Smell training or olfactory training is the act of regularly sniffing or exposing oneself to robust aromas with the intention of regaining a sense of smell. The stimulating smells used are often selected from major smell categories, such as aromatic, flowery, fruity, and resinous. Using strong scents, the patient is asked to sniff each different smell for a minimum of 20 seconds, no less than two times per day, for three to six months or more. It is used as a rehabilitative therapy to help people who have

anosmia Anosmia, also known as smell blindness, is the loss of the ability to detect one or more smells. Anosmia may be temporary or permanent. It differs from hyposmia, which is a decreased sensitivity to some or all smells. Anosmia can be due to a num ...

or post-viral olfactory dysfunction, a symptom of

COVID-19 Coronavirus disease 2019 (COVID-19) is a contagious disease caused by a virus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The first known case was COVID-19 pandemic in Hubei, identified in Wuhan, China, in December ...

. It was considered a promising experimental treatment in a 2017

meta-analysis A meta-analysis is a statistical analysis that combines the results of multiple scientific studies. Meta-analyses can be performed when there are multiple scientific studies addressing the same question, with each individual study reporting me ...

Efficacy

Along with olfactory implants, smell training is a promising but experimental treatment option. Several individual studies have indicated that smell training can increase olfactory sensitivity. In 2021 a meta-analysis was published that examined research studies of olfactory training for treating loss of smell as a consequence of a viral infection. It found clinically significant improvements and supported its use as a treatment option. As of March 2021, there have been no studies of smell training's efficacy for children. In 2017, the International and European Rhinologic Societies recommended smell training for treating loss of smell due to various conditions. In 2020, the

British Rhinological Society The British Rhinological Society is a learned society that encourages education and research in topics related to rhinology, the nose, sinuses Paranasal sinuses are a group of four paired air-filled spaces that surround the nasal cavity. The ma ...

published consensus guidelines for the treatment of smell loss due to

. Although no specific studies were available at that time, the expert panel made recommendations regarding treatment options and concluded that "olfactory training was recommended for all OVID-19patients with persistent loss of sense of smell of more than 2 weeks duration." Critics such as

Richard Doty Mirage Men is a 2013 documentary film directed by John Lundberg, written by Mark Pilkington and co-directed by Roland Denning and Kypros Kyprianou. ''Mirage Men'' suggests there was conspiracy by the U.S. military to fabricate UFO folklore in o ...

have pointed to the small sample sizes in the studies and the potential for the observed improvements to have been the result of nerve regeneration that would have occurred without intervention as reason to be skeptical.

Mechanism

Smell training likely achieves results because the

olfactory nerve The olfactory nerve, also known as the first cranial nerve, cranial nerve I, or simply CN I, is a cranial nerve that contains sensory nerve fibers relating to the sense of smell. The afferent nerve fibers of the olfactory receptor neurons tr ...

and

olfactory bulb The olfactory bulb (Latin: ''bulbus olfactorius'') is a grey matter, neural structure of the vertebrate forebrain involved in olfaction, the sense of odor, smell. It sends olfactory information to be further processed in the amygdala, the orbitof ...

have

neural plasticity Neuroplasticity, also known as neural plasticity, or brain plasticity, is the ability of neural networks in the brain to change through growth and reorganization. It is when the brain is rewired to function in some way that differs from how it ...

and are able to regenerate.

History

The idea was first written about by Thomas Hummel, a German psychologist at the

Dresden University of Technology TU Dresden (for german: Technische Universität Dresden, abbreviated as TUD and often wrongly translated as "Dresden University of Technology") is a public research university, the largest institute of higher education in the city of Dresden, th ...

, in his 2009 paper "Effects of olfactory training in patients with olfactory loss". In his original study, Hummel instructed patients with olfactory dysfunction to follow a twice-a-day routine for twelve weeks. The routine included inhaling the odor of rose, lemon, clove, and eucalyptus ( phenyl ethyl alcohol,

citronellal Citronellal or rhodinal ( C10 H18 O) is a monoterpenoid aldehyde, the main component in the mixture of terpenoid chemical compounds that give citronella oil its distinctive lemon scent. Citronellal is a main isolate in distilled oils from the pl ...

eugenol Eugenol is an allyl chain-substituted guaiacol, a member of the allylbenzene class of chemical compounds. It is a colorless to pale yellow, aromatic oily liquid extracted from certain essential oils especially from clove, nutmeg, cinnamon, basil ...

, and

eucalyptol Eucalyptol is a monoterpenoid. A colorless liquid, it is a bicyclic ether. Eucalyptol has a fresh mint-like smell and a spicy, cooling taste. It is insoluble in water, but miscible with organic solvents. Eucalyptol makes up ~70% - 90% of eucaly ...

respectively) essential oils for ten seconds each. These intense odors each correspond to a different odor category in Henning's odor prism. Hummel's paper built on a 1989 study by the

Monell Chemical Senses Center The Monell Chemical Senses Center is a non-profit independent scientific institute located at the University City Science Center campus in Philadelphia, in Pennsylvania. Monell conducts and publishes interdisciplinary basic research on taste, Olfa ...

in Philadelphia. The study showed that after repeated exposure to

androstenone Androstenone (5α-androst-16-en-3-one) is a 16-androstene class steroidal pheromone. It is found in boar's saliva, celery cytoplasm, and truffle fungus. Androstenone was the first mammalian pheromone to be identified. It is found in high concent ...

, a chemical which half of all humans cannot detect, some subjects gained the ability to smell it.

Alternatives

In addition to smell training, other treatments for anosmia that have been researched include

systemic Systemic fundamental to a predominant social, economic, or political practice. This refers to: In medicine In medicine, ''systemic'' means affecting the whole body, or at least multiple organ systems. It is in contrast with ''topical'' or ''loc ...

steroidal and non-steroidal oral medications,

topical medication A topical medication is a medication that is applied to a particular place on or in the body. Most often topical medication means application to body surfaces such as the skin or mucous membranes to treat ailments via a large range of classes ...

s, and

acupuncture Acupuncture is a form of alternative medicine and a component of traditional Chinese medicine (TCM) in which thin needles are inserted into the body. Acupuncture is a pseudoscience; the theories and practices of TCM are not based on scientifi ...

Efficacy

Mechanism

History

Alternatives

References

Further reading