Hubbry Logo
OdorOdorMain
Open search
Odor
Community hub
Odor
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Odor
Odor
Odor
View on Wikipedia
from Wikipedia

"Smell", from Allegory of the Senses by Jan Brueghel the Elder, Museo del Prado

An odor (American English) or odour (Commonwealth English; see spelling differences) is a smell or a scent caused by one or more volatilized chemical compounds generally found in low concentrations that humans and many animals can perceive via their olfactory system. While smell can refer to pleasant and unpleasant odors, the terms scent, aroma, and fragrance are usually reserved for pleasant-smelling odors and are frequently used in the food and cosmetic industry to describe floral scents or to refer to perfumes.

Odor physiology

[edit]
Odor control covers at a sewage treatment plant: Under these covers, grit and gravel are settled out of the wastewater.

Sense of smell

[edit]

The perception of odors, or sense of smell, is mediated by the olfactory nerve. The olfactory receptor (OR) cells are neurons present in the olfactory epithelium, which is a small patch of tissue at the back of the nasal cavity. There are millions of olfactory receptor neurons that act as sensory signaling cells. Each neuron has cilia in direct contact with the air. Odorous molecules bind to receptor proteins extending from cilia and act as a chemical stimulus, initiating electric signals that travel along the olfactory nerve's axons to the brain.[1]

When an electrical signal reaches a threshold, the neuron fires, which sends a signal traveling along the axon to the olfactory bulb, a part of the limbic system of the brain. Interpretation of the smell begins there, relating the smell to past experiences and in relation to the substance(s) inhaled. The olfactory bulb acts as a relay station connecting the nose to the olfactory cortex in the brain. Olfactory information is further processed and forwarded to the central nervous system (CNS), which controls emotions and behavior as well as basic thought processes.

Odor sensation usually depends on the concentration (number of molecules) available to the olfactory receptors. A single odorant is usually recognized by many receptors. Different odorants are recognized by combinations of receptors. The patterns of neuron signals help to identify the smell. The olfactory system does not interpret a single compound, but instead the whole odorous mix. This does not correspond to the concentration or intensity of any single constituent.[2][3]

Most odors consist of organic compounds, although some simple compounds not containing carbon, such as hydrogen sulfide and ammonia, are also odorants. The perception of an odor effect is a two-step process. First, there is the physiological part. This is the detection of stimuli by receptors in the nose. The stimuli are recognized by the region of the human brain which handles olfaction. Because of this, an objective and analytical measure of odor is impossible. While odor feelings are personal perceptions, individual reactions are usually related. They relate to things such as gender, age, state of health, and personal history.

Smell acuity by age and gender

[edit]

The ability to identify odor varies among people and decreases with age. Studies claim that there are sex differences in odor discrimination, and that women usually outperform men.[4] Conversely, there are some studies claiming a male advantage.[5][6][7] A 2019 meta-analysis claimed that the differences in olfaction are extremely small, but confirmed a small advantage for women.[8]

Pregnant women have increased smell sensitivity, sometimes resulting in abnormal taste and smell perceptions, leading to food cravings or aversions.[9] The ability to taste also decreases with age as the sense of smell tends to dominate the sense of taste. Chronic smell problems are reported in small numbers for those in their mid-twenties, with numbers increasing steadily, with overall sensitivity beginning to decline in the second decade of life, and then deteriorating appreciably as age increases, especially once over 70 years of age.[10]

Smell acuity compared to other animals

[edit]
Two dogs sniffing each other

For most untrained individuals, the act of smelling acquires little information concerning the specific ingredients of an odor. Their smell perception primarily offers information that elicits an emotional response.[citation needed] Experienced individuals, however, such as flavorists and perfumers, can identify discrete chemicals in complex mixtures using only their sense of smell.

Odor perception is a primary evolutionary sense. The sense of smell can induce pleasure or subconsciously warn of danger, which may, for example, help to locate mates, find food, or detect predators. Humans have an unusually good sense of smell considering they have only 350 functional olfactory receptor genes compared to the 1,300 found in mice, for example. This is despite an apparent evolutionary decline in the sense of smell.[11][12] The human sense of smell is comparable with many animals, able to distinguish between a diverse range of odors. Studies have reported that humans can distinguish in the region of one trillion unique aromas.[13][14]

Habituation or adaptation

[edit]

Odors that a person is used to, such as their own body odor, are less noticeable than uncommon odors. This is due to "habituation." After continuous odor exposure, the sense of smell is fatigued, but recovers if the stimulus is removed for a time.[15] Odors can change due to environmental conditions: for example, odors tend to be more distinguishable in cool dry air.[16]

Habituation affects the ability to distinguish odors after continuous exposure. The sensitivity and ability to discriminate odors diminishes with exposure, and the brain tends to ignore continuous stimulus and focus on differences and changes in a particular sensation. When odorants are mixed, a habitual odorant is blocked. This depends on the strength of the odorants in the mixture, which can change the perception and processing of an odor. This process helps classify similar odors as well as adjust sensitivity to differences in complex stimuli.[17]

Genetic component

[edit]

The primary gene sequences for thousands of olfactory receptors are known for the genomes of more than a dozen organisms. They are seven-helix-turn transmembrane proteins. But there are no known structures for any olfactory receptor. There is a conserved sequence in roughly three quarters of all ORs. This is a tripodal metal-ion binding site,[18] and Suslick has proposed that the ORs are in fact metalloproteins (most likely with zinc, copper, and manganese ions) that serve as a Lewis Acid site for the binding of many odorant molecules. In 1978, Crabtree suggested that Cu(I) is "the most likely candidate for a metallo-receptor site in olfaction" of strong-smelling volatiles. These are also good metal-coordinating ligands, such as thiols.[19] In 2012, Zhuang, Matsunami, and Block confirmed the Crabtree/Suslick proposal for the specific case of a mouse OR, MOR244-3, showing that copper is essential for detection of certain thiols and other sulfur-containing compounds. Thus, by using a chemical that binds to copper in the mouse nose, so that copper was not available to the receptors, the authors showed that the mice could not detect the thiols without the copper. However, these authors also found that MOR244-3 lacks the specific metal ion binding site suggested by Suslick, instead showing a different motif in the EC2 domain.[20]

Evolutionary impact

[edit]

Gordon Shepherd proposed that the retro-nasal route of olfaction (odorants introduced to the olfactory mucosa through the oral cavity often as food) was partially responsible for the development of human olfactory acuity. He suggested the evolutionary pressure of diversification of food sources and increased complexity of food preparation presented humans with a broader range of odorants, ultimately leading to a "richer repertoire of smells". Animals such as dogs show a greater sensitivity to odors than humans, especially in studies using short-chain compounds. Higher cognitive brain mechanisms and more olfactory brain regions enable humans to discriminate odors better than other mammals despite fewer olfactory receptor genes.[21]

Measuring techniques

[edit]

Concentration

[edit]

Odor concentration refers to an odor's pervasiveness. To measure odor sensation, an odor is diluted to a detection or recognition threshold. The detection threshold is the concentration of an odor in air when 50% of a population can distinguish between the odorous sample and an odor-free reference sample. The recognition odor threshold is usually a factor of two to five higher than the detection threshold.[22]

The measurement of odor concentration is the most widespread method to quantify odors. It is standardized in CEN EN 13725:2003.[23] The method is based on dilution of an odor sample to the odor threshold. The numerical value of the odor concentration is equal to the dilution factor that is necessary to reach the odor threshold. Its unit is the "European Odour Unit", OUE. Therefore, the odor concentration at the odor threshold is 1 OUE by definition.

Olfactometer

[edit]

To establish odor concentration, an olfactometer is used which employs a group of human panelists. A diluted odorous mixture and an odor-free gas—n-Butanol—as a reference are presented from sniffing ports to a group of panelists who are sensitive in odor perception. To collect an odor sample, the samples are collected using specialized sample bags, which are made from an odor free material, e.g., Teflon. The most accepted technique for collecting odor samples is the lung technique, where the sample bag is placed in a sealed drum, where a vacuum is created outside the bag, which fills under expansion, and draws into itself the sample from the source. Critically, all components which touch the odor sample, must be odor free, which includes lines and fittings.

In comparing the odor emitted from each port, the panelists are asked to report if they can detect a difference between the ports. The gas-diluting ratio is then decreased by a factor of 1.4 or two (i.e., the concentration is increased accordingly). The panelists are asked to repeat the test. This continues until the panelists respond with certainty and correctly twice in a row. These responses are used to calculate the concentration of the odor in terms of European odor units (OUE/m3, where 1 OUE/m3≡40 ppb/v n-butanol).[24]

Humans can discriminate between two odorants that differ in concentration by as little as 7%.[25] A human's odor detection threshold is variable. Repeated exposure to an odorant leads to enhanced olfactory sensitivity and decreased detection thresholds for a number of different odorants.[26] It was found in a study that humans who were unable to detect the odor of androstenone developed the ability to detect it after repeated exposure.[27] People who cannot smell are said to be anosmic.

There are a number of issues which have to be overcome with sampling, these include:

  1. If the source is under vacuum
  2. if the source is at a high temperature
  3. If the source has high humidity

Issues such as temperature and humidity are best overcome using either pre-dilution or dynamic dilution techniques.

Other analytical methods

[edit]

Other analytic methods can be subdivided into the physical, the gas chromatographical, and the chemosensory method.

When measuring odor, there is a difference between emission and immission measurements. Emission measurement can be taken by olfactometry using an olfactometer to dilute the odor sample. Olfactometry is rarely used for immission measurement because of low odor concentrations involved. The same measuring principles are used, but the judgment of the air-assay happens without diluting the samples.

Odor measurement is essential for odor regulation and control.[28] An odor emission often consists of a complex mixture of many odorous compounds. Analytical monitoring of individual chemical compounds present in such an odor is usually not practical. As a result, odor sensory methods, instead of instrumental methods, are normally used to measure such odor. Odor sensory methods are available to monitor odor both from source emissions and in the ambient air. These two contexts require different approaches for measuring odor. The collection of odor samples is more easily accomplished for a source emission than for odor in the ambient air.[29]

Field measurement with portable field olfactometers can seem more effective, but olfactometer use is not regulated in Europe, while it is popular in the U.S. and Canada, where several states set limits at the receptor sites or along the perimeter of odor-emitting plants, expressed in units of dilution-to-threshold (D/T).[30]

Intensity

[edit]

Odor intensity refers to the perceived strength of an odor sensation. This intensity property is used to locate the source of odors and perhaps most directly related to odor nuisance.[3]

The perceived strength of an odor is measured alongside its concentration and can be mathematically modeled using the Weber-Fechner law: I = a × log(c) + b,[31] where:

  • I represents the perceived psychological intensity at a given dilution step on the butanol scale,
  • a is the Weber-Fechner coefficient,
  • C is the chemical concentration, and
  • b is the intercept constant (0.5 by definition).[31]

Odor intensity can be expressed using an odor intensity scale, which is a verbal description of an odor sensation to which a numerical value is assigned.[31]

Odor intensity can be divided into the following categories according to intensity:

0 – no odor
1 – very weak (odor threshold)
2 – weak
3 – distinct
4 – strong
5 – very strong
6 – intolerable

Odor intensity is determined in a laboratory by specialists who have been trained to accurately define intensity.

Hedonic tone assessment

[edit]

Hedonic assessment is the process of rating odors according to a scale ranging from extremely unpleasant to extremely pleasant. Intensity and hedonic tone, whilst similar, refer to different things: that is, the strength of the odor (intensity) and the pleasantness of an odor (hedonic tone). The perception of an odor may change from pleasant to unpleasant with increasing concentration, intensity, time, frequency, or previous experience with a specific odor—all factors in determining a response.[32]

FIDOL factors

[edit]

The overall set of qualities are sometimes identified as the "FIDOL (Frequency, Intensity, Duration, Offensiveness, Location) factors".[33]

The character of an odor is a critical element in assessing an odor. This property is the ability to distinguish different odors and is only descriptive. First, a basic description is used—such as sweet, pungent, acrid, fragrant, warm, dry, or sour. The odor is then referenced to a source such as sewage or apple which can then be followed by a reference to a specific chemical such as acids or gasoline.[3]

Most commonly, a set of standard descriptors is used, which may range from "fragrant" to "sewer odor".[34] Although the method is fairly simplistic, it is important for the FIDOL factors to be understood by the person rating the smell. This method is most commonly used to define the character of an odor which can then be compared to other odors. It is common for olfactometry laboratories to report character as an additional factor post sample-analysis.

Categorization

[edit]

Different categorizations of primary odors have been proposed, including the following, which identifies 7 primary odors:[22][35][36]

  1. Musky – perfumes
  2. Putrid – rotten eggs
  3. Pungent – vinegar
  4. Camphoraceous – mothballs
  5. Ethereal – dry cleaning fluid
  6. Floral – roses (see also floral scent)
  7. Pepperminty – mint gum

Though the concept of primary odors is not universally accepted.[36]

Interpretive dispersion modeling

[edit]

In many countries odor modeling is used to determine the extent of an impact from an odor source. These are a function of modeled concentration, averaging time (over what time period the model steps are run over, typically hourly), and a percentile. Percentiles refer to a statistical representation of how many hours per year the concentration C may be exceeded based on the averaging period.

Sampling from area sources

[edit]

There are two main odor sampling techniques: direct and indirect odor sampling techniques.

Direct sampling

[edit]

Direct refers to the placement of an enclosure on or over an emitting surface from which samples are collected, and an odor emission rate is determined.

The most commonly used direct methods include the flux chamber[37] and wind tunnels such as the one at the University of New South Wales (UNSW).[38] There are many other available techniques, and consideration should be given to a number of factors before selecting a suitable method.

A source which has implications for this method are sources, such as bark bed biofilters, that have a vertical velocity component. For such sources, consideration must be given as to the most appropriate method. A commonly used technique is to measure the odor concentration at the emitting surface, and combine this with the volumetric flow rate of air entering the biofilter to produce an emission rate.

Indirect sampling

[edit]

Indirect sampling is often referred to as back calculation. It involves the use of a mathematical formula to predict an emission rate.

Many methods are used, but all make use of the same inputs which include surface roughness, upwind and downwind concentrations, stability class (or other similar factor), wind speed, and wind direction.

Health risks

[edit]

The human sense of smell is a primary factor in the sensation of comfort. Olfaction as a sensory system brings awareness of the presence of airborne chemicals. Some inhaled chemicals are volatile compounds that act as a stimulus, triggering unwanted reactions such as nose, eye, and throat irritation. Perception of odor and of irritation is unique to each person, and varies because of physical conditions or memory of past exposures to similar chemicals. A person's specific threshold, before an odor becomes a nuisance, depends also on the frequency, concentration, and duration of an odor.

The perception of irritation from odor sensation is hard to investigate because exposure to a volatile chemical elicits a different response based on sensory and physiological signals, and interpretation of these signals is influenced by experience, expectations, personality, or situational factors. Volatile organic compounds (VOCs) may have higher concentrations in confined indoor environments, due to restricted infiltration of fresh air, as compared to the outdoor environment, leading to greater potential for toxic health exposures from a variety of chemical compounds. Health effects of odor are traced to the sensation of an odor or the odorant itself. Health effects and symptoms vary—including eye, nose, or throat irritation, cough, chest tightness, drowsiness, and mood change—all of which decrease as an odor ceases. Odors may also trigger illnesses such as asthma, depression, stress-induced illness, or hypersensitivity. The ability to perform tasks may decrease, and other social/behavioral changes may occur.

Occupants should expect remediation from disturbing and unexpected odors that disturb concentration, diminish productivity, evoke symptoms, and generally increase the dislike for a particular environment. It is important to set occupational exposure limits (OELs) to ensure the health and safety of workers, as well as comfort, because exposure to chemicals can elicit physiological and biochemical changes in the upper respiratory system. Standards are hard to set when exposures are not reported and can also be hard to measure. Workforce populations vary in terms of discomfort from odors because of exposure history or habituation, and they may not realize possible risks of exposure to chemicals that produce specific odors.[39][40]

Types

[edit]

Some odors are sought after, such as from perfumes and flowers, some of which command high prices. Whole industries have developed around products that remove or mask unpleasant odors, such as deodorant.

Odor molecules transmit messages to the limbic system, the area of the brain that governs emotional responses. Some believe that these messages have the power to alter moods, evoke distant memories, raise spirits, and boost self-confidence. This belief has led to "aromatherapy", wherein fragrances are claimed to cure a wide range of psychological and physical problems. Aromatherapy claims that fragrances can positively affect sleep, stress, alertness, social interaction, and general feelings of well-being. Evidence for the effectiveness of aromatherapy is mostly anecdotal and controlled scientific studies to substantiate its claims are lacking.

Some people are allergic to the fragrances found in perfume, scented shampoo, scented deodorant, or similar products. Reactions, as with other chemical allergies, can range from slight headaches to anaphylactic shock, which can result in death.[citation needed]

Unpleasant odors play various roles in nature, often to warn of danger, though this may not be known to the subject who smells it.[41] The natural gas industry uses odor to enable consumers to identify leaks. Natural gas in its native state is colorless and almost odorless. To help users detect leaks, an odorizer with the scent of rotten eggs, tert-Butylthiol (t-butyl mercaptan), is added. Sometimes a related compound, thiophane, may be used in the mixture.

An odor that is viewed as unpleasant by some people or cultures can be viewed as attractive by others where it is more familiar or has a better reputation.[41] It is commonly thought that those exuding an unpleasant body odor are unattractive to others. But studies have shown that a person who is exposed to a particular unpleasant odor can be attracted to others who have been exposed to the same unpleasant odor.[41] This includes odors associated with pollution.[41]

What causes a substance to smell unpleasant may be different from what one perceives. For example, perspiration is often viewed as having an unpleasant odor, but it is odorless. It is bacteria in the perspiration that causes the odor.[42]

Unpleasant odors can arise from specific industrial processes, adversely affecting workers and even residents downwind of the source. The most common sources of industrial odor arise from sewage treatment plants, refineries, animal rendering factories, and industries processing chemicals (such as sulfur) which have odorous characteristics. Sometimes industrial odor sources are the subject of community controversy and scientific analysis.

Body odor is present both in animals and humans and its intensity can be influenced by many factors (behavioral patterns, survival strategies). Body odor has a strong genetic basis both in animals and humans, but it can be also strongly influenced by various diseases and psychological conditions.

Study

[edit]

The study of odors is a growing field but is a complex and difficult one. The human olfactory system can detect many thousands of scents based on only minute airborne concentrations of a chemical. The sense of smell of many animals is even better. Some fragrant flowers give off odor plumes that move downwind and are detectable by bees more than a kilometer away.

The study of odors is complicated by the complex chemistry taking place at the moment of a smell sensation. For example, iron-containing metallic objects are perceived to have a distinctive odor when touched, although iron's vapor pressure is negligible. According to a 2006 study, this smell is the result of aldehydes (for example, nonanal) and ketones: 1-octen-3-one) released from the human skin on contact with ferrous ions that are formed in the sweat-mediated corrosion of iron. The same chemicals are also associated with the smell of blood, as ferrous iron in blood on skin produces the same reaction.[43]

Pheromones

[edit]

Pheromones are odors that are used for communication, and are sometimes called "airborne hormones". A female moth may release a pheromone that can entice a male moth that is several kilometers downwind. Honeybee queens constantly release pheromones that regulate the activity of the hive. Worker bees can release such smells to call other bees into an appropriate cavity when a swarm moves into new quarters, or to "sound" an alarm when the hive is threatened.

Advanced technology

[edit]

Most artificial or electronic nose instruments work by combining output from an array of non-specific chemical sensors to produce a fingerprint of whatever volatile chemicals in the local environment.[44] Most electronic noses need to be "trained" to recognize chemicals of interest before it can be used.[45][46] Many current electronic-nose instruments suffer from problems with reproducibility subject to varying ambient temperature and humidity. An example of this type of technology is the colorimetric sensor array, which visualizes odor through color change and creates a "picture" of it.[47][48]

Behavioral cues

[edit]

Odor perception is a complex process involving the central nervous system and can evoke psychological and physiological responses. Because the olfactory signal terminates in or near the amygdala, odors are strongly linked to memories and can evoke emotions. The amygdala participates in the hedonic or emotional processing of olfactory stimuli.[49] Odors can disturb our concentration, diminish productivity, evoke symptoms, and in general increase a dislike for an environment. Odors can impact the liking for a person, place, food, or product as a form of conditioning.[50] Memories recalled by odors are significantly more emotional and evocative than those recalled by the same cue presented visually or auditorily.[51] Odors can become conditioned to experiential states and when later encountered have directional influences on behavior. Doing a frustrating task in a scented room decreases performance of other cognitive tasks in the presence of the same odor.[52] Nonhuman animals communicate their emotional states through changes in body odor, and human body odors are indicative of emotional state.[53]

Human body odors influence interpersonal relationships and are involved in adaptive behaviors, such as parental attachment in infants or partner choice in adults. "Mothers can discriminate the odor of their own child, and infants recognize and prefer the body odor of their mother over that of another woman. This maternal odor appears to guide infants toward the breast and to have a calming effect."[citation needed] Body odor is involved in the development of infant–mother attachment and is essential to a child's social and emotional development and evokes feelings of security. Reassurance created by familiar parental body odors may contribute significantly to the attachment process.[54] Human body odors can also affect mate choice. Fragrances are commonly used to raise sexual attractiveness and induce sexual arousal. Researchers found that people choose perfume that interacts well with their body odor.[55]

Body odor is a sensory cue critical for mate selection in humans because it is a signal of immunological health. Women prefer men with major histocompatibility complex (MHC) genotypes and odor different from themselves especially during ovulation. Different MHC alleles are favorable because different allele combinations would maximize disease protection and minimize recessive mutations in offspring. Biologically females tend to select mates "who are most likely to secure offspring survival and thus increase the likelihood that her genetic contribution will be reproductively viable."[56]

Studies have suggested that people might be using odor cues associated with the immune system to select mates. Using a brain-imaging technique, Swedish researchers have shown that gay and straight males' brains respond in different ways to two odors that may be involved in sexual arousal, and that the gay men respond in the same way as straight women, though it could not be determined whether this was cause or effect. The study was expanded to include lesbian women; the results were consistent with previous findings that lesbian women were not as responsive to male-identified odors, while their response to female cues was similar to that of straight males.[57] According to the researchers, this research suggests a possible role for human pheromones in the biological basis of sexual orientation.[58]

An odor can cue recall of a distant memory. Most memories that pertain to odor come from the first decade of life, compared to verbal and visual memories which usually come from the 10th to 30th years of life.[59] Odor-evoked memories are more emotional, associated with stronger feelings of being brought back in time, and have been thought of less often as compared to memories evoked by other cues.[59]

Use in design

[edit]
Further information: Visual merchandising § Scent

The sense of smell is not overlooked as a way of marketing products. The deliberate and controlled application of scent is used by designers, scientists, artists, perfumers, architects, and chefs. Some applications of scents in environments are in casinos, hotels, private clubs, and new automobiles. For example, "technicians at New York City's Sloan-Kettering Cancer Center disperse vanilla-scented oil into the air to help patients cope with the claustrophobic effects of MRI testing. Scents are used at the Chicago Board of Trade to lower the decibel level on the trading floor."[60]

If ingredients are listed on a product, the term "fragrance" can be used in a general sense.

Scent preferences

[edit]

See also

[edit]
  • Aroma compound – Chemical compound that has a smell or odor
  • Chemoreceptor – Sensory receptor that detects chemicals
  • Flavor – Sense of chemicals on the tongue
  • Geosmin – Chemical compound responsible for the characteristic odour of earth
  • Machine olfaction – Simulation of the sense of smell
  • Olfaction – Sense that detects smells
  • Olfactometer – Instrument used to detect and measure odor dilution
  • Olfactory fatigue – Inability to distinguish an odor after prolonged exposure
  • Perfume – Mixture of fragrant substance
  • Petrichor – Earthy smell when rain falls on dry soil
  • Phantosmia – Smelling of an odor not actually there
  • Scented water – Lightly scented perfume

References

[edit]

Further reading

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Odor

View on Grokipedia
from Grokipedia
Odor is the perceptual quality arising from the stimulation of olfactory receptors by volatile chemical compounds, typically present in low concentrations in , which the interprets as a distinct smell or . These odorants, often organic molecules with sufficient to evaporate and reach the , bind to specialized protein receptors embedded in the cilia of olfactory sensory neurons located in the —a thin patch of tissue high in the nasal passages. The olfactory system's detection mechanism relies on the activation of approximately 400 functional receptor types in humans, each tuned to specific molecular features, generating electrical signals transmitted via the to the brain's and higher processing centers like the . This enables the discrimination of an estimated one trillion distinct odor mixtures, far exceeding earlier assumptions of mere thousands, and integrates with , , and other senses to influence and . Comprising around five to six million olfactory sensory neurons, the system regenerates throughout life but declines with age, contributing to conditions like that impair hazard detection and appetite regulation. Evolutionarily, odor perception serves critical survival functions, from identifying nutritious foods and spoilage to signaling dangers like smoke or toxins and facilitating social cues via pheromones, though human reliance on it has diminished relative to vision and audition in modern environments. In applied contexts, odors underpin industries like perfumery, food science, and environmental monitoring, where volatile compounds are analyzed for quality control or pollution assessment, underscoring olfaction's role in both biological adaptation and technological innovation.

Fundamentals of Odor

Definition and Subjective Perception

Odor is the perceptual quality evoked by volatile organic compounds, termed odorants, that interact with s in the nasal , triggering neural signals interpreted by the as smell. These odorants must be sufficiently volatile to reach the olfactory mucosa in airborne concentrations typically ranging from parts per million to , depending on the compound's and . The sensation arises not from the chemical itself but from the 's decoding of receptor activation patterns, which can involve hundreds of distinct types in humans. Perception of odor is subjective, shaped by individual genetic polymorphisms in approximately 400 functional genes, which influence detection thresholds, intensity ratings, and qualitative descriptors for specific odorants. For example, functional variants in a single receptor can modify for about 13% of tested odors across diverse populations, leading to differences in sensitivity or even to particular scents. Demographic factors, including age, , and geographic ancestry, further contribute to variability, with older adults exhibiting reduced acuity due to cumulative receptor loss estimated at 1% annually after age 20. Cognitive and experiential elements, such as familiarity and contextual associations, modulate interpretation, rendering odor qualities as constructed rather than purely physicochemical properties. Olfactory adaptation exemplifies subjectivity's dynamic aspect, wherein prolonged exposure to an odorant diminishes perceived intensity through receptor desensitization and central neural , allowing detection of novel stimuli amid constants. This process varies interindividually, influenced by receptor density and neural processing efficiency, and can lead to temporary . While universal thresholds exist for strong irritants, hedonic judgments—pleasant versus unpleasant—diverge widely, often tied to evolutionary or learned aversions rather than odorant chemistry alone.

Chemical and Physical Properties

Odorants, the chemical compounds responsible for odors, are predominantly volatile organic molecules that evaporate readily into the gaseous phase under ambient conditions, allowing them to reach olfactory receptors in the . These molecules typically exhibit low molecular weights, ranging from approximately 26 to 300 daltons, which facilitates their through air and in the aqueous layer overlying . Higher molecular weights, exceeding 400 daltons, generally reduce volatility and perceptibility, as seen with the largest known odorant, a labdane diterpenoid at 296 daltons. Key physical properties include high and low boiling points, enabling spontaneous at and contributing to odor persistence and detectability. correlates with odor intensity and detection thresholds; compounds with lower vapor pressures often require lower concentrations for detection due to slower rates. , influenced by molecular size and shape, affects and search patterns in odor detection, as demonstrated in canine behavioral studies where higher-diffusivity odorants lead to broader . , quantified by logP values, enhances interaction with lipid-rich receptor environments, with most odorants showing moderate hydrophobicity to balance air-mucus partitioning. Chemically, odorants encompass a diverse array of structures, including aliphatic hydrocarbons, alcohols, aldehydes, ketones, esters, and heterocyclic compounds often featuring functional groups such as carbonyls or thiols that influence receptor binding affinity. Sulfur-containing odorants, like mercaptans, exhibit particularly low detection thresholds (e.g., 0.00047 ppm for ) due to high receptor sensitivity, while the presence of double bonds or aromatic rings modulates odor quality and potency. These properties collectively determine an odorant's threshold concentration for human detection, typically measured in parts per million or billion, with variability arising from molecular interactions rather than bulk physical traits alone.

