The pandemic might feel like it's over, yet alarmingly, millions of people around the world are continuing to suffer with an altered sense of smell. Read on for the latest research and techniques that might help restore it
Anosmia – the sudden, mysterious loss of the sense of smell – was without doubt, one of the most recognisable and baffling symptoms during the Covid-19 pandemic. The curious phenomenon appeared to have no rhyme or reason, affecting mild, severe, or even asymptomatic patients indiscriminately.
While most people had their sense of smell restored within a matter of weeks, others are not so lucky. Today, an estimated 15 million people worldwide are continuing to experience an altered sense of smell, months, or even years after contracting the virus.
An estimated 15 million people worldwide are continuing to experience an altered sense of smell.
What makes it all the more puzzling is that there is nothing straightforward about Covid-related smell loss, or post-viral olfactory dysfunction (PVOD), as it is known.
It can present in a number of ways: as a reduced sense of smell (hyposmia), a complete loss of the sense of smell (anosmia), or even a distorted sense of smell (dysosmia), which can result in (parosmia), peculiar condition that makes once pleasurable aromas smell intolerable – such as coffee that smells like sewage, or shampoo that smells like spoiled milk. There's also phantosmia, where you can perceive a smell that isn’t actually there. The loss of taste, known as ageusia, can also be a symptom.
Covid-related smell loss can present in peculiar ways - as a partial or complete loss of smell, or a distorted sense of smell. ©Appellation
Inflammation, Not The Virus, May Be To Blame
Some of the recent research into Covid-related smell loss suggests that an ongoing immune response in the olfactory epithelium – the delicate nasal lining of the nose – could be to blame.
In one study published in JAMA Neurology, a Johns Hopkins Medicine-led team found that inflammation, rather than the virus itself, may weaken odor signals to the brain. Another study in 2022 led by Duke University, Harvard Medical School and the University of California San Diego, analysed nasal tissue samples from 24 Covid patients, including nine affected by long-term smell loss. As well as observing notably fewer olfactory sensory neurons, they discovered “widespread” inflammatory immune cells – even in patients with no detectable Covid virus.
“The findings are striking,” remarked Dr Bradley Goldstein, an associate professor in neurobiology at Duke University and the study's lead author. “It’s almost resembling a sort of autoimmune-like process in the nose.”
How Smell Loss Affects The Brain
Scientists have long known about the correlation between the sense of smell and brain activity. An impaired sense of smell is after all, one of the earliest symptoms of neurodegenerative conditions such as Alzheimer's and Parkinson's disease. Today, as well as research into Parkinson's smell loss, there's a groundswell of studies investigating how Covid-related smell loss impacts the brain.
Advanced brain imaging has revealed that people with an altered sense of smell have different patterns of brain activity ©Appellation
In one study at University College London (UCL), researchers used advanced neuroimaging methods to compare the brain activity of people with long Covid who lost their sense of smell, to those whose smell had returned to normal. Their startling findings reveal that people with an altered sense of smell have different patterns of brain activity in two areas of the brain responsible for processing smell: the orbitofrontal cortex and the pre-frontal cortex.
People with an altered sense of smell have different patterns of brain activity.
The good news? The reduced brain activity was not present in those whose sense of smell had returned, which suggests that although Covid-19 does impact neurological function, it is clinically reversible.
“Our study gives reassurance that, for the majority of people whose sense of smell comes back, there are no permanent changes to brain activity,” remarked Dr Jed Wingrove, the lead author of the study from UCL Department of Medicine.
In more encouraging news, the study's joint author, Professor Claudia Wheeler-Kingshott, also suggests it is possible to retrain the brain to recover its lost sense of smell. “Olfactory training – that is, retraining the brain to process different scents – could help the brain to recover lost pathways, and help people with long Covid recover their sense of smell,” she said.
How Does Olfactory Training Work?
Olfactory therapy, or smell training, is a bit like rehabilitation or physiotherapy for your nose. The method, invented by one of the world's leading olfaction experts, Professor Thomas Hummel, who leads the Smell and Taste Clinic at the University of Dresden in Germany, is an evidence-based protocol that has been used therapeutically – even before Covid – to treat smell disorders caused by brain injuries or viral infections like influenza.
Professor Hummel's technique involves repeated and deliberate exposure to a variety of aromas from the “odor prism”, a classification system that categorises scents as either spicy, floral, fruity, resinous, burnt or foul – similar to the way we classify flavours (sweet, bitter, sour, salty or umami). Repeatedly smelling these different scents multiple times each day appears to restore neural connections disrupted by the virus, retraining, or reawakening, the essential communication pathways that enable us to smell.
While the technique doesn't work on everyone, it has yielded fairly impressive results with multiple studies to back it up. In just one study on post-viral patients at European hospitals, 95 percent were able to recover their sense of smell within six months. All were encouraged to do smell training at home.
In just one study on post-viral patients at European hospitals, 95 percent were able to recover their sense of smell within six months.
Can I Train My Sense of Smell At Home?
You certainly can scent train at home, even using common ingredients found in your kitchen pantry or garden, but just like any training regime, it requires commitment – and there's no guarantee. To give yourself the best chance at regaining your sense of smell, experts advise a daily protocol, usually for a minimum of three to four months.
