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 flavour, and with the peppers and the guacamole, there’ll be some mouthfeel there.”

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 Johnson’s worked at for the past 10 years. Nugget Nectar used to be his favourite 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.

People who can no longer smell report a strong sense of loss

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 behaviour. 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.

Johnson, 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.”

After making a rapid recovery, he was cleared to drive again six weeks later and returned to work as regional sales manager at Troegs 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 flavour’… 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 realise 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.

More complex flavours – like grapefruit or barbecued steak – depend on smell

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 flavours – like grapefruit or barbecued steak – depend on smell. But for Johnson, as for many people who can’t smell, there’s another category of loss altogether.

At the time of his accident, Johnson’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.”

How it happens

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.

Even the common cold can cause total loss of smell

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.

In Johnson’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.

How smell works

Johnson takes a deep sniff of his glass of Nugget Nectar, the beer that was once his favourite. Volatile chemicals from the liquid are drawn high up into his nostrils, to the roof of his nasal cavity, the part specialised 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.

For something to have a smell, its molecules must easily evaporate

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.

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 recognise it as ‘beer’. Johnson 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, Johnson 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.

By one estimate, we can detect more than a trillion smells

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, Johnson 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?

Emotional state

One reason to enhance our sense of smell would be to help us better navigate our social world. Some people born without being able to smell may have trouble identifying the emotional states of others, says 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.”

Researchers have also found that smell can change your mood and behaviour. George Preti at Monell and colleagues have found that extracts of male underarm odours 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. And along with Pam Dolton, he has also found evidence that people can detect “stressed” body odours, often without realising.

Read more about how smell influences how we perceive the emotional state of other people.

So, those with no smell may be missing out on many subtle social signals. Can anything be done for them? There are some effective treatments. 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 Johnson, 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.

Researchers recommended to actively smell a few different things a couple of times a day

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 10 years.

For now, the glimmer of good news for Johnson 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.

Johnson 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).

He recognises 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 is an edited version of an article originally published by Mosaic, and is reproduced under a Creative Commons licence. For more about the issues around this story, visit Mosaic’s website here.

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