I am not normally a fan of lotions and potions, but right now, four small vials sit on my bedside table.
Every morning and every night, I open and sniff each one in turn, savouring the scents as they hit my nostrils. First, the sharp tang of lemon, followed by the abrasive chill of eucalyptus, the sweetness of rose and the metallic warmth of clove.
My new routine – training what is, for most of us, a neglected sense – was inspired by some striking research linking our noses’ sensitivity to our cognitive performance. For one thing, studies have shown that the worse your sense of smell is, the worse you perform in cognitive assessments. Olfactory dysfunction has also been linked to more than 100 conditions, including ALS (motor neurone disease), multiple sclerosis, Parkinson’s disease, Alzheimer’s disease and general age-related cognitive impairment.
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And though some of that olfactory impairment is undoubtedly the result of neurological damage, more recent research suggests that smell loss could actually contribute to some conditions. Failing the sniff test may even limit our overall lifespan – a frightening thought, given how many people lost their sense of smell as a result of the covid-19 pandemic.
“By middle age, your all-cause mortality can be predicted by your olfactory ability,” says Michael Leon, a neurobiologist at the University of California, Irvine.
Evidence like that has prompted a host of investigations into whether “smell training” can reawaken this often-ignored sense and sharpen our minds. So far, research and real-world experience suggest that it can. Our olfactory ability seems to work like a muscle – the more we use it, the stronger it gets. “When you’re confronted by the data, it’s really hard not to be motivated to do it,” says Leon.
The olfactory myth
Some olfactory impairment is, of course, unavoidable. Global rates of olfactory dysfunction are somewhat difficult to pin down – one pre-pandemic study of people in the US, for example, found that just over 12 per cent had some olfactory impairment, while a similar study of people in Sweden found that 19 per cent were dealing with some level of smell loss. Both studies, along with others, find that the incidence of olfactory impairment rises dramatically with age.
Like many people, I had always taken my nose for granted. I certainly didn’t think I should be exercising it regularly to preserve its health. “In surveys, smell is always considered the least important sense,” says Anna Oleszkiewicz, a psychologist at the University of Wroclaw in Poland. “People would rather give up on their smell than their smartphone.”
Our lack of interest may arise from the persistent myth that human “olfaction”, the scientific term for smell, is simply less developed than that of other animals. This idea first emerged in the late 19th century, when neuroanatomist Paul Broca divided mammals into two categories – osmatiques, such as dogs, which are guided by their snouts, and anosmatiques, which aren’t. The latter category included cetaceans – dolphins, porpoises and whales – and primates.
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The mechanism of smell is roughly the same for all mammals: molecules enter the nose and dissolve in the mucus that coats the olfactory epithelium, specialised tissue in the nasal cavity that is densely packed with olfactory receptor neurons. The molecules bind to these receptors, which then send signals through the olfactory nerve to the “olfactory bulb” in the brain, the structure right above the nasal cavity where the scent is processed and identified.
Broca’s principal evidence for his categorisation was variations in the sizes of that olfactory bulb. He had a point: after accounting for relative brain size, a dog’s olfactory bulb can be 30 to 40 times larger than a human’s, suggesting that scents play a much bigger role in canine lives. By the 1920s, scientists had started referring to smell as a “vestigial” sense in humans.
Our sensitivity is in a similar range to dogs, pigs and other animals that are considered ‘super-smellers’
This notion went more or less unchallenged for a century, until modern researchers began putting the human nose to the test in the 2000s. Studies revealed that, with a healthy olfactory system, humans can detect many gases at extremely low concentrations – we need just a few molecules to reach the receptors in our nostrils. “Our sensitivity is in a similar range to dogs, pigs and other animals that are considered ‘super-smellers’,” says Oleszkiewicz.
The oft-repeated wisdom that humans can discern only around 10,000 odours was also shown to be dramatically wrong. In 2014, researchers at Rockefeller University in New York used 128 odorous molecules to produce various scents to test whether participants could notice changes in composition. It turned out that they could do so to a remarkable degree; based on the participants’ sensitivity to these different combinations of chemicals, the researchers estimated that the average person could differentiate among 1 trillion odours.
Loss and depression
Unfortunately, many of us don’t appreciate this rich “smellscape” until it is gone. The most common causes of smell loss are traumatic brain injury, viral infection and sinonasal disease. The olfactory epithelium and the nerves linking our nose to the brain are directly exposed to pollutants and pathogens, making them especially vulnerable to damage.
The results of that damage can be devastating. “You’re encased in a bubble and the world is going on out there without you,” says Chrissi Kelly, who lost her sense of smell in 2012 following a sinus infection. She soon fell into what she calls a “profound depression”, a very common consequence of olfactory damage. “I felt like I’d undergone a personality change,” she says.