Physiology of Olfaction

Anatomy and Mechanism of Smell Detection

The , located in the superior on the of the , serves as the primary site for odor detection in humans, covering approximately 5 cm² bilaterally. This consists of three main cell types: neurons (ORNs), sustentacular (supporting) cells, and basal cells. ORNs, which are bipolar sensory neurons numbering around 6 million per side, extend dendrites apically into the layer and axons basally through the to form the (cranial nerve I). Sustentacular cells provide structural support and secrete , while basal cells act as stem cells for regenerating ORNs, which have a lifespan of about 30-60 days. Odor detection begins when volatile odorant molecules enter the nasal cavity via inhalation and dissolve in the aqueous mucus layer overlying the epithelium, which is enriched with odorant-binding proteins that facilitate transport to receptors. Each ORN expresses one of approximately 400 functional olfactory receptor (OR) genes, encoding G-protein-coupled receptors (GPCRs) localized on the cilia extending from the dendritic knob into the mucus. Binding of an odorant to its specific OR induces a conformational change, activating the heterotrimeric G-protein G_olf. This activation stimulates type III to produce (cAMP) from ATP, elevating intracellular cAMP levels. The increased cAMP binds to and opens cyclic nucleotide-gated (CNG) cation channels, primarily composed of CNGA2 and CNGA4 subunits, allowing influx of Na⁺ and Ca²⁺ ions, which depolarizes the ORN membrane. The entering Ca²⁺ further amplifies the signal by activating Ca²⁺-gated Cl⁻ channels (such as Ano2), leading to Cl⁻ efflux due to elevated intracellular Cl⁻ concentrations maintained by the Na⁺-K⁺-2Cl⁻ cotransporter NKCC1; this efflux contributes up to 80-90% of the receptor current, generating a that, if sufficient, triggers action potentials in the ORN . These action potentials propagate unmyelinated axons, bundled into fila olfactoria, through the to in the olfactory bulb's , where each receives input from ORNs expressing the same OR type, enabling initial spatial coding of odor quality. The transduction process is highly sensitive, with detection thresholds as low as for some odorants, and involves amplification via second messengers to achieve reliable signaling from sparse odorant bindings.

Variations in Olfactory Acuity

Olfactory acuity, encompassing sensitivity to odor detection thresholds, , and identification, exhibits significant inter-individual variability influenced primarily by genetic, physiological, and environmental factors. Genetic polymorphisms in (OR) genes, which number over 400 in humans, account for much of this diversity, with variants altering receptor function and leading to differences in . For instance, a specific polymorphism in the OR2J3 has been associated with variation in sensitivity to , a compound in sweat and , where certain alleles result in either heightened detection or specific . Genome-wide association studies further reveal sex-specific genetic variants linked to olfactory identification, underscoring a heritable basis for acuity differences. Sex-based differences are consistently observed, with meta-analyses of thousands of participants demonstrating that females outperform males across multiple olfactory tasks, including absolute detection thresholds, odor , and identification, with effect sizes ranging from small to medium. This advantage persists from childhood through adulthood and is evident in every tested aspect of olfaction, potentially linked to hormonal influences or greater neuronal density in olfactory regions, though central processing differences remain under investigation. Recent studies confirm females' superior performance in odor intensity ratings and familiarity, while hedonic judgments show less divergence. Age represents a major modulator of olfactory function, with acuity peaking in early adulthood and declining progressively thereafter, particularly after age 60. Longitudinal population-based studies tracking participants over 3–10 years document this trajectory, attributing it to degenerative changes in , reduced OR neuron regeneration, and central neural atrophy. By ages 65–80, over 50% of individuals experience measurable or , correlating with increased risks of , frailty, and neurodegeneration, as olfactory loss predicts faster cognitive decline and volume reduction in dementia-free older adults. Factors such as , chronic diseases, and nasal exacerbate this decline, though baseline genetic acuity influences susceptibility.

Neural Processing and Adaptation

Olfactory signals originate in the olfactory epithelium, where odorant molecules bind to G-protein-coupled receptors on the cilia of olfactory sensory neurons (OSNs), triggering a transduction cascade that generates action potentials. These axons project ipsilaterally to the olfactory bulb (OB), converging in a topographically organized manner onto approximately 1,800-2,400 glomeruli per bulb in humans, with each glomerulus receiving input exclusively from OSNs expressing one of about 400 olfactory receptor types. This convergence enables initial odor coding through sparse, combinatorial activation patterns across glomeruli, where specific odors evoke distinct spatial maps of glomerular activity, as observed in optical imaging studies of rodent OBs. Mitral and tufted cells, the primary output neurons of the OB, receive excitatory input within these glomeruli and integrate lateral inhibition via periglomerular and granule cells, refining representations through gain control and contrast enhancement before projecting to the piriform cortex and other limbic structures. Higher-order processing in the piriform cortex involves distributed, overlapping ensembles that support odor object recognition and contextual integration, with evidence from electrophysiological recordings indicating that odor identity is encoded in both rate and temporal patterns of neural firing. Olfactory adaptation, a reduction in perceived intensity during sustained or repeated exposure to an odorant, occurs via both peripheral and central mechanisms to prevent and enhance detection of novel stimuli. At the peripheral level in , adaptation involves calcium-dependent feedback: odorant-induced influx through cyclic nucleotide-gated (CNG) channels elevates intracellular Ca²⁺, which activates and Ca²⁺/-dependent kinase II to phosphorylate and desensitize the channels, reducing responsiveness within seconds to minutes. This process is complemented by guanylyl cyclase activation to restore cAMP levels and receptor dephosphorylation via phosphodiesterases. Central adaptation in the OB includes presynaptic inhibition of OSN inputs to glomeruli and postsynaptic adjustments in mitral cell excitability, mediated by slow and neuromodulatory circuits that normalize firing rates over prolonged stimulation. Human electro-olfactogram recordings confirm that adaptation manifests as decreased epithelial responses to repeated pulses, with recovery times scaling with stimulus duration and intensity, distinguishing it from short-term which may involve higher cortical feedback. These mechanisms ensure , though excessive adaptation can contribute to conditions like specific , where sensitivity to certain odors is selectively impaired.

Evolutionary and Comparative Perspectives

Olfaction in Non-Human Animals

Olfaction serves critical functions in non-human animals, including , predator avoidance, mate selection, and social communication, often surpassing human capabilities in . Across species, the exhibits conserved principles such as receptor activation by odorants and central processing in analogous structures, yet adaptations reflect ecological niches. In mammals , the olfactory epithelium contains up to 300 million receptors compared to approximately 6 million in humans, enabling detection of odorants at concentrations as low as parts per . The canine olfactory bulb is proportionally larger, occupying about 12.5% of brain volume versus 0.01% in humans, supporting discrimination of complex scent profiles for tasks such as tracking and . Aquatic vertebrates, particularly , possess olfactory rosettes with numerous lamellae, allowing detection of at dilutions of one part per million, equivalent to sensing one drop in an Olympic-sized . This sensitivity aids prey location in turbid waters, though directional flow and concentration gradients influence efficacy more than absolute distance myths suggest. Insects rely on antennal sensilla housing olfactory receptor neurons tuned to pheromones, enabling species-specific detection over kilometers; for instance, silkmoth males respond to bombykol at femtogram levels. Accessory proteins and sensillar lymph enhance binding efficiency, facilitating rapid behavioral responses in and . Birds display wide variation in olfaction, with procellariiforms like albatrosses using enlarged olfactory bulbs for odor-guided over oceans, detecting prey volatiles from afar. Contrary to prior assumptions of olfactory reduction due to flight, genomic analyses reveal olfactory retention in many taxa, supporting roles in nestling recognition and selection.

