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Olfactory fatigue, also known as odor fatigue, olfactory adaptation, and noseblindness, 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. Since odor detection may be an indicator that exposure to certain chemicals is occurring, olfactory fatigue can also reduce one's awareness about chemical hazard exposure.
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'.
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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.
An ORN or an Olfactory Receptor Neuron alert goes off to detect the smell. When the nose is covered taste is a lot harder because the air we breathe goes into our mouth as well. A common idea is that vanilla smells sweet and that is because we taste sweet when we eat vanilla flavorings.
Improving olfactory sense
Olfactometric method and clinical studies
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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.
Rejuvenation through coffee
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:
- "Smelling coffee aroma between perfume samples, as compared to smelling unscented air, actually works. The perceived odor intensity of the perfume from sample to sample stayed the same after smelling coffee aroma while it decreased when smelling air between samples. The pleasantness of the perfume, however, was similar after smelling coffee or air."
- "Fragrance sellers often provide coffee beans to their customers as a "nasal palate cleanser," to reduce the effects of olfactory adaptation and habituation. To test this idea, college students smelled three fragrances multiple times, rating odors each time. After completing nine trials, participants sniffed coffee beans, lemon slices, or plain air. Participants then indicated which of four presented fragrances had not been previously smelled. Coffee beans did not yield better performance than lemon slices or air."
Rejuvenation through other means
Another commonly believed way to reduce olfactory fatigue is to smell one's own unperfumed skin (typically the crook of the elbow) on the theory this is the "baseline" smell for each individual.
- Adaptive system
- American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)
- Building Indoor Environment
- Thermal comfort
- Odors chapter, Fundamentals volume of the ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, 2005
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