Nick Johnson skims the lunch menu at the White Dog Cafe, a warren of little rooms and ante-rooms in Philadelphia's university district. "Beef empanadas… I would have loved those. But all that braised beef would just get lost on me. Fish and chips I avoid: all fried foods taste the same. I'm looking at the fish tacos. I know I'll get the spicy heat and a little bit of pineapple flavor, and with the peppers and the guacamole, there'll be some mouthfeel there."

Losing your sense of smell takes away more than scents and flavors — it can fundamentally change the way you relate to other people, writes Emma Young.

He orders the tacos, and we get a beer that's on tap. It's called Nugget Nectar, and it's produced by the local craft brewery that Nick's worked at for the past ten years. Nugget Nectar used to be his favorite beer. "It has a real nice balance of sweetness and hops. But now," he says, and his face falls, "it's a shell of its former self to me." He can describe what it smells like: "piney", "citrusy", "grapefruity". But he can't smell it any more.

We don't think of ourselves as being particularly good smellers, especially compared with other animals. But research shows that smells can have a powerful subconscious influence on human thoughts and behavior. People who can no longer smell — following an accident or illness — report a strong sense of loss, with impacts on their lives they could never have imagined. Perhaps we don't rank smell very highly among our senses because it's hard to appreciate what it does for us — until it's gone.

Nick, who's 34, can pinpoint the moment he lost his sense of smell. It was 9 January 2014. He was playing ice hockey with friends on the frozen pond at his parents' place in Collegeville, Pennsylvania. "I've done it millions of times," he says. "I was skating backwards, slowly, and I hit a rut on the ice. My feet went out from under me. I hit the back right side of my head. I was out. I came to in the ambulance, people surrounding me, blood pouring out of my ear." He had ruptured an eardrum and fractured his skull in three places. He had blood on his brain, and was suffering from seizures. "I had no idea what was going on."

Smells can have a powerful subconscious influence on human thoughts and behavior.

After making a rapid recovery, he was cleared to drive again six weeks later and returned to work as regional sales manager at Tröegs brewery. Before long, he found himself in a meeting about a new beer. "We were tasting it, and the others were saying, 'Can you smell the hops in the beer?'… and I couldn't. Then I tasted it. There were guys saying, 'It's got this pale biscuity flavor'… and I couldn't taste it. Then I went and tried one of the hoppier ones… and I couldn't smell it. That's when it clicked."

The stress of the injury and all the medication perhaps explain why he didn't realize he had lost his sense of smell sooner. It came as a shock, he says. Now, though, he is acutely aware of the effects it has had.

Losing enjoyment of food and drink is a common complaint for people who lose their sense of smell. You can taste sweet, salty, bitter, sour and umami with your tongue. More complex flavors – like grapefruit or barbecued steak – depend on smell. But for Nick, as for many people who can't smell, there's another category of loss altogether.

At the time of his accident, Nick's wife was eight months pregnant with their second child. Over lunch, he says: "I joke I can't smell my daughter's diaper. But I can't smell my daughter. She was up at 4 o'clock this morning. I was holding her, we were laying in bed. I know what my son smelt like as a little baby, as a young kid. Sometimes not so good, but he still had that great little kid smell to him. With her, I've never experienced that."



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Nick takes a deep sniff of his glass of Nugget Nectar, the beer that was once his favorite. Volatile chemicals from the liquid are drawn high up into his nostrils, to the roof of his nasal cavity, the part specialized for smelling. Then he takes a sip, and those same chemicals travel up from the back of his mouth to the same part of his nose. So far, so good.

Next, the molecules are absorbed into the mucus inside his nose. This is critical for something to be smelly: at the moment, no one can look at a molecule and say, based solely on its structure, how it will smell, or even whether it will smell at all. All we know is that for something to have a smell, its molecules must easily evaporate so they can be carried in air and inhaled, but they must also dissolve in mucus to be detected.

'I joke I can't smell my daughter's diaper. But I can't smell my daughter.'

For a healthy person sniffing a beer, or their child, or a T-shirt belonging to their partner, exactly what happens next, leading to a perception of the beer or the person as a complex aromatic whole, is only hazily understood. Lurking within the mucus of the nose are the tips of olfactory receptor cells. These nerve cells lead directly to the brain. While we have millions of these cells, there seem to be only about 400 types, each of which binds to a specific molecule (the number is debated; some argue it could be closer to a mere 100). Based on the pattern of activation of the various receptor types, when I sniff Nugget Nectar, I recognize it as 'beer'. Nick smells nothing – the impact of his fall probably damaged or even killed his olfactory nerve cells, and his brain receives no information about the smell of his drink.

Before his injury, Nick had a highly sensitive nose. Unlike me, he would have been easily able to distinguish Nugget Nectar from other beers. That ability comes with experience. After the incoming smell signal pattern is processed, this information is sent to different parts of the brain, including regions involved in memory and emotion, as well as to the cortex, where thinking takes place. We can then quickly learn to pair patterns of receptor activation with the source of the smelly molecules.

What difference would it make if we all put more effort into smelling the world around us?

Until recently, it was thought humans could detect perhaps only 10,000 different scents. But there's been a radical rethink, according to Joel Mainland, who's researching the fundamentals of smell at the Monell Chemical Senses Center, a world-leading institute for research on smell and taste in Philadelphia. A recent paper in the journal Science estimated that we can detect more than a trillion smells. A few problems have been raised with the methodology of that study and there's still a lot of debate about the true figure, but Mainland certainly thinks that we've underestimated our abilities.

Because of the nature of his job, Nick underwent all kinds of sensory training to improve his smell and taste. The rest of us probably have untapped potential, too. Yes, dogs are renowned for being able to track a person's scent across a field. When Mainland was a PhD student, his supervisor suggested investigating whether humans could be trained to do the same thing. It turned out they could.

Dogs have more types of olfactory receptor than we do – but as Mainland points out, cows have more than dogs (about 1,200 compared with 800) and it's not clear that cows are significantly better at smelling. The poor reputation of humans may be down to the fact that we spend comparatively little time actively sniffing and so training our sense. What difference would it make if we all put more effort into smelling the world around us?



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George Preti's lab is lined with fume cupboards, machinery for analyzing gases, and freezers packed with saliva and sweat samples and even archived T-shirts worn by people during experiments into scents produced from our underarms while we're stressed.

He waves the jar under my nose. It smells… bad. He smiles. "Dirty laundry? It's from a Japanese group. They've isolated the smell from dirty laundry. How about this?" He removes the stopper from another jar. It smells… bad, but in a different way. "That's 3M2H. It's one of the principal components of human underarm odor!"

The lab technician grins from a bench where he's busy preparing sample bottles for a series of new stress and fatigue experiments. "You've just been officially Preti-labbed!" he calls.

'They've isolated the smell from dirty laundry.'

Preti has been studying smell at Monell for decades. He specializes in the scents we humans produce. Our breath, our urine and even our blood contain molecules that smell. But the main source of body odor is the underarm region. It's rich in apocrine glands, which produce small secretions that ultimately generate underarm odor. When we're stressed, we produce more of these secretions.

Individual body odor is influenced by genes, and by one group in particular: the genes that determine which proteins combine to form your unique major histocompatibility complex (MHC). This group of proteins is on the surface of almost every cell in your body. It indicates to the immune system that a cell is 'self', and so not to be attacked. Back in 1995, researchers found that women prefer the smell of men whose suites of MHC genes are more different from their own. In 2013, a study found that people can even recognize their own scent, based on their particular MHC.

But it seems there are other smelly components of human body odor that we can detect and that can influence us. Preti and his colleagues have found that extracts of male underarm odors can not only influence female physiology, altering levels of a hormone involved in regulating the menstrual cycle, but also make women feel more relaxed and less tense. "None of the women smelled anything different between the control and the experimental extract," he says. "There was an impact. But it was not conscious."

With Pam Dalton, Preti has also investigated the impact of stress on body odor. In a study published in 2013, they collected body odor from people who'd been made to feel stressed in the lab. Another group smelled these odors while they watched videos of women doing something that might potentially be stressful – such as getting kids ready for school while trying to cook breakfast – but where the women didn't actually look stressed. ("We combed through hundreds – if not more – videos to try to find a set that worked!" Dalton remembers.)

Friends who can smell somehow seem to be picking up on signals [those without smell are] missing, signals that are so powerful they can override the emotional information contained in a smile or a frown.

The people watching the videos rated the women in them as being more stressed when they were exposed to the 'stressed' body odors than when they were exposed to a mild neutral fragrance or to body odor samples collected from volunteers who'd been exercising. Male viewers (but not female ones) also rated the women as being less trustworthy, less competent and less confident with the stressed odors. Yet the viewers didn't rate any of the three odors as being more or less pleasant or even as being very different from the others. The team concluded that some kind of subconscious signaling was going on.

While no one yet knows which specific chemicals convey information about our emotional state in this way, Dalton says just knowing that they exist means we can allow for them. Because she doesn't feel she produces much body odor, Dalton doesn't usually wear deodorant. However, if she knows she'll be going into a stressful situation, she puts some on – she wants to protect herself from any potentially psychologically damaging scent signals her own body may produce. She thinks we could all benefit from a better understanding of how smells can affect us: "If we're not aware we're being influenced," she explains, "we can't guard against it."

