attention. We saw a push to develop a quick, simple, and affordable test for smell loss. (The Monell Chemical Senses Center in Philadelphia delivered a 1-minute, $1 SCENTinel rapid smell test.)
COVID-19 also produced a unique smell loss, with less than a third of those with the disease having any nasal blockage. We used to think congestion caused smell loss, but now we know that’s not always the case. Scientists are re-examining how viruses damage the olfactory system, both short- and long-term.
These are heady times for otolaryngologists, those who study and treat disorders of the nose. It was not always so. Darwin regarded smell as a rudimentary sense because its use in hunting, detecting danger, and other primal activities is now far less critical. And in a 2019 U.K. survey, 250 adults effectively turned up their noses at smell, ranking it the least valued of our five senses.
But smell remains crucial to our well-being. We’re learning more and more that our sense of smell is closely linked to our health — and could one day be used for monitoring our health and predicting disease.
What Happens When You Smell Something
Consider what happens when we detect an odor — something we do thousands of times a day. For something to have a smell, it must give off molecules. We inhale these molecules into the tops of our noses, where 6 million to 10 million specialized receptor cells await. Some molecules contain multiple chemicals, so they bind to families of receptors, creating a scent pattern.
Once this happens, the receptor cells message the olfactory bulb at the base of the brain, and the process of recognition and reaction begins. Sometimes, that happens right away (rotting flesh). Other times, it takes a bit. (“Hmm, what notes are you finding in the chardonnay?”) And sometimes, nothing registers at all. (When you have a cold, mucus prevents the molecules from fighting through.)
Joel Mainland, PhD, a neuroscientist and researcher at the Monell Center, estimates there are that 40 billion molecules that can have an odor. Some of these odors may smell the same or be undetectable by humans. It’s impossible to know for sure how many we can detect, but Doty puts it at “tens or even hundreds of thousands.”
Our sense of taste operates in a similar fashion, which is why people often confuse the two senses. Sensory cells in taste buds that line the tongue, back of the mouth, and palate detect chemicals in food molecules and relay that information to the brain. As we chew and swallow, some of these molecules are forced up through the nasal cavity to our old friends, the olfactory receptors, who contribute to the process.
To grasp this, Doty recommends pinching your nose shut while chewing a piece of chocolate. Closing the passageway between the oral and nasal cavities prevents food molecules from getting through, and you from tasting anything. The same thing happens when you have a cold.
“Most things we think of as taste depend upon the smell system,” he says. “That’s why a significant number of people who come into our clinic complaining of being unable to taste anything actually have a smell problem.”
The same receptors found in the nose have also turned up in the kidneys, heart, and lungs. Why that’s so isn’t clear, but Mainland believes it’s because these cells have multiple functions. For example, some appear in the “carotid body,” a small cluster of cells near the carotid artery in the neck, and “seem to be able to sense how much oxygen and carbon dioxide is present – basically measuring and responding to lactate as if it were an odor.”
Smells can also trigger memories, such as a sniff of a perfume that makes you think of the person who wore it, or the aroma of food cooking that reminds you of your grandmother. These “olfactory flashbacks” happen because the brain’s smell-processing center links to its emotion and memory hub. A study from Wheeling Jesuit University in West Virginia found that the smell of cinnamon improved brain function and working memory, a finding that could help us treat dementia.
What Your Sense of Smell Can Tell You About Your Health
Jayant Pinto, MD, a professor of surgery at University of Chicago Medicine, likens smell loss to a canary in a coal mine. “It doesn’t directly cause death,” he says, “but it’s a harbinger.”
In a 2014 study, he gave smell tests to 3,000 people ages 57 to 85. Five years later, he checked back. Nearly 40% of those who did poorly on the original test had died, compared to 19% who scored moderately and 10% who tested well. He calculated that a significant loss of smell is a stronger predictor of 5-year mortality (the likelihood of dying in the next 5 years) than emphysema, cancer, heart attack, stroke, diabetes, or congestive heart failure.
This study has since been replicated, so the association isn’t a one-time fluke. Other research links olfactory dysfunction to many neurodegenerative diseases (Parkinson’s, Alzheimer’s, epilepsy) and autoimmune disorders (multiple sclerosis, Crohn’s, myasthenia gravis).
“Smell dysfunction can be a very early indicator of these things,” says Doty, so if anything seems amiss, it can’t hurt to test for olfactory problems. Test options include the SCENTinel rapid smell test mentioned earlier or the gold-standard University of Pennsylvania/Sensonics’ Smell Identification Test (around $30). Both are self-administered and involve scratching and sniffing cards with various scents. The results provide a baseline for an individual’s sense of smell and may also indicate disease. For example, “about 90% of people who develop Parkinson’s had demonstrable smell loss early on,” Doty says.