Evolutionary Role in Survival and Reproduction

Olfaction has played a pivotal role in animal survival by enabling the detection of food sources and avoidance of predators, with evolutionary pressures shaping olfactory systems to prioritize chemical cues for foraging efficiency. In competitive environments, such as those faced by Drosophila larvae, reliance on smell provides a survival advantage by facilitating rapid localization of food amid rivals, as demonstrated in experiments where olfactory mutants exhibited reduced foraging success and higher mortality rates. Similarly, in invertebrates, food odors trigger behavioral responses that enhance resource utilization and digestive preparation, underscoring olfaction's adaptive value in optimizing energy intake across taxa. Predator detection via olfactory cues is innate and widespread, as evidenced in birds where chemical signals from predators elicit avoidance behaviors without prior learning, and in lizards where olfaction aids in distinguishing reptilian threats from non-predatory scents. In evolutionary terms, the conservation of olfactory pathways across reflects selection for these survival functions, with s evolving neural architectures responsive to environmental chemical gradients for and assessment. For instance, aquatic like Neolamprologus pulcher integrate olfaction with other senses to discriminate dangerous heterospecifics, highlighting its role in multimodal predator avoidance under varying ecological pressures. These adaptations likely arose early in , as olfaction's primacy allowed ancestral organisms to exploit chemical information for basic survival before visual or auditory systems dominated in diurnal lineages. Regarding reproduction, olfaction facilitates mate selection and , particularly in mammals where chemical signals convey genetic compatibility and health status. Experimental evidence from shows females preferentially select males based on olfactory cues linked to (MHC) diversity, promoting heterozygous offspring with enhanced immune responses—a mechanism conserved across mammalian lineages. In , rapid of genes correlates with shifts in social and mating behaviors, enabling detection of pheromones that signal fertility and kin avoidance to prevent . This reproductive utility extends to broader processes, as seen in birds where olfactory differences contribute to and isolation between populations. Overall, the expansion of olfactory families—comprising up to 1% of mammalian genomes—underscores selection for scent-based in , balancing attraction to dissimilar genotypes against familiarity for . Such traits persist despite reductions in olfactory reliance in visually dominant species, affirming olfaction's foundational evolutionary role in ensuring .

Chemical Communication and Pheromones

Evidence for Pheromones in Animals

Pheromones were first empirically demonstrated in insects, where they elicit stereotyped behavioral and physiological responses. The sex pheromone bombykol, a 16-carbon alcohol ((10E,12Z)-hexadeca-10,12-dien-1-ol), was isolated from female silkworm moths (Bombyx mori) in 1959 by Adolf Butenandt and colleagues after screening over 500,000 individuals, triggering oriented flight, landing, and precopulatory wing fanning in males at concentrations as low as 10^{-10} g. This discovery established pheromones as species-specific chemical signals modulating reproduction, with subsequent identifications in over 1,500 insect species confirming releaser effects like aggregation and alarm signaling. In social insects such as , trail pheromones provide robust evidence of coordination. Forager deposit hydrocarbons like (Z)-9-hexadecenal from their poison glands onto substrates, which recruits nestmates to food sources by eliciting trail-following behavior; experiments show trails persist for hours and amplify recruitment via positive feedback, with trail-laying rates increasing exponentially with colony size in species like (Linepithema humile). Alarm pheromones, such as 4-methyl-3-heptanone in fire ants (Solenopsis invicta), induce rapid escape or attack responses, detected via antennal sensilla and quantified in field assays where exposure reduces efficiency by up to 50%. Evidence in mammals, while more complex due to multimodal signaling, includes urinary and salivary volatiles processed via the (VNO). In mice (Mus musculus), major urinary proteins (MUPs) bound to sulfated steroids like 2,5-dimethylpyrazine act as male pheromones, accelerating female onset by 5-10 days through VNO-mediated hypothalamic activation, as shown in studies where VNO abolishes the effect. Male mouse alarm pheromones, such as (R)- and from stressed individuals, trigger avoidance freezing via VNO TRPC2 channels, with neural recordings confirming discriminatory sensitivity to 10^{-9} M concentrations. In pigs (Sus scrofa), the steroidal (5α-androst-16-en-3-one) from boar functions as a primer , inducing the standing reflex () in estrous sows within seconds of exposure, facilitating ; bioassays demonstrate detection thresholds below 1 ppb via the VNO, supporting its role in synchronizing . Similar "male effect" in goats and sheep, involving axillary volatiles, trigger ovulatory surges in anestrous females, with plasma elevations measurable within 30 minutes post-exposure in controlled trials. These findings, derived from sensory ablations, receptor genetics, and chromatographic identifications, affirm pheromonal mediation across taxa, though mammalian responses often integrate with learned cues unlike the innate rigidity in .

Debates on Human Pheromones

The existence of human pheromones—defined as species-specific chemicals that elicit innate, stereotyped behavioral or physiological responses in conspecifics—remains highly debated, with empirical evidence failing to meet the rigorous criteria established for pheromones in other mammals. Unlike insects or rodents, where pheromones trigger precise responses via the vomeronasal organ (VNO), humans lack a functional VNO; the organ is vestigial, with pseudogenized receptor genes and no demonstrable chemosensory role in adults. Early claims of VNO responsiveness to steroids in the 1990s were not replicated under controlled conditions, and histological studies confirm its structural degeneration post-infancy. This anatomical deficit undermines arguments for pheromone-like signaling, as human olfaction relies primarily on the main olfactory epithelium without evidence of dedicated pheromone pathways. Proponents have focused on axillary steroids such as (AND, derived from male testosterone ) and estratetraenol (EST, from female estrogens), citing over 40 studies since 2000 reporting subtle effects on mood, , or attraction when subliminally presented. For instance, AND exposure has been linked to increased in women and enhanced hedonic ratings of emotional faces, while EST reportedly elevates metrics in men. However, these findings suffer from methodological flaws, including small sample sizes (often n<50), lack of double-blind replication, and positive publication bias, where null results are underreported. Independent attempts to verify gender-specific signaling by AND and EST failed to show differential perception or behavioral modulation, suggesting effects may stem from general olfactory processing rather than pheromonal specificity. Skeptics, including bioassay experts, argue that no molecule qualifies as a human pheromone absent bioassay-led identification—starting from observed behavioral changes traced to isolated volatiles—followed by cross-context validation. Claims for AND and EST originated from anecdotal commercial products rather than natural contexts, and their concentrations in sweat are orders of magnitude below those evoking responses in lab settings. Recent reviews (2023–2025) emphasize the replication crisis in this field, with meta-analyses revealing inconsistent effects across cultures and no causal link to reproductive outcomes like mate choice or ovulation synchrony (the latter debunked by larger datasets). While human body odors influence preferences via major histocompatibility complex (MHC) dissimilarity—evidenced in shirt-sniffing paradigms—these appear modulated by learned hedonic valence, not innate pheromonal triggers. Emerging research on sweat-borne volatiles (e.g., from apocrine glands) and tears explores broader chemosignaling, but effects remain context-dependent and non-specific, akin to emotional contagion via rather than pheromones. As of 2025, no consensus exists; while some detect minor autonomic shifts, the field lacks the robust, reproducible evidence required to affirm pheromones, with calls for preregistered, large-scale trials to resolve ambiguities. This debate highlights tensions between anecdotal or underpowered studies—often from labs with commercial ties—and stricter evidential standards, underscoring that human chemical communication prioritizes volatile mixtures over singular pheromones.

Measurement and Analytical Techniques

Psychophysical and Olfactometric Methods

Psychophysical methods in olfaction evaluate the perceptual responses to odor stimuli, encompassing detection thresholds, discrimination abilities, and odor identification through controlled human testing. These approaches rely on quantitative scaling techniques, such as magnitude estimation or category rating, and forced-choice paradigms to reduce subjective bias and establish reliable sensory metrics. Staircase adaptive procedures, which iteratively adjust stimulus intensity based on subject responses, are commonly used to determine thresholds efficiently. A prominent example is the Sniffin' Sticks test, which utilizes odor-impregnated felt-tip pens presented in blinded trials. The threshold subtest involves staircased dilutions of n-butanol, where subjects select the odorous pen from triplets of blanks or weaker concentrations, yielding scores from 0 to 16 based on reversal points. Discrimination assesses the ability to differentiate odors in similar triplets, while identification tests recognition of 16 common scents with verbal cues, providing composite TDI (threshold-discrimination-identification) scores for clinical olfactory function. This battery demonstrates high reliability, with test-retest correlations exceeding 0.7, though performance declines with age and is influenced by factors like nasal airflow. Olfactometric methods quantify odor intensity and concentration using standardized human panels as detectors, distinct from individual psychophysical profiling by focusing on sample dilution to imperceptibility. Dynamic olfactometry, governed by the European standard EN 13725 (2003, with updates), dilutes gaseous odor samples with purified air in ratios presented via olfactometers to trained assessors, who report detection in yes/no responses. The odor concentration is calculated as the reciprocal of the dilution at which 50% of the panel (minimum 4 assessors) detects the odor, expressed in European odour units per cubic meter (ou_E/m³), with panel screening ensuring low personal odor sensitivity variance (geometric standard deviation < 2.3). This technique supports environmental monitoring, as validated in inter-laboratory trials showing coefficients of variation around 50% due to inherent human variability. Olfactometers, such as triangular or sequential sampling devices, deliver precise, humidity- and temperature-controlled stimuli to isolate olfactory from trigeminal responses. Limitations include assessor fatigue after 15-20 trials and the need for pre-screening to exclude anosmics, ensuring method validity over instrumental alternatives like gas chromatography, which correlate poorly with perceived odor strength. Recent advancements incorporate automated delivery for self-administered thresholds, reducing examiner bias while maintaining comparability to manual tests.