Nick Johnson is painfully aware that he can no longer smell himself, his daughter or anyone else. It's not just because he knows what he is missing. Some people born without being able to smell may have trouble identifying the emotional states of others, says Joel Mainland. They're aware that while they rely heavily on facial expressions, for example, friends who can smell somehow seem to be picking up on signals they're missing, signals that are so powerful they can override the emotional information contained in a smile or a frown.

They'll talk about meeting up with a group of friends, Mainland explains, and one friend might say of another: "Oh, she wasn't happy at all." And they'll say, "She looked happy." And the other will say, "Yeah, she looked happy, but she clearly was not happy."

Estimates of just how many people can't smell usually range around a few per cent of adults. That means millions of people living without smell – some born without it, others who have lost it. Chronic nasal sinus disease is one of the most common causes of loss in younger people. Another risk stems from the fact that our olfactory receptor neurons dangle down into our nostrils, leaving them exposed to damage from environmental toxins and infections.

In older, but not elderly, people, viral infections are often to blame. Even the common cold can do it, though why it should wipe out smell in some people but not others, no one knows. By the time we get to our 70s and 80s, very few of us will have escaped significant deterioration in our sense of smell. The system has a capacity to regenerate: the nerve cells are dying all the time and being replaced. But as we get older, this process slows, and the patches of nasal tissue without any olfactory receptors get bigger.

'I wanted an answer. And he said basically there isn't an answer.'

In Nick's case, the cause was probably catastrophic damage to his olfactory receptor neurons. Heading from the nose to the brain, these neurons pass through a bony sieve-like structure. When he hit his head on the ice, the sudden movement of his brain inside his skull could have crushed or even cut them against the bone, preventing signals from his nose from reaching his brain.

Once he realized his sense of smell was gone, Nick went back to his neurologist and was surprised to be told there was nothing they could do to help. "He told me, 'You may get your smell back in six to eight months, a year. Or you could never get it back.'

"With everything that had happened to me, I wanted an answer. And he said basically there isn't an answer."

There are effective treatments for some people who have lost their sense of smell. If the loss is due to chronic sinus disease, you can treat that condition and reverse the smell loss – sometimes very rapidly. But for patients like Nick, there's very little that can be done. He came to Monell to ask the researchers here if they had any advice, and the main recommendation was to actively smell a few different things a couple of times a day, because there's evidence this can help to stimulate the system and may aid recovery.

Things may be different in the future. There is a team at Monell experimenting with nasal stem cells. Right now, they're investigating the most effective ways of converting these stem cells into nerve cells. The hope is that this approach will provide new olfactory receptor neurons for people whose own have been permanently damaged or defective since birth. The team hope to start animal trials in around September 2015, and if those studies go well, to move to people in five to ten years.



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For now, though, Nick has to try to live with the knowledge that his sense of smell may never return.

Life is certainly different, he says. His co-workers are supportive. But he has to rely on their descriptions of how the new beers smell and taste. He really misses the scent of the brewery, and even of the ice rink, and, now he notices what he's lost, the smell of other familiar places.

"I walk into my parents' house or my wife's family's house – and it doesn't have that smell. And I miss the ambience and the smells when there's an Eagles game, and everyone sets up grills in all the parking lots in south Philly, and grill up all kinds of crazy food items, and drink beer, hours before the game starts. Stuff you are used to… it's just gone."

Nick used to smoke all kinds of meat and regularly barbecue for family and friends. "I used to cook a lot more," he says. "This has slowed things down." But while he can't detect complex flavors, he can still get sweet, salty, bitter, sour and umami, and also the heat of chili. "I love salt now. I put a lot of heat into things too, because I can get that. I put lots of cayenne in… At times, my wife will say, 'This is ridiculous, I can't even eat it!'"

'Stuff you are used to… it's just gone.'

There are some more serious consequences of not being able to smell, though. The safety valve on his gas cooker broke recently. He was in the kitchen at a time when it must have stunk of gas, but he went to bed oblivious to the danger to his sleeping family.

The glimmer of good news for Nick is that there are some promising signs. Strong-smelling things sometimes do produce a smell sensation, though it's always the same. It used to be an awful burning-oil-type smell, he says. A few months ago, it shifted to something sweeter. This may be a sign that some kind of repair to the system is going on.

Nick says he's determined to be positive, and to live life as close as possible to the way it was before the injury. He's gone back to playing ice hockey (though he says, smiling, that he now has the best helmet money can buy). The day of our lunch at the White Dog Cafe, it's his son's third birthday. On the way home, he's going to pick up some kid-sized ice skates as a gift.

He recognizes how serious the accident was, but also just how much worse things could have turned out. "I had blood on my brain. I could have died. My outlook is: I'm glad I'm not dead. If the loss of my sense of smell is what's happened because of this, I'll take it."​

This story first appeared on Mosaic and is republished here under a Creative Commons license.