A low mood is a natural response to losing a sense that, whether we realise it or not, plays a significant role in how we experience the world. We may not recognise how important aroma is to the flavour of food, for instance, until it vanishes.
“Your day becomes a lot less colourful,” says Thomas Hummel at the Dresden University of Technology in Germany. Hummel’s research has also found how deeply olfactory dysfunction can affect people dealing with it: in 2022, Hummel and his colleagues tracked 171 participants with a damaged sense of smell over 11 months. They found a clear correlation between patients’ olfactory ability and their depressive symptoms over this period – and when their olfactory function improved, so did their mood.
Olfactory dysfunction also appears to be connected to a host of other conditions that cannot be so easily explained, however. Reviewing the evidence, Leon and his colleagues recently documented 139 conditions associated with smell loss, including cardiovascular disease, arthritis, polycystic ovary syndrome and a host of neurological conditions, such as Parkinson’s and dementia.
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In many of these conditions, smell loss may be a symptom – high blood pressure might damage the vessels in our nose, for example. When it comes to dementia, however, the direction of causality is less clear, with a wealth of research demonstrating a strong link between olfactory ability and cognitive function.
Between 2011 and 2014, for example, the LIFE-Adult-Study in Leipzig, Germany, asked about 7000 participants to take various health assessments, including tests of their sensitivity to different odours, their capacity to distinguish between them, and their mental abilities. A clear correlation emerged between the participants’ olfactory capacity and their results on the cognitive tests, even after researchers adjusted for factors such as age, education and depressive symptoms – the weaker the nose, the worse participants scored in verbal fluency, attention, memory and learning. The results prompted researchers to suggest that olfactory testing could be useful in early screening for cognitive impairment.
Other studies have come to similar conclusions. In 2021, psychologists at San Diego State University in California examined the health records of 497 people. They found that people’s performance on odour sensitivity and identification tests could predict who developed mild cognitive impairment and who developed Alzheimer’s. Quite remarkably, the olfactory tests were shown to be a better predictor of the condition’s progression than the Mini Mental State Examination, an assessment that is widely used to identify who is at risk of dementia.
A major risk factor
Based on these findings, some researchers have come to suspect that lost smell sensitivity is actively contributing to the brain’s deterioration. Leon offers two good reasons why.
The first concerns the brain’s wiring. Vision, hearing and touch first pass through a neural relay station known as the thalamus, located deeper within the brain, before reaching the outer layers associated with higher-level thinking. “The thalamus is like a railroad hub,” says David Vance, a psychologist at the University of Alabama at Birmingham. “Everything arrives there, then it’s processed and distributed through the brain.”
Smells, however, go straight to the olfactory bulb. Embedded in the forebrain, the bulb has direct links to many other areas of the cortex that are involved in emotional processing and regulation, decision-making, memory and learning, including the amygdala, the orbitofrontal cortex and the hippocampus. “The olfactory system is the only sense with a ‘superhighway’ to the memory centres and the emotional centres of the brain,” says Leon.
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This fast track to the cortex may explain why certain smells can be so evocative. And when that superhighway is broken, we lose a potent source of mental stimulation that would otherwise help keep our neurons happy and healthy.
Brain scans support this idea. Olfactory dysfunction is accompanied by a widespread loss of our brain’s grey matter and the wiring that passes messages within it. As you might expect, the most pronounced changes can be seen in the olfactory bulb itself – but many other regions shrink alongside it, including the orbitofrontal cortex and the hippocampus. Such changes may explain both the cognitive decline and the rising depression in such people: “Smell loss interferes with the affective and emotion control systems,” says Hummel.
Leon’s second hypothesis concerns inflammation. The body naturally raises inflammation to protect us from infection. If it remains too high for too long, however, it can cause damage to our organs – and is known to be a major risk factor for the development of Alzheimer’s.
Pleasant odours – such as eucalyptus, lavender, ginger, citrus and peppermint – have been shown to suppress inflammation
He points to evidence showing that certain odours can rapidly provoke the immune system. Mats Olsson at the Karolinska Institute in Sweden and his colleagues asked participants to sniff and then rate the chemicals that give rotten eggs, fermented herrings, urine and vomit their disgusting odours. Afterwards, they swabbed the participants’ saliva and measured the levels of certain inflammatory molecules. The resulting paper, published in December 2022, showed a clear connection between the two: the fouler the stench, the higher the levels of inflammation, suggesting that the scents primed the oral cavity with a “preparatory immune response” to potential pathogens. Meanwhile, pleasant odours – such as eucalyptus, lavender, ginger, citrus and peppermint – have been shown to suppress inflammation, although exactly why is unclear.
