A particular kind of terror in losing a sense you never thought about. Not blindness, which the imagination rehearses. Not deafness, which literature has made noble. But anosmia, the total absence of smell, which, until recently, most people could not even name.
10 min read
In the spring of 2020, millions of people woke up to a world drained of its invisible architecture. Coffee had no warmth. Garlic had no threat. Their own children smelled of nothing. The loss was not painful in the way a broken bone is painful. It was ontological. The world was still there, visually intact, but it had been emptied of a dimension that no one had bothered to tell them was holding everything together.
What happened next is one of the stranger episodes in the history of medicine. A rehabilitation protocol developed in a German university lab, obscure, unglamorous, studied by perhaps two dozen researchers worldwide, became a global phenomenon overnight. Millions of people, desperate and largely abandoned by their physicians, began sitting at their kitchen tables twice a day, holding small jars of essential oil under their noses, sniffing with the deliberate concentration of someone learning to walk again.
They were smell training. And in doing so, they accidentally stumbled into one of the most striking demonstrations of neuroplasticity that modern science has produced.
Thomas Hummel and the backwater of olfactory research
The protocol has a name and a father. Thomas Hummel, a professor of otorhinolaryngology at the Technische Universitat Dresden, had been studying olfactory disorders since the early 1990s, a period when telling colleagues you researched smell was roughly equivalent to announcing you collected bottle caps. The olfactory system was, in the hierarchy of neuroscience, a backwater. Vision had the cortex. Hearing had cochlear implants. Smell had anecdote and resignation.
Hummel was not resigned. He had noticed something that the clinical literature had mostly ignored: some patients with post-viral anosmia recovered their smell over time, and those who reported actively seeking out odors during their recovery seemed to recover more completely. The observation was informal. It was also, as it turned out, the seed of everything.
By 2009, Hummel and his team had formalized the intuition into a protocol. Four essential oils, rose, eucalyptus, lemon, and clove, were selected not arbitrarily but according to a classification system proposed by the German psychologist Hans Henning in his 1916 work Der Geruch. Henning had described odor perception as organized along a geometric prism with six vertices: flowery, fruity, resinous, spicy, putrid, and burnt. Hummel's four oils were chosen to represent four of these six categories. Rose for flowery. Lemon for fruity. Eucalyptus for resinous. Clove for spicy. The putrid and burnt categories were excluded for obvious reasons of domestic harmony.
The instructions were disarmingly simple. Twice a day, morning and evening, the patient would open each jar in sequence and inhale gently for ten to fifteen seconds, concentrating on the smell, or on the memory of the smell, if the smell itself was absent. The minimum duration was twelve weeks. Improvement, when it came, often continued for months afterward.
The first randomized controlled trial, published by Hummel's group in 2009 in The Laryngoscope, showed statistically significant improvement in olfactory function compared to controls. Subsequent studies replicated the finding. A 2017 systematic review by Sorokowska and colleagues in Rhinology, confirmed it. By the standards of olfactory medicine, a field where therapeutic nihilism was the default posture, the results were notable. Here was an intervention that cost almost nothing, had no side effects, and produced measurable structural and functional changes in the nervous system.
Almost nobody noticed.
Olfactory neurons regenerate throughout adult life
To understand why smell training works, you need to understand something unusual about the olfactory system: it is the only sensory system in the human body that continuously regenerates its primary neurons throughout adult life.
The olfactory epithelium, a postage-stamp-sized patch of tissue high in the nasal cavity, contains roughly six million olfactory receptor neurons, each genetically unique to the individual. Each neuron expresses a single type of odorant receptor on its surface, chosen from a repertoire of approximately four hundred functional receptor genes, as mapped by the Nobel Prize-winning work of Linda Buck and Richard Axel, published in Cell in 1991. When an airborne molecule binds to one of these receptors, the neuron fires. The signal travels along the neuron's axon, through tiny perforations in the cribriform plate, a sieve-like bone at the base of the skull, and into the olfactory bulb, the brain's first relay station for smell.