Instrumental Analysis and Modeling

Instrumental analysis of odors primarily relies on techniques that separate, identify, and quantify volatile organic compounds (VOCs) responsible for olfactory perception, bypassing subjective human assessment. Gas chromatography-mass spectrometry (GC-MS), often coupled with headspace solid-phase microextraction (HS-SPME), enables the extraction and detection of trace VOCs in odor samples by vaporizing analytes, separating them via a chromatographic column, and ionizing them for mass spectral identification. This method has been applied to human body odor analysis, achieving detection limits in the parts-per-billion range for compounds like aldehydes and ketones. Gas chromatography-olfactometry (GC-O) integrates chromatographic separation with human sensory detection, where effluents are sniffed at the detector outlet to pinpoint odor-active compounds based on dilution thresholds and perceived intensity. Recent advancements, such as rapid GC and comprehensive two-dimensional GC (GC×GC), enhance resolution of complex mixtures, allowing identification of low-concentration odorants that contribute disproportionately to overall aroma due to high odor activity values (OAVs), calculated as concentration divided by odor threshold. GC-O has been validated in food and environmental samples, correlating instrumental peaks with sensory descriptors like "fruity" or "sulfurous." Electronic noses (e-noses) employ arrays of gas sensors—typically metal oxide semiconductors, quartz crystal microbalances, or conducting polymers—that generate pattern-based responses to odor profiles, processed via machine learning algorithms for classification and quantification. Portable e-nose systems, such as those using high-bandwidth readouts, achieve real-time odor monitoring with spatial resolution comparable to dynamic olfactometry, though slightly lower accuracy in complex matrices like landfill emissions. Training models often involve principal component analysis or neural networks to discriminate odor sources, with applications in environmental compliance where sensor drift is mitigated through periodic recalibration. Odor modeling complements instrumental analysis by simulating dispersion and concentration predictions using atmospheric transport equations. Gaussian plume models, like AERMOD, treat odors as non-reactive pollutants, estimating downwind concentrations from emission rates, meteorology, and topography, with odor units expressed in dilutions to threshold (D/T). Lagrangian models track particle trajectories stochastically, improving accuracy for variable winds over short ranges, as in wastewater treatment facilities. These models incorporate empirical odor emission factors derived from flux chamber measurements, validated against field olfactometry, but require site-specific validation due to uncertainties in source characterization and boundary layer turbulence. Hybrid approaches combining e-nose data with dispersion simulations enable predictive mapping of odor plumes, aiding regulatory assessments.

Factors Influencing Odor Assessment

Individual differences in olfactory sensitivity profoundly affect odor assessment outcomes, particularly in human-based methods like dynamic olfactometry. Age is a primary determinant, with olfactory thresholds increasing (indicating reduced sensitivity) as individuals age; for instance, panel sensitivity to environmental odors decreases by approximately one dilution step per 25-year age increment. Sex also plays a role, with females generally demonstrating lower odor detection thresholds than males across various odorants, a pattern observed in large-scale population studies explaining about 0.3% of perceptual variance. Genetic polymorphisms in olfactory receptor genes account for much of this inter-individual variability, altering binding affinities and perceptual thresholds for specific odorants; functional assays have validated such effects for at least 10 odorant-receptor pairs. Health status and lifestyle factors further modulate assessment reliability. Conditions impairing nasal airflow, such as allergies or post-viral damage, elevate detection thresholds, as evidenced in cohorts with persistent olfactory dysfunction following respiratory infections. Smoking and certain medications suppress olfactory function, while demographic elements like ethnic background correlate with differential detection sensitivities due to population-level genetic variations. In sensory panels, pre-screening for hyposmia or exclusion of individuals with acute illnesses (e.g., colds) is standard to mitigate these effects, though residual variability persists across assessors. Environmental and stimulus-related variables introduce additional confounding in odor evaluation. Temperature and humidity alter odorant volatility and dispersion; elevated temperatures enhance evaporation rates, potentially lowering perceived thresholds, while high humidity can dampen diffusion. Sniffing parameters—odorant concentration (molecule count), inhalation volume, and duration—directly scale perceptual magnitude, with empirical models linking response intensity to the product of these factors. Background odors and sample storage duration exacerbate variability, as aging samples yield lower odor unit values in panel tests due to compound degradation. Procedural standardization, including use of reference standards like n-butanol and trained panels, reduces but does not eliminate high inter-laboratory discrepancies, often exceeding 50% in concentration estimates.

Classification and Types of Odors

Hedonic Tone and Intensity

Odor hedonic tone, also known as hedonic valence, describes the affective quality of an odor along a continuum from unpleasant to pleasant, representing one of the primary perceptual dimensions in olfaction. This dimension captures the immediate emotional response elicited by the stimulus, often preceding identification or intensity judgments in cognitive processing. In contrast, odor intensity quantifies the perceived strength or concentration of the odor, independent of its emotional connotation, and is typically rated on unipolar scales ranging from undetectable to extremely strong. These two attributes are assessed separately in psychophysical paradigms, such as magnitude estimation or category scaling, where panelists rate odors using standardized tools like visual analog scales or labeled magnitude scales (e.g., 0–10 for intensity, with descriptors like "barely detectable" to "extremely intense"). Although hedonic tone and intensity are conceptually distinct, empirical studies reveal moderate correlations influenced by odor type and context; for instance, higher intensity often amplifies unpleasantness in malodors but may optimize pleasantness at intermediate concentrations before declining at higher levels due to sensory overload or irritation. A 2019 analysis of odor concentration effects found that hedonic tone peaks at moderate dilutions for many volatile compounds, shifting negatively as intensity exceeds optimal thresholds, reflecting a quadratic relationship rather than strict independence. Field assessments of environmental odors, such as those near industrial sites, consistently show negative correlations between rated intensity and hedonic tone among exposed residents, where stronger odors are deemed more annoying regardless of source familiarity. Individual variability in hedonic ratings arises from genetic, experiential, and cultural factors, with intensity perceptions showing less inter-subject dispersion but still modulated by adaptation and context; for example, familiar odors tend to receive higher intensity and more positive hedonic scores in laboratory settings. Neuroimaging evidence indicates partial independence, as amygdala activation responds to intensity irrespective of valence, while orbitofrontal cortex integrates both for affective appraisal. In applied contexts like odor annoyance modeling, combining hedonic tone with intensity via weighted indices (e.g., intensity × hedonic scale) better predicts human response than either alone, as validated in European field studies from 2003 onward.

Categorization by Source and Composition

Odors are categorized by their sources into biological, environmental, and anthropogenic origins, with each category featuring distinct chemical compositions dominated by volatile organic compounds (VOCs) that evaporate readily to interact with olfactory receptors. Biological sources encompass emissions from humans and animals, plants, and microorganisms, where odors typically stem from metabolic byproducts or secondary metabolites; for example, human axillary odor primarily consists of branched-chain fatty acids such as (E)-3-methyl-2-hexenoic acid and 3-hydroxy-3-methylhexanoic acid, produced via microbial transformation of apocrine sweat gland secretions. Plant odors arise from terpenoids like limonene and linalool, along with green leaf volatiles such as hexanal, emitted for defense or attraction. Microbial odors, from bacterial or fungal activity, often include amines and sulfides generated during decomposition. Environmental sources, such as soil bacteria or algal blooms, produce geosmin and 2-methylisoborneol, earthy sesquiterpenes responsible for musty smells in water and air. Anthropogenic odors from industrial emissions feature a broader range of synthetic or process-derived VOCs, including aldehydes, benzene derivatives like toluene, and sulfur compounds such as hydrogen sulfide from wastewater or refining. Chemically, odorants are classified by molecular structure and functional groups, which dictate their volatility and perceptual qualities, with most having carbon chains of 4–12 atoms and vapor pressures allowing detection at parts-per-billion levels. Aldehydes (R-CHO) contribute green or nutty notes, as in from lipid oxidation; ketones (R-CO-R') yield fruity or caramel scents, exemplified by derivatives; and esters (R-COOR') evoke fruity aromas, common in fermentation products. Terpenoids, isoprene-derived hydrocarbons, dominate floral and resinous plant odors, while nitrogenous amines (R-NH2) produce fishy smells in decaying matter, and sulfur-containing thiols (R-SH) impart pungent, garlic-like qualities in both biological decay and industrial effluents.
Chemical ClassFunctional GroupTypical Odor ProfilePrevalent SourcesKey Examples
Aldehydes-CHOGreen, fatty, almondPlants, food oxidation, industrialHexanal, benzaldehyde
Ketones>C=OFruity, buttery, Acetone, 2-butanone
Esters-COORFruity, sweetFruits, microbial fermentation,
TerpenoidsIsoprene units, pine, essential oils,
Fatty Acids-COOHRancid, cheesyBody secretions, dairyIsovaleric acid, 3M2H
Sulfur Compounds-SH, -S-Rotten, garlicDecay, industry,
Amines-NH2Fishy, Protein breakdown
These classifications overlap, as mixtures rather than single compounds typically produce perceived odors, with composition varying by concentration and interactions.

Behavioral and Psychological Effects

Habituation and Sensory Adaptation

Olfactory sensory refers to the physiological diminution of neural responsiveness in olfactory sensory neurons (OSNs) during sustained exposure to an odorant, enabling the to recalibrate sensitivity for detecting changes in stimulus intensity or . This process occurs primarily at the peripheral level within the olfactory cilia, where odorant binding to G protein-coupled receptors triggers cyclic nucleotide-gated (CNG) channel opening and calcium influx, but subsequent feedback mechanisms—such as calcium-calmodulin-mediated and channel desensitization—reduce the transduction current, leading to decreased firing rates. For instance, in and mammalian models, adaptation manifests within seconds to minutes, with recovery upon stimulus removal, as demonstrated by patch-clamp recordings showing reversible suppression of CNG currents by intracellular calcium levels exceeding 10 μM. In contrast, to odors involves a central nervous system-mediated decrease in behavioral or perceptual response to repeated, non-aversive stimuli, distinct from by its reliance on learning-like processes rather than direct receptor . Empirical psychophysical studies in humans reveal that repeated odor presentations elevate detection thresholds by 20-50% and attenuate suprathreshold intensity ratings, with effects persisting beyond stimulus offset but recoverable through interspersed odors or breaks exceeding 30 seconds. This form of enhances discrimination of foreground odors against backgrounds, as shown in models where circuits habituate to sustained inputs via synaptic depression, improving signal-to-noise ratios for transient stimuli by up to 2-fold in spike train analyses. The interplay between and underlies everyday olfactory experience, such as rapid desensitization to one's own or ambient scents, but their independence is evident in dissociated effects: peripheral persists under , while requires conscious and exhibits characteristics like stimulus specificity and dishabituation to intensified or novel cues. Human studies confirm modulates early olfactory event-related potentials (e.g., N1/P2 components) within 200-400 ms post-stimulus, reducing amplitudes by 30-40% after 5-10 repetitions, independent of peripheral receptor fatigue. Dysfunctions in these processes contribute to conditions like or , where impaired prolongs perceived intensity, as observed in clinical thresholds deviating 1.5-2 standard deviations from norms in affected populations.