A healthy nose may, therefore, help keep our immune system in check, raising inflammation when it perceives a potential threat to our health and lowering it when it senses we are in a safe environment. Olfactory dysfunction throws this out of balance, leading to chronically high inflammation that ultimately damages the brain.
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The ability to smell our surroundings can also help us avoid environments that trigger inflammation. “Bad odours predict bad things and so it’s a good idea to stay away from them,” says Leon. If our noses lose their sensitivity, we may not be so good at making those calls, potentially increasing our exposure to pollutants and pathogens that might cause more damage to our nose and brain.
Animal studies are helping to pick apart these threads – and they strongly suggest that the olfactory system plays a crucial role in the development of dementia. Researchers at the University of Navarra in Spain, for instance, found that exposing mice that were genetically programmed to develop the equivalent of Alzheimer’s to the sweet smell of menthol reduced brain inflammation and improved their memory. These benefits were completely absent, however, in animals with a damaged nasal cavity.
Mohammad Reza Raoufy at Tarbiat Modares University in Tehran, Iran, and his colleagues also recently reported that electrically stimulating rats’ olfactory bulbs can slow the accumulation of amyloid plaques, improving cognition and preventing dementia.
Get a brain boost
Could we achieve the same results simply by paying a bit more attention to our noses? That’s the aim of various commercially available smell-training programmes. These kits usually require users to sniff a variety of aromatic compounds – such as clove, eucalyptus, rose and lemon – for a few minutes each day, over a few weeks or months.
There is good evidence that smell training of this kind can improve olfactory abilities. “When we expose ourselves to odours, we become more sensitive,” says Hummel, who pioneered studies into smell training. In 2009, he and his colleagues recruited 56 people with olfactory dysfunction. Forty were assigned smell training over 12 weeks, during which they were told to sniff each of the four odours described above for at least 10 seconds twice a day and to keep a weekly diary of their experiences. “If you ask people to do something on a routine basis, it needs to be super simple,” says Hummel. Despite this very low time investment, subsequent tests revealed increased odour sensitivity, while a control group saw no change.
Other, subsequent trials suggest that patience is key. A 2016 study of people with post-infectious olfactory dysfunction, for example, found that more than 70 per cent of people who trained for 56 weeks experienced significant improvement, compared with 58 per cent of those who trained for 15 weeks.
Crucially, the training also brings a brain boost. Vance and his colleagues recently reviewed the evidence for smell training’s effects on mental performance, finding 18 controlled trials that offered promising evidence that it really can slow or even reverse some signs of cognitive decline. A few of these experiments also revealed neurological changes, including growth in regions like the hippocampus. “Neurons that fire together, wire together,” he says. “So you have all that neuroplastic input that creates a lot of changes in the brain.”
We may not even need to be awake to reap the benefits. Leon recently invented a commercially available device called Memory Air, which puffs out 40 different scents as the user sleeps. A small trial that tested an early prototype on 43 people aged 60 to 85 found a 226 per cent improvement in verbal memory after six months of nightly use. “There have been billions of dollars spent on trying to find ways to improve memory, and it turns out that you can improve it with just giving olfactory enrichment,” says Leon.
In trying to regain her lost sense of smell, Kelly trialled an experimental treatment that involved injecting blood cells called platelets into her nose to help regenerate the tissue. The platelet injections did improve her sense of smell slightly, she says, but consistent smell training had already helped return some of her lost sensitivity: “Smell training and smell awareness is a constant part of my daily life, and I have no doubt that it has played an important role.”
The restoration of her sense of smell hasn’t just recoloured Kelly’s world; it has brightened and enlivened it. “Once you are in the habit, your world is so full,” she says. “Everything in your house smells. Every book has a different smell, your tea towel, even your keys.” In line with the scientific research, this has enhanced her sensory memory, says Kelly. “It’s like getting into Narnia, you know? You walk into the wardrobe, move the coats out of the way and pretty soon, you can feel the snow crunching under your feet.” Based on her experiences, she has founded the CKOS Network, an online community for people with smell loss.
It was Kelly’s experience that finally convinced me to give smell training a go myself. When I looked back on my childhood, I realised that smell was as vivid as any other sense. Cut grass, wet earth, the instant coffee I’d sprinkle on top of my hot chocolate – I lived in a vibrant smellscape. As the years have passed, however, my sensitivity seems to have waned, and I notice odours less and less. It is as if my world has moved from multicolour through to greyscale – and that doesn’t bode well for my brain as I enter my fifth decade.
I purchased my vials of lemon, clove, eucalyptus and rose online and now sniff them in both the morning and evening.
After a few weeks, I’ve already noticed more flashes of odour entering my consciousness in the hours between each exercise, a glimpse, I hope, of a richer – and smellier – future.