It may even be possible to smell disease. My wife, “the nose,” is a registered nurse at our local hospital. Many years ago, when she was pregnant, she walked into a patient’s room and had to turn around right away. “There was an overwhelming smell of decomposition,” she recalls, “like if you were hiking and came upon a dead animal.”
The patient in that room had stage IV cancer, and she insists she could smell it. (Her hypersensitivity ended with the birth of our child.)
“In the 19th century, the smell of acetone [nail polish remover] in the breath was regarded as indicative of diabetes,” says Doty. “There’s also a metabolic disorder called maple syrup disease where the urine is sweet-smelling. There are many other examples in the medical literature where skin odor is related to certain diseases. There’s no reason why changes in the body that occur with certain diseases, if they end up in our saliva or blood, couldn’t be discerned by a dog or even a human.”
Indeed, scientists are training dogs (and even ants) to sniff out diseases like cancer, and at least one woman has shown that she’s able to detect Parkinson’s by smell.
Whether “super-smellers” — people with a hypersensitive sense of smell – exist is controversial. But Doty and Mainland agree that the smeller spectrum varies widely. Just as with everything else, some people are better at it than others.
You May Not Be Sick — Just Aging
Like hearing and vision, our sense of smell gradually weakens over time. Doty’s research shows that 75% of people over 80 have “some demonstrable deficit.” Between ages 65 and 80, this is true for half the population. And among those over 65, 5% to 15% have no sense of smell at all (a condition called anosmia).
This decline is believed to be caused, at least in part, by the colds we catch over the years. “The olfactory endothelium, or membrane at the top of the nose, accumulates little islands of damage called metaplasia every time we have a bad cold,” explains Doty. “So, by the time we’re in our 60s and 70s, something that otherwise would be innocuous, like the common cold, can take us over the waterfall.”
A big area of research right now is whether age-related smell loss can be slowed or stopped, and if our sense of smell can be improved. Not really: Receptor cells can’t be strengthened by working them out, so to speak. And once they’re damaged by viruses, accident, or aging, they can’t be regenerated.
But you can learn to smell better. The trendy notion of “olfactory training” is a little misleading; it’s the brain that can be trained. We can teach ourselves to recognize and identify new scents. Mainland tries to smell new things every day, even going as far as to order unique scents from perfumers to sample. Wine sommeliers essentially do the same thing, exposing themselves to many varieties of wine to learn their scent nuances.
The Future of Smell Research
As Darwin noted, we no longer spend our days sniffing the ground and tracking prey. But the genes that governed these ancient behaviors are still with us, explains neuroscientist Marissa Kamarck, PhD. And not only can scientists detect the original or ancestral version of these genes (so-called non-functional or pseudo genes), but they can also identify variants or newer versions of them. In a recent study that she co-authored with Mainland, Kamarck found evidence for the theory that our sense of smell, as a species, may be degrading.
“We found that most often, the variants that predicted lower intensity [for smells] were the newer variants,” she says. “And in olfaction, our genes are mutating faster than in other families of genes,” Mainland says.
If anything is happening, it will take centuries to unfold. And any ability to smell that we have lost may have been replaced or compensated for by our gaining new ones. (Like smelling a gas leak, something prehistoric man had no need to recognize or fear.) Like every other part of us, our sense of smell is always evolving.
Even more intriguing discoveries lie ahead. Mainland points out that the mapping from chemical structure to olfactory perception is unknown (unlike in vision, where wavelength translates into color, and in hearing, where frequency predicts pitch).
“There is not a scientist or perfumer in the world who can view a novel molecular structure and predict how it will smell,” he says. His research goal is to develop that.
Indeed, a recent global survey by the Ericsson ConsumerLab found that most consumers expect to be able to smell movies and even products digitally through an “Internet of Senses” by 2030.
Imagine that. If my wife is at work, I could text her a sample scent of the milk to see if it’s safe to drink.
Richard Doty, PhD, director, Smell and Taste Center, Hospital of the University of Pennsylvania, Philadelphia.
Joel Mainland, PhD, neuroscientist and researcher, Monell Chemical Senses Center, Philadelphia.
Jayant Pinto, MD, professor of surgery, University of Chicago Medicine.
Marissa Kamarck, PhD, neuroscientist and researcher, Monell Chemical Senses Center, Philadelphia.
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