Losing your sense of smell, known as anosmia, is a phenomenon that, whilst previously rarely discussed, has come under the spotlight since the arrival of the Covid-19 pandemic.

Anosmia can refer to either the partial or full loss of a person’s sense of smell, and this can be permanent or temporary. There are many causes of anosmia, from a blockage in the nose or poor air quality, to exposure to toxic chemicals or brain and nerve damage. Furthermore, the simple factor of age can play a role in anosmia; it is believed that a person’s sense of smell is best between the ages of 30 and 60 and then declines thereafter.⁽¹⁾

The prolonged anosmia that occurs after infection with Covid-19 is thought to be due to olfactory nerve damage. Around 50% of people who have tested positive for Covid-19 have reported a loss of their sense of smell.
The sense of smell, which is powered by the olfactory system, is a highly complex and sophisticated part of human physiology. When a molecule is released from something in our environment, like a flower, it is detected by special nerve cells known as olfactory neurones⁽²⁾. Some of these olfactory cells act as a docking station for very specific molecules, whilst others respond to a wider range of aromatic chemicals.

The olfactory nerve cells are found at the top of the nasal cavity in the olfactory membrane, which has a remarkably large surface area for absorbing different molecules. These nerve cells connect directly to the brain through a porous bone plate called the cribriform plate. When a fragrant molecule is detected, messages are carried along the olfactory neurones to the brain so that we can recognise the smell and its source.

With respect to Covid-19, it is believed that the coronavirus damages the olfactory nerve cells by invading the cells that support them, which are called sustentacular cells. These sustentacular cells contain many angiotensin converting enzyme 2 (ACE2) receptors, which the virus can notoriously use to gain entry to the cell.⁽³⁾

One of the most obvious and catastrophic consequences of anosmia, is the associated loss of taste. Between 75% and 95% of what we taste is derived from, and involves, our sense of smell⁽⁴⁾. While it might seem bad enough that losing our ability to smell may lead us to lose our appetite, anosmia can have more sinister consequences. Without our smell, we are less likely to detect burning, gas leaks or food that has spoiled. Missing such vital signs could prove fatal. Anosmia can also lead to self-esteem issues and depression associated with the lower quality of life.⁽⁵⁾

Whilst anosmia caused by congestion or blockage can be treated with decongestants, antihistamines, steroid nasal sprays, and antibiotics, or by surgical removal (in the case of nasal polyps for example), it is much harder to treat nerve and brain-related anosmia (which includes that caused by Covid-19). More permanent and severe anosmia usually occurs when a person has suffered some form of head trauma that has caused some of the olfactory nerve fibres to shear or tear at the cribriform plate.

Fortunately, the olfactory nerve is a unique cranial nerve (nerve that arises directly from the brain)⁽⁶⁾ in that it is one of very few nerves with a natural regeneration capability. However, Covid-induced anosmia appears to persist for more than 6 months in around 10% of patients who have reported the symptom⁽⁷⁾. Therefore, there is a great need to find ways of treating this more permanent olfactory nerve damage.

This is where we can unlock the true power of essential oils using a treatment called “smell training.” Smell training involves subjecting anosmia patients to repeated short-term exposures of different odours over long periods of time. For example, in 2009, Professor Thomas Hummel’s research group⁽⁸⁾ carefully selected four distinct essential oils (rose, eucalyptus, clove and lemon) to help re-train, regrow, and activate the olfactory receptors and their supporting cells in human volunteers suffering from anosmia.

It is vital that the scents chosen for retraining are powerful, stimulating and recognisable. The four different essential oils chosen by Hummel are associated with different bitter, sour, pungent and sweet tastes, and contrasting flowery, fruity, spicy and resinous scents. The selected essential oils also have additional benefits, such as in the form of their antimicrobial properties⁽⁹⁾.

Hummel’s anosmic volunteers sniffed the four different odours for 10 to 20 seconds twice a day for twelve weeks. A critical part of the training was to ensure that the volunteers thought about how the scent smelled during the sniffing. The aim of this was to improve the olfactory nerve’s neuroplasticity—its ability to change and make new connections. The results of the study were astonishingly successful. Overall, 30% of the volunteers who underwent training showing an improvement in their sense of smell, while no improvements were seen for volunteers who did not undergo the smell training.

Professor Hummel’s study involved people who had suffered anosmia due to both head trauma and severe upper respiratory infections. This provides great hope that these methods, or more developed methods that involve the use of a wider array of odours, may offer an invaluable lifeline for people suffering from a loss of smell, and particularly for the large number of people who will be suffering these after-effects due to Covid-19.

In the meantime, we can all keep our noses healthy by treating them well. We can avoid blowing our noses too enthusiastically or sniffing toxic substances. We can also treat ourselves to regular steam treatments or a saline flush, which both help to maintain a healthy mucosal membrane in our noses.

(1) Smell Disorder (English) (howdenmedicalclinic.com)
(2) Anosmia: Symptoms, Causes, and Treatments (webmd.com)
(3) Olfactory training and COVID-19-related loss of smell | Perio-Implant Advisory (perioimplantadvisory.com)
(4) Just how much of what we taste derives from the sense of smell? | Flavour | Full Text (biomedcentral.com)
(5) (PDF) Self-esteem as an important factor in quality of life and depressive symptoms in anosmia: A pilot study (researchgate.net)
(6) Cranial Nerves | Boundless Anatomy and Physiology (lumenlearning.com)
(7) Coronavirus Smell Therapy for Anosmia Recovery – Full Text View – ClinicalTrials.gov
(8) Effects of olfactory training in patients with olfactory loss – Hummel – 2009 – The Laryngoscope – Wiley Online Library
(9) Olfactory training and COVID-19-related loss of smell | Perio-Implant Advisory (perioimplantadvisory.com)