Body Odor Preferences and MHC Influence

Research indicates that preferences for human body odor may be influenced by genetic dissimilarity in the (MHC), a cluster of genes encoding proteins, with the hypothesis that such preferences promote offspring heterozygosity for enhanced disease resistance. In a seminal 1995 experiment, 49 women rated the attractiveness of body odors collected from T-shirts worn by 44 men over two nights; women preferred odors from MHC-dissimilar men, an effect absent or reversed in those using oral contraceptives, suggesting hormonal modulation. Subsequent studies have yielded mixed results, with some replicating female preferences for dissimilar male odors during fertile phases of the , while others found no such pattern or preferences for similarity. A 2020 meta-analysis of 10 studies on odor preferences (23 effect sizes) found no significant overall association between MHC dissimilarity and human body odor attractiveness (Zr = -0.020, p = 0.360), though small effects appeared in subgroups like pill non-users. Men's preferences for female body odors show even weaker evidence, with a 2017 study of 113 men detecting no attraction to HLA-dissimilar, heterozygous, or rare-allele odors. Criticisms include small, predominantly Western samples prone to publication bias, confounding from diet or hygiene, and failure of lab preferences to predict actual partner MHC dissimilarity in genomic analyses of couples. These limitations question the robustness of MHC effects beyond controlled settings. Proposed mechanisms involve MHC-bound peptides volatilizing into odorants detectable by the , as evidenced by peptide-specific activation in vomeronasal neurons and self-recognition studies, rather than microbiota mediation, which lacks direct support in germ-free models. Despite evolutionary rationale, empirical translation to remains unsubstantiated, with no MHC-dissimilarity link in real couples from large-scale genetic data. A 2025 study noted women preferring MHC-similar male odors but observed higher from dissimilar sources , hinting at post-copulatory mechanisms independent of odor.

Scent in Human Attraction and Social Behavior

Humans detect and respond to body odors subconsciously, which can influence perceptions of attractiveness and compatibility in potential mates. Experimental studies using worn t-shirts to capture natural body odors have shown that women often rate the scents of men with dissimilar major histocompatibility complex (MHC) genotypes as more pleasant, suggesting an olfactory mechanism favoring genetic diversity to enhance offspring immunity. This preference aligns with heterozygote advantage hypotheses, where dissimilar MHC alleles reduce inbreeding risks and broaden immune responses, though results vary by population and methodology, with some replication challenges noted in meta-analyses. In contrast, a 2025 study found women preferred odors from MHC-similar men, yet sperm from MHC-dissimilar men showed higher motility in exposure tests, indicating potential dissociation between odor preference and gamete-level compatibility. Olfactory cues also modulate social judgments beyond mate selection, affecting interpersonal and . Individuals with similar body odors report stronger initial connections and formation, as demonstrated in assessments where odor similarity predicted liking between same-sex strangers. MHC heterozygotes tend to produce less intense body odors, potentially signaling genetic fitness and influencing social perceptions of . Crossmodal effects occur wherein pleasant odors enhance ratings of facial attractiveness and trustworthiness, while unpleasant ones diminish them, underscoring olfaction's role in multisensory social evaluation. In parent-offspring interactions, facilitates recognition and critical for . Mothers can distinguish their newborn's odor from others with high accuracy, often preferring it, which supports and attachment formation. Exposure to maternal in reduces neural responses to fearful stimuli, promoting calmer and brain-to-brain synchrony during interactions. Infant body odors, distinct from adults due to unique volatile compounds, elicit protective and affiliative behaviors in caregivers, reinforcing dyadic bonds. Claims of pheromones—specific chemical signals triggering fixed attraction or social responses—lack robust empirical support, differing from well-documented effects in and . Compounds like may subtly alter mood or aggression in sex-dependent ways, but effects are context-specific and not reliably replicable as "love potions." Overall, olfactory social signaling relies more on complex, individually variable body odors than discrete pheromones, with cultural and experiential factors modulating responses.

Applications and Technological Advances

Industrial and Environmental Uses

In the natural gas industry, odorless is intentionally odorized with organosulfur compounds such as tert-butyl mercaptan or ethyl mercaptan to enable human detection of leaks, a practice standardized since the mid-20th century following incidents like the 1937 New London explosion that highlighted the risks of undetected gas accumulation. These odorants are added at concentrations of approximately 1 pound per million cubic feet of gas, producing a distinctive rotten egg smell detectable at parts-per-million levels, thereby serving as a mechanism in pipelines and distribution systems regulated by bodies like the U.S. Pipeline and Hazardous Materials Safety Administration. Wastewater treatment facilities employ odor detection and control technologies to mitigate emissions of (H2S) and volatile organic compounds (VOCs), which arise from anaerobic decomposition processes. Biofilters and biotrickling filters achieve removal efficiencies exceeding 90% for H2S in full-scale operations, while chemical target and mercaptans, ensuring compliance with environmental standards such as those set by the U.S. Environmental Protection Agency for management. Electronic noses, mimicking biological olfaction through arrays, monitor these odors in real-time at treatment plants, correlating responses with panel assessments for concentrations as low as 1 odor unit per cubic meter. Environmentally, odor sensors facilitate air quality surveillance by identifying pollutants from industrial emissions and landfills, with applications in detecting VOCs and reduced compounds indicative of anaerobic conditions. Portable systems have been deployed for perimeter monitoring around sites, enabling predictive modeling of odor plumes via dispersion algorithms integrated with meteorological data. In ecological contexts, such technologies support remediation efforts by quantifying odor intensity and hedonic tone, aiding in the assessment of restoration efficacy in contaminated sites where volatile emissions signal ongoing .

Recent Developments in Machine Olfaction

Machine olfaction, the emulation of biological smell sensing through electronic noses (e-noses), has advanced significantly through sensor miniaturization, biomimetic designs, and AI integration. In November 2024, researchers introduced a high-speed miniaturized e-nose featuring high-bandwidth sensor readouts, precise parameter control, and machine learning algorithms, enabling rapid odor classification with response times under milliseconds, surpassing traditional systems limited by diffusion delays. Concurrently, data-centric approaches using eigengraph methods improved odor identification by preserving critical sensor data, achieving higher accuracy in distinguishing complex mixtures compared to conventional dimensionality reduction techniques. Biomimetic innovations have incorporated natural olfaction principles, such as active sniffing mechanisms in arrays, yielding faster response times and enhanced in dynamic environments; a January 2025 study demonstrated these designs outperform passive s by factors of 2-5 in accuracy for detection. In May 2025, an AI-powered e-nose was developed to mimic human-like scent differentiation, analyzing diverse odors for healthcare diagnostics via breath volatiles, with classification accuracies exceeding 90% in preliminary tests against pathogens and diseases. models, including the Principal Odour Map, have bridged molecular structures to perceptual qualities, predicting smell profiles from chemical data with reduced reliance on empirical training sets. By August 2025, commercial applications emerged, exemplified by Ainos' patented AI Nose device, deployed in fabrication for real-time defect detection via odor signatures, targeting sub-ppm sensitivity in environments. Comprehensive reviews from 2020-2025 underscore the shift from lab prototypes to field-deployable systems, driven by advances in nanomaterial sensors, advanced sampling (e.g., micro-preconcentrators), and for , though challenges persist in cross-environment drift compensation. Neuromorphic computing principles are increasingly applied to emulate processing, promising energy-efficient, real-time olfaction for , with prototypes showing 10-fold reductions in power consumption relative to von Neumann architectures. These developments position machine olfaction for broader integration in , , and medical diagnostics, contingent on standardized datasets to mitigate in AI models.