Here is the critical detail: olfactory receptor neurons live for only thirty to sixty days. They are born from a population of basal stem cells in the epithelium, mature, extend their axons through the cribriform plate, form synaptic connections in the olfactory bulb, function for a few weeks, and die. The cycle never stops. You are, quite literally, rebuilding your sense of smell every month.
This constant regeneration is the system's great strength and its great vulnerability. Under normal conditions, newly born neurons follow chemical guidance cues to find their correct targets in the olfactory bulb. Neurons expressing the same receptor type converge on the same glomerulus, a spherical cluster of synapses, creating a precise spatial map. The map is rewritten continuously, but because the guidance cues are stable, each new generation of neurons recreates the same topography. The result is seamless. You never notice the renovation because the blueprint stays the same.
When a virus damages the olfactory epithelium, which is exactly what SARS-CoV-2 does, along with influenza, rhinoviruses, and others, the regeneration process can go wrong. The stem cells still divide. New neurons still emerge. But the guidance cues may be disrupted. The newly born neurons, like commuters in a city where all the street signs have been removed, extend their axons into the olfactory bulb and connect to the wrong glomeruli. A neuron that should wire to the rose-encoding glomerulus instead arrives at the one for sulfur. The subjective experience of this miswiring is parosmia, the terrifying condition in which familiar smells become distorted, usually into something nauseating. Coffee smells like sewage. Chocolate smells like gasoline. Your partner's skin smells like burnt rubber.
Smell training intervenes at precisely this point. By repeatedly presenting the same four odors, and crucially, by asking the patient to attend to each one deliberately, to recall its true character from memory even if the current perception is distorted or absent, the protocol appears to provide a form of guided neuroplasticity. The repeated stimulation encourages regenerating neurons to find their correct glomerular targets. The attention component may increase top-down neural feedback that helps reinforce correct connections and prune incorrect ones. Over weeks and months, the spatial map in the olfactory bulb is gradually restored.
The mechanism is not fully understood. No one has performed serial biopsies of the human olfactory epithelium during smell training, for obvious reasons. But the convergent evidence from functional brain imaging, psychophysical testing, and animal models is compelling. Smell training accelerates and directs natural recovery.
Attention is the buried word in the protocol
A word buried in the protocol that deserves more scrutiny than it typically receives. The word is attention.
Hummel's instructions do not say: expose yourself to four odors twice a day. They say: concentrate on each odor. Focus. Try to recall what it should smell like. The distinction is not incidental. Multiple studies have shown that passive odor exposure, ambient scenting of a room, for instance, produces significantly less improvement than the same exposure paired with deliberate, focused attention. The act of trying to smell, of directing conscious awareness toward the olfactory signal, appears to be pharmacologically active in a way that mere proximity to molecules is not.
This is a deeply strange finding if you think of smell as a passive sense, which most people do. We tend to imagine olfaction as something that happens to us, an odor drifts by, we register it, end of story. But the neuroscience tells a different tale. Olfactory perception is a construction, assembled in real time from the interaction of bottom-up sensory signals and top-down expectations, memories, and attentional states. When you focus on an odor, you are not merely receiving more of it. You are changing the neural computation that transforms a chemical signal into a percept.
The philosopher Alva Noe has argued, in his 2004 book Action in Perception, that perception is not undergone by organisms but performed by them. Smell training is perhaps the most literal embodiment of this thesis in clinical medicine. The patient is not a passive recipient of a treatment administered by someone else. The patient is the treatment. Their attention is the active ingredient.
This is also, incidentally, why smell training is so difficult. Not physically: opening a jar and sniffing requires no special equipment or ability. But attentionally. Sustaining focused olfactory attention for even fifteen seconds is genuinely hard work for most people. The mind wanders. The visual system, accustomed to its dominance, reasserts itself. The temptation to go through the motions, to hold the jar under the nose while thinking about something else, is overwhelming. And going through the motions does not work nearly as well.
Post-Covid hunger for olfactory education
The post-Covid explosion of smell training revealed a vast, unmet hunger for olfactory education. Before 2020, the only people who deliberately trained their sense of smell were professionals, perfumers, flavorists, sommeliers, oenologists, tea tasters, and a handful of sensory scientists. These disciplines had always recognized that olfactory acuity is not a fixed trait but a skill, developed through years of systematic practice. A trainee perfumer spends months learning to identify raw materials blindfolded, not because their nose is anatomically different from anyone else's but because they have built, through repetition and attention, a cognitive architecture for discriminating and categorizing olfactory information.
The Covid anosmia crisis democratized this knowledge. Suddenly, ordinary people were learning things that perfumery students learn in their first year: that smell requires active engagement; that naming an odor helps you perceive it; that the same molecule can smell different depending on concentration, context, and expectation; that olfactory memory is more durable and more emotionally charged than visual or auditory memory; that the nose adapts quickly and must be rested between exposures; that some days your sense of smell is sharper than others, for reasons that remain largely mysterious.
A cottage industry materialized overnight. Smell training kits, small boxes containing the four canonical oils, appeared on Amazon, Etsy, and in pharmacies across Europe. Charities like AbScent in the UK, founded by the smell loss advocate Chrissi Kelly, became lifelines for hundreds of thousands of people. Facebook groups swelled into communities of mutual support where members tracked their progress in obsessive, moving detail. The medical establishment, which had never taken olfactory complaints particularly seriously (there is no smell equivalent of an audiologist, no insurance code for olfactory rehabilitation), was forced, belatedly, to pay attention.
Some of the smell training converts went further. Having recovered their olfaction, they did not stop training. They found that deliberate olfactory practice had sharpened their perception beyond its pre-illness baseline. They could detect subtleties they had never noticed before. Their vocabulary for describing smells had expanded. They had become, in a modest but real sense, more present in the olfactory world.
What directed attention does for an undamaged nose
This is perhaps the most interesting implication of the smell training story, and the one least discussed in the medical literature. If directed olfactory attention can repair a damaged sense of smell, what can it do for an undamaged one?
The answer, based on decades of evidence from sensory science, is: quite a lot. Olfactory training studies in healthy subjects have shown improvements in odor discrimination, identification, and sensitivity. The gains are not enormous, and they require sustained effort. But they are real. The human nose is not a fixed instrument with immutable specifications. It is more like a muscle, or, more precisely, like a neural network whose discriminative power increases with structured input and feedback.
The broader principle is one that applies across all sensory modalities but is most dramatic in olfaction, because olfaction is the sense we neglect the most. We live in a culture that is visually saturated and olfactorily impoverished. We have a hundred words for colors and almost none for smells, a poverty that synesthetes who see scent as color navigate more fluently than the rest of us. We can describe the exact shade of blue in a painting but struggle to articulate the difference between two white wines. This is not a limitation of the nose. It is a limitation of attention.
Smell training, whether performed by an anosmia patient with four jars of essential oil, or by a perfumer with a thousand raw materials, or by anyone who simply pauses to notice what the air actually smells like, is fundamentally an exercise in reversing that neglect. It is the practice of paying attention to information that has always been there, arriving at the same neurons, triggering the same molecular cascades, but processed at the periphery of awareness rather than its center.
Humans have been smell training for millennia
A final irony is worth noting. For all the scientific validation of the Hummel protocol, for all the randomized trials and imaging studies and neurobiological models, the core mechanism of smell training is something that humans have been doing for millennia without calling it that. The incense rituals of ancient temples. The spice markets of medieval trade routes. The garden walks prescribed for melancholia in eighteenth-century medicine. The sommelier swirling a glass and inhaling with closed eyes. All of these are, in their way, structured practices of olfactory attention.
Thomas Hummel did not invent smell training. He measured it. He formalized it. He proved it. And in doing so, he gave a name and a mechanism to what the human olfactory system had been waiting for all along: the simple, radical act of being asked to pay attention to what it was telling us.
It took a pandemic to make us listen.