It's possible to scent train at home using common ingredients found in your kitchen pantry or garden
Rose, lemon, clove and eucalyptus essential oils are the ones traditionally used in olfactory training but you're free to use other oils – and other ingredients – to create your own personal scent kit. Simply choose a variety of scents from the fragrance prism. Think lavender essential oil (floral), frankincense essential oil (resinous), cinnamon (spicy), some fresh lemon or orange peel (fruity), crushed herbs and even fresh flowers.
If you're using essential oils, it's best not to smell directly from the bottles, which are designed to preserve, rather than emit the scent of the oil. Instead, use dark amber glass jars with a screw-on lid and a wide rim to enable a better smelling experience. Add several drops of your chosen essential oil to a small piece of absorbent paper, and place it inside the jar. After each use, screw the lid back on tightly. Store the jars somewhere cool and dark, or in the fridge. Ideally, the scents should be smelled twice daily, in the morning and the evening.
Think Of A Specific Scent Memory
Many olfaction experts now recommend recalling a memory associated with a particular scent as you smell it, to further encourage your sense of smell to return.
It's well known that scent can evoke powerful memories, thanks to the unique link between the olfactory system and the hippocampus, the so-called memory centre of the brain. A fascinating study at Northwestern University Feinberg School of Medicine in Chicago determined that unlike vision, hearing and touch, smell travels directly and rapidly on a "superhighway" to the hippocampus, which may explain why scent can trigger such sudden – as well as vivid – memories.
According to research by Professor Hummel and colleagues, it is the memory of the scent of a rose – not the word “rose” – that fires up the limbic system, which includes the hippocampus and the amygdala. For French novelist Marcel Proust, it was the recollection of the scent of a delicate madeleine pastry that revived a childhood memory, which he famously describes in À la recherche du temps perdu (Remembrance of Things Past).
In short, the idea is to create your own personal Proust Effect. If you smell coffee, for example, try to think of a cafe that you frequent, or a perhaps a cafe you visited while on holiday. As well as trying to recall the smell and the taste of the coffee, try to visualise the table setting, the sunlight through the window, the waiter – anything at all that might help bring the memory to life.
Recalling a memory linked to a particular scent while you smell it can enhance olfactory training
Rotate The Scents Every 12 Weeks
If after several months, there's no improvement in your sense of smell, try expanding the scents in your kit, as regularly switching around the scents can enhance the outcome of olfactory training. In a 2015 study published in Laryngoscope, Professor Hummel found that participants who changed the original four odors every 12 weeks with a variety of other scents had better results.
For those using essential oils, be sure to clean your jars thoroughly with dish soap followed by several rinses with boiling water. Allow to dry fully before replacing the paper with a new oil.
Are There Any Other Natural Solutions?
Aside from olfactory training, there's currently no clinically endorsed therapy for smell loss. Nevertheless, there's no harm in considering natural and behavioural interventions.
With so much data pointing to inflammation, an anti-inflammatory diet, getting more exercise and quality sleep, and managing stress will benefit anyone.
Cold and heat exposure is also known to reduce inflammation. This was discussed in detail on a recent Huberman podcast with guest Dr Susanna Søberg, a leading expert on the benefits of deliberate cold and heat exposure on human physiology. According to Dr Søberg's studies, cold and heat exposure significantly decrease inflammation markers in the body, including interleukin-6 (IL-6). “I think it's very important to look at cold exposure and heat exposure as something that lowers inflammation in the body,” she said. "If we can do that, we will have an open door for preventing lifestyle diseases.”
Acupuncture has also shown promising results for many patients with inflammation and olfactory dysfunction, while the Institute for Functional Medicine (IFM) lists some helpful advice and evidence-backed supplements and lifestyle-based interventions to boost mitochondrial health, which is impaired in those with long Covid.
Though not included in IFM's list, Vitamin A nasal drops were researched as a potential treatment for smell loss at the University of East Anglia in the UK. Previous research in Germany also linked Vitamin A to improved smell, with experts hypothesising Vitamin A can repair tissues in the nose damaged by viruses.
Other (More Extreme) Smell Loss Solutions
In the interest of information, we will list a few other less conventional methods being trialled to restore smell loss - but be warned – they're not for the squeamish.
A recent Stanford Medicine-led study found that post-viral smell loss improved when patients were given injections platelet-rich plasma injections in their nasal tissue. The treatment, given once every two weeks for six weeks, was shown to improve symptoms in nearly 60% of participants after three months.
Another experimental procedure tipped as a potential solution to smell loss is the stellate ganglion block, which involves injecting a temporary local anaesthetic into a bundle of nerves in the neck. The procedure is being trialled at the Cleveland Clinic with positive reports. Further study is required to understand the science, though experts hypothesise that it may increase blood flow to the brain, or trigger the sympathetic nervous system.
Recovery Is A Long Road
While the majority of post-viral patients will have their sense of smell restored, the road to recovery is long. Others are coming to terms with the fact their sense of smell may never return.
It's difficult to comprehend the profound impact of losing the ability to smell. Scent connects us with the world. It is intrinsically tied to our wellbeing – to our experience of pleasure, our mental and emotional health, and even our physical safety (imagine not being able to smell gas leak or a fire, for example). No one should have to endure this silent, debilitating condition.
Helen Keller once called smell the “fallen angel” of our senses because it is the sense we appreciate the least. So for those unaffected, please celebrate your fallen angel. Don’t take it for granted.