Olfactory Interfaces and Medical Applications

Olfactory interfaces encompass technologies designed to generate, deliver, and control odors for human perception, often integrated with virtual or systems to enhance multisensory immersion. These devices typically employ miniaturized odor generators (OGs) using principles such as or piezoelectric ejection to release volatile compounds with low latency and precise spatiotemporal control. A 2023 study demonstrated a soft, olfactory interface attached to the skin, capable of synchronizing scents with 3D virtual environments via connectivity, enabling applications like 4D movie experiences and emotion modulation through targeted odor delivery. Such interfaces have shown potential in extending human-computer interaction beyond visual and auditory cues, with prototypes achieving odor release volumes as low as microliters for sustained delivery without user discomfort. In medical contexts, olfactory interfaces facilitate therapeutic interventions, particularly olfactory training protocols aimed at restoring olfactory function in patients with or . Olfactory training involves repeated, structured exposure to distinct odorants—such as , , , and —typically twice daily for 20-30 seconds per scent over several months, leveraging to reactivate olfactory pathways. A review of 13 years of research, encompassing over 20 studies, confirmed that olfactory training yields statistically significant improvements in threshold, , and identification (TDI) scores, with a mean gain of 10.3 points on standardized tests compared to no intervention. For post-viral , including cases following infection, training reduced anosmia prevalence from baseline rates of around 40-50% to under 10% after 12 weeks in intervention groups, outperforming controls in randomized trials. Emerging olfactory brain-computer interfaces (BCIs) further refine this by adapting odor stimuli based on real-time neural feedback, potentially aiding rehabilitation in alongside smell loss. Beyond training, olfactory interfaces support diagnostic and monitoring applications by interfacing with electronic noses (e-noses) for (VOC) analysis in clinical settings, though e-nose hardware primarily handles detection while interfaces handle human validation or . In breath analysis for phenotyping, e-noses integrated with olfactory feedback systems have demonstrated diagnostic accuracies exceeding 80% for conditions like and inflammatory diseases, via of exhaled VOCs. Wearable olfactory displays have been prototyped for on-demand scent delivery in therapeutic scenarios, such as alleviating phantom odors in or enhancing in neurological disorders, with pilot studies reporting improved patient adherence due to portable, non-invasive designs. These advancements underscore causal links between targeted odor exposure and neural recovery, grounded in empirical measures of olfactory thresholds rather than subjective reports alone.

Health Risks and Empirical Evidence

Toxicology of Odorants

Odorants, comprising a diverse array of volatile organic compounds (VOCs) and other volatile substances, display varied toxicological profiles primarily through exposure, with effects ranging from sensory to systemic . Acute exposure to many odorants at concentrations exceeding odor thresholds but below lethal levels often manifests as of the eyes, , throat, and , alongside headaches, , and , due to their interaction with mucous membranes and neural pathways. For instance, common indoor VOC odorants from sources like paints, solvents, and cleaners can induce these symptoms at low parts-per-million levels, though overlap between odor detection and toxic thresholds is infrequent, with typically preceding overt . Certain odorants exhibit higher , particularly sulfur-containing compounds used as gas odorants such as ethyl mercaptan and tert-butyl mercaptan. studies report an LC50 (lethal concentration for 50% of subjects) of approximately 2,770 ppm for ethyl mercaptan in , with lower concentrations (e.g., LC01 at 2,250 ppm) causing significant lethality, underscoring risks in confined spaces despite oral LD50 values exceeding 2,000 mg/kg body weight indicating lower ingestion hazard. odorants, including mercaptans and , show limited evidence from scoping reviews of peer-reviewed human and animal data for severe outcomes, with transient nonspecific symptoms like headaches predominant, though some case reports suggest potential neurotoxic effects such as without conclusive causation. Chronic exposure to specific toxic odorants poses risks of organ damage and oncogenicity. , with its characteristic sweet odor, is a known leukemogen linked to via following prolonged low-level , as evidenced by epidemiological data associating with increased incidence. , emitting a pungent odor, acts as a respiratory irritant and Group 1 , with meta-analyses confirming associations with nasopharyngeal cancer and at occupational exposure levels above 1 ppm. Unpleasant or bitter-smelling odorants, such as those with fishy or profiles, empirically correlate with lower LD50 values (e.g., ln(LD50) significantly reduced compared to pleasant floral or fruity scents), suggesting an evolutionary signaling mechanism where aversive odors flag higher toxicity potential. Regulatory thresholds, such as OSHA permissible exposure limits (e.g., 1 ppm for , 1 ppm for ), reflect these risks, prioritizing ventilation and monitoring to mitigate effects, though individual variability in and sensitivity influences outcomes. Empirical data indicate that while some odorants like d-limonene from cleaning products show low , secondary reactions indoors can amplify irritancy or form more hazardous byproducts. Overall, toxicological assessments emphasize dose-response relationships, with most odorants non-toxic at ambient environmental levels but hazardous in industrial or accidental high-exposure scenarios.

Associations with Symptoms and Controversies

Exposure to certain odors, particularly those emanating from volatile organic compounds (VOCs) such as and , has been empirically linked to acute symptoms including eye, , and , headaches, and in controlled chamber studies where participants were intentionally exposed to mixtures at levels typical of indoor environments. These effects are dose-dependent, with higher indoor VOC concentrations—often 2-5 times outdoor levels—correlating with increased reports of irritation and reduced , though long-term causality remains under investigation due to confounding factors like ventilation and individual variability. Industrial odors from sources like or have shown associations with symptoms in systematic reviews of 13 studies, where 11 reported poorer outcomes, including elevated stress, anxiety, and disturbances, strongest in communities with chronic low-level exposure below toxic thresholds. High odor exposure also correlates with physical symptoms such as loss of appetite (odds ratio 4.27) and respiratory issues, though these are often self-reported and may involve responses amplified by perceived annoyance. Controversies arise prominently in conditions like (MCS), where individuals report diverse symptoms—headaches, fatigue, dizziness—triggered by low-level odors from everyday chemicals, yet blinded provocation studies fail to consistently reproduce effects, suggesting psychological mechanisms such as chronic anxiety or expectation bias as primary drivers rather than direct . MCS lacks recognition from major bodies like the and due to inconsistent diagnostic criteria and absence of objective biomarkers, with reviews attributing symptoms to rather than verifiable chemical causation. Sick building syndrome (SBS), often tied to building odors from outgassing materials or poor ventilation, presents similar debates: while symptoms like mucosal irritation and fatigue cluster in affected environments, causality is contested, as interventions improving air quality yield mixed results, and psychosocial factors—including odor sensitivity and job stress—explain variance not accounted for by measured pollutants alone. Empirical data indicate that SBS symptoms may persist even after odor mitigation, underscoring the role of perceptual and expectancy effects over purely physiological pathways.

History of Odor Research

Pre-Modern Observations

Ancient civilizations recognized odors as integral to daily life, ritual, and medicine, often associating pleasant scents with purity and foul ones with decay or disorder. In around 3000 BCE, aromatic resins like and cedar were employed in mummification processes to preserve bodies and evoke the "scent of eternal life," comprising woody, spicy notes from , cloves, and , as evidenced by chemical analyses of balms. Sweet smells symbolized ma'at (cosmic order), while stenches denoted chaos, with texts and artifacts indicating deliberate use of perfumes to mask body odors and in temple offerings. Similarly, in ancient and from circa 10,000 BCE, scented oils and ointments served to cleanse and conceal natural body smells, reflecting early empirical awareness of odor's social and hygienic roles. Greek philosophers provided foundational theoretical observations on olfaction. (384–322 BCE), in De Anima II.9 and De Sensu 5, posited that odors are dry exhalations perceived through air or water as a medium, akin to but distinct from flavors detected by touch in moist conditions; he classified basic odor qualities as sweet, pungent, bitter, and succulent, though subordinate to sight as a "less noble" due to its reliance on vaporous transmission via to the brain. (427–347 BCE) earlier differentiated odors as pleasant or unpleasant arising from elemental shifts, such as water to air, while (371–287 BCE) enumerated seven distinct odor categories, advising against conflating them with tastes to avoid conceptual errors. Roman naturalist (23–79 CE) documented extensive practical observations in Naturalis Historia, cataloging aromatic plants' medicinal applications while expressing ambivalence toward perfumes as indulgent luxuries fostering moral laxity, contrasting their utility against excess. This echoed broader Roman sensory experiences of urban odors—, , decay, and —integrated into and , as in Plautus's plays referencing everyday smells. In the medieval period, (129–216 CE) localized the olfactory organ to brain ventricles and bifurcated odors into beneficial and harmful, influencing (980–1037 CE), who synthesized these with Aristotelian frameworks to emphasize smell's role in memory and humoral balance. Hildegard von Bingen (1098–1179 CE) further applied fragrant herbs empirically to rectify bodily humors, as detailed in her Physica, underscoring odor's therapeutic potential amid miasmatic disease theories. These pre-modern accounts blended sensory phenomenology with proto-empirical cataloging, laying groundwork for later inquiry without quantitative measurement.

Modern Scientific Foundations

The modern scientific understanding of olfaction began with the identification of odorant receptors as a multigene family of G-protein-coupled receptors (GPCRs) in the early . Linda Buck and demonstrated that these receptors, encoded by approximately 1,000 genes in mice, are expressed in olfactory sensory s and selectively bind volatile odorant molecules, initiating the perceptual process. This discovery revealed that each neuron expresses a single receptor type, with axons from neurons sharing the same receptor converging onto specific glomeruli in the , establishing a for odor coding. Their work, awarded the 2004 in or , shifted olfaction from phenomenological descriptions to molecular mechanisms, emphasizing combinatorial receptor activation to distinguish thousands of odors. Olfactory occurs in the cilia of bipolar sensory within the of the . Odorants dissolve in and bind to receptors, activating the G-protein G_olf, which stimulates to produce cyclic AMP (cAMP). This second messenger opens cyclic nucleotide-gated (CNG) cation channels, allowing influx of Na⁺ and Ca²⁺, depolarizing the and generating action potentials that propagate via unmyelinated axons forming cranial I. Calcium-dependent channels amplify the signal through Cl⁻ efflux, enhancing sensitivity, while adaptation mechanisms, including activity and receptor desensