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Olfactory fatigue, also known as odor fatigue or olfactory adaptation, is the temporary, normal inability to distinguish a particular odor after a prolonged exposure to that airborne compound. For example, when entering a restaurant initially the odor of food is often perceived as being very strong, but after time the awareness of the odor normally fades to the point where the smell is not perceptible or is much weaker. After leaving the area of high odor, the sensitivity is restored with time. Anosmia is the permanent loss of the sense of smell, and is different from olfactory fatigue.
It is a term commonly used in wine tasting, where one loses the ability to smell and distinguish wine bouquet after sniffing at wine(s) continuously for an extended period of time. The term is also used in the study of indoor air quality, for example, in the perception of odors from people, tobacco, and cleaning agents.
Olfactory fatigue is an example of neural adaptation or sensory adaptation. The body becomes desensitized to stimuli to prevent the overloading of the nervous system, thus allowing it to respond to new stimuli that are 'out of the ordinary'.
After olfactory neurons depolarize in response to an odorant, the G-protein mediated second messenger response activates adenylyl cyclase, increasing cyclic AMP (cAMP) concentration inside a cell, which then opens a cyclic nucleotide gated cation channel. The influx of Ca2+ ions through this channel triggers olfactory adaptation immediately because Ca2+/calmodulin-dependent protein kinase II or CaMK activation directly represses the opening of cation channels, inactivates adenylyl cyclase, and activates the phosphodiesterase that cleaves cAMP. These series of actions by CaMK, desensitizes olfactory receptors to prolonged odorant exposure.
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Sachs mentions the earliest reference as old as early nineteenth century. The primitive stages in curing the condition in humans were through the clinical experiments. The Elsberg method was one of the first to determine the threshold value for odorous substances. To date, his olfactometric method is known as one of the best methods to stimulate the threshold of olfactory senses and determine the decaying of the senses once there has been a continuous supply of same smell.
Some perfumers stock coffee beans near their displays as it is commonly believed that smelling coffee between perfume testing can limit or reverse olfactory fatigue.
This view remains controversial, with some researchers citing positive reactions while subsequent research seems to indicate no benefit from sniffing coffee beans compared to lemons scents or plain air: