Scientists once compared the abilities of humans versus canines in tracking a trail of chocolate essential oil laid down in an open field. Though the humans weren't nearly as proficient as the dogs, they did get better with practice. Vladimir Godnik/Getty Images/fStop
About 25 years ago, after a particularly bad cold, I suddenly lost my sense of smell — I could no longer sense the difference between sweaty tennis shoes and a fragrant rose. Since then, my olfactory discernment comes and goes, and most of the time it's just gone. I always figured there wasn't much I could do about that, and it hasn't been terrible. My taste buds still work, and I adore fine chocolate.
But when COVID-19 hit, the inability to detect odors and fragrances became a diagnostic symptom that upset a lot of COVID-19 sufferers, many of whom also lost their sense of taste. That got me thinking — what does it really mean to have a disordered sense of smell? Does it matter that with my eyes closed I can't tell if I'm in an overripe gym or a perfume store? And is there hope that I'll ever again be able to smell a wet dog or freesia or a gas leak or a raw onion?
I went searching for answers.
What enables us to detect aromas, anyway?
Scientists explain that when you put your nose in the way of steam rising from a hot cup of coffee, molecules called odorants rise up and land high up in your nose. And when you take a swig of that same joe, as the liquid goes down your throat, some molecules rise upward and hit that sweet spot. Nerve cells there have receptors that recognize specific molecules, and those nerve cells extend directly into the brain. "That's how you tell you're smelling coffee as opposed to pizza," says Pamela Dalton of the Monell Chemical Senses Center in Philadelphia, who studies how we perceive good smells and bad. When the coffee "odorants" connect with their nerve cells, she says, your brain knows that you've just enjoyed your morning brew.
It's a pretty incredible system. Back in the 1920s, based on what was known about nasal anatomy at the time, researchers calculated that humans could discriminate among 6,561 odors. That number eventually got rounded up to 10,000, and while it was a calculation and not a measurement, the number became accepted wisdom.
At Rockefeller University recently, scientists tested people with combinations of different chemicals and from those studies estimated that people actually can sense greater than 1 trillion smells — though the researchers were also quick to note that their study doesn't mean there are a trillion smells to be smelled, just that humans could tell the difference among a trillion scents. For now, the true number of odor molecules that humans can detect remains a mystery.
How does that measure up to your pet dog's abilities? Conclusive head-to-head comparisons between humans and animals are hard to find. But people might do better than you think. Neuroscientist John McGann of Rutgers University claimed in Science a few years ago that the human olfactory bulb, where those nerves from the nose end up, is actually quite astute. He compiled a half dozen studies showing that people are better than animals at detecting some smells, and worse at others, leading him to conclude that "our sense of smell is similar to that of other mammals."
In 2006, scientists from the University of California, Berkeley reported that they'd trained humans to track a trail of chocolate essential oil laid down in an open field. (Seriously. The cover of Nature Neuroscience that week actually had a photo of a blindfolded volunteer facedown in a field, tracking a scent.) The humans weren't nearly as good at the task as the dogs were, but did get better with practice.
So how come I can't even smell a freshly opened bar of chocolate?
How common are smell disorders?
I'm far from alone in my deficit. The National Institute on Deafness and Other Communication Disorders, the division of the National Institutes of Health that deals with taste and smell, says 23% of Americans over age 40 report some alteration in their sense of smell, as do 32% of those over 80 — and that's from data gathered long before the COVID-19 pandemic. Some folks can't smell anything at all — that's called anosmia. Others, like me, have only a partial sense of accurate odor detection — hyposmia. Some smell one thing for another — that's parosmia. And then there's phantosmia, where people smell things that aren't there at all.
What else besides COVID-19 can damage the sense of smell?
Lots of things! Aging is a risk factor, as are swollen sinuses, nasal polyps, heavy drinking, brain damage, and a history of dry mouth, according to NIDCD. Nerve damage caused by Alzheimer's disease, diabetes, poor nutrition, brain tumors, Parkinson's disease and other conditions can interrupt the normal flow of information from your nose to your brain. My disorder followed a bad cold — probably because swollen sinuses covered up my odorant receptors, the experts tell me.
Why my problem didn't go away when the swelling resolved is anyone's guess, but the original insult likely came from a virus.
"It's been really hard to put various viruses into humans and see what parts of the olfactory system they actually disrupt," says Dalton.
There are a bunch of possible mechanisms, she says. By attracting immune chemicals known as cytokines, a virus might disrupt the chemical balance that allows the receptors to work. The same or another virus might kill olfactory nerves, or kill the cells responsible for regenerating those nerves. And viruses could even enter the olfactory nerves and travel right up into the brain — the olfactory bulb — and do damage there. Research with the virus that causes COVID-19 might soon explain how it works to disturb smell — but other viruses might act differently.
Head trauma can actually tear the nerves. "When those nerves shear off, sometimes they don't grow back," says John Hayes, professor of food science at Pennsylvania State University. "Other times they grow back and actually get miswired." That's parosmia, he says — which can leave a poor soul thinking a fresh banana smells like burning rubber.
What's the connection between smell and taste?
While there's a fair amount of overlap between taste and smell disorders, most people who can't smell food can still taste it. I enjoy an orange or a piece of chocolate or a nice frittata as well as I did when I could still smell.
Here's why: The taste and smell systems operate independently. Smell relies on the neurons that start at the top of the nose and go straight to the brain. Taste arises from the 2,000 to 10,000 sweet, sour, salt, bitter and umami taste buds in your mouth and elsewhere in your oral cavity. The connecting nerves take their own independent path into your noggin. They do not use the nose's olfactory nerves.
Hayes says the misperception that people who can't smell things can't taste them either stems from the way scientists use the word "taste" versus the way the rest of us do. In science, he says, "taste is strictly defined as nonvolatile chemicals triggering sensations in the oral cavity." You don't need a sense of smell to taste sweet, sour, salt, bitter, and umami. While a COVID-related loss of smell appears to track closely with a loss of taste, other causes of a sudden inability to detect or discern odors and fragrances rarely do.
The taste of food may blossom when you add in its aroma, but even people with anosmia can still taste an onion. Plus texture, and "chemesthesis" — the tingle or burning or cooling effects of foods like hot peppers or mint — contribute to the experience of taste, as well.
Dalton says some people who've lost their sense of smell still have 'olfactory memory,' which allows them to conjure up aromas the way some people can hear a piece of music in their heads.
Given that the loss of smell is not so terrible for me, I'm probably among them, Dalton tells me. "You can take joy and pleasure in what remains, even if it's a fill-in from memory."
"I wouldn't call it a lesser experience," Hayes says. "I would say it's a different experience." And I would agree with Hayes.
Once you've lost your sense of smell, can you ever get it back?
That depends on the cause. Nerves damaged by the pressure of swollen sinuses sometimes repair themselves, the evidence suggests, though my doctor says mine are unlikely to at this late date. Anti-inflammatory drugs, or surgery to remove swollen tissue or polyps, can help some people, maybe by allowing odorants to reach their receptors. And many smell and taste deficits caused by COVID-19 in the last year appear to be clearing up over time.
But how often that happens for people who lose their sense of smell for other reasons remains one of the mysteries of the — osmias — there just hasn't been all that much research.
The pandemic has increased interest in "smell training," where people practice by sniffing five or six strong scents twice a day. I tried that earlier this year. But three months of training and using a nasal steroid solution to reduce swelling didn't help me. My otolaryngologist co-authored a literature review of olfactory training that concluded that it's a low-risk intervention that is effective in a minority of people (21%, in one of the studies), but it generally doesn't work for long-termers like me.
What's life like without a sense of smell?
People's reactions are all over the map. For some, not being able to smell a salty ocean breeze or brownies baking is an immense loss — it's even kicked off some cases of depression. Some people lose interest in food: A members-only Facebook support group run by AbScent, a U.K.-based membership organization of people who have a disordered sense of smell, has many reports of people who have real trouble finding foods they enjoy eating.
For others, like me, there's just the occasional pang of nostalgia — I especially miss the smell of fresh cut grass and little babies. Those brief, random moments once or twice a month when I can detect an odor can sometimes be disturbing, too — a bit too much information coming in about when passengers in my subway car had their last shower.
It's worth noting that smell disturbances also can be downright dangerous. In a review of patient histories done between 1983 and 2001, nearly half of patients with anosmia reported at least one "olfactory-related hazardous event" such as missing a gas leak. My doctor long ago ordered me to switch to an electric stove instead of gas, for safety's sake, and to keep the piercing smoke alarms in my house in good working order.
Then there's the 2014 study that found that roughly 15% of people with disturbances in their sense of smell had mistakenly eaten rancid or otherwise spoiled foods. Ugh. Luckily I have a husband willing to smell a quart of milk that's near its pull date and will let me know if there's a problem before I take a swig.
Has COVID-19 taught us more about a lost sense of smell and how to get it back?
As with all things COVID-19, the virus is setting its own path. Most other smell disorders are not related to taste disorders, but the two problems seem entwined for people who trace their loss to SARS-CoV-2. According to NIDCD, suddenly not being able to smell anything is a better predictor of infection with this coronavirus than fatigue, fever or cough. And loss of smell can serve as a sentinel — it often happens before other symptoms develop. The Monell Center reports that 77% of people with COVID-19 fail "smell tests," and the connection has brought heightened interest to smell disorders in general.
NIH reports it supported 205 studies on the sense of smell in 2020; NIDCD'S budget for such topics was $65 million in fiscal year 2020, and just three months ago, the institute announced four new projects aimed at using taste and smell testing to screen for early signs of COVID-19.
Meanwhile, AbScent is working to raise research funds and help scientists collect data on its members.
Researchers still have more questions than answers, but all this increased attention on the loss of smell because of the pandemic has given me some hope.
"We've always been the poor orphan of the senses," says Monell's Dalton. "This sensory system has finally begun to receive the attention it deserves. And anything we learn from COVID could potentially help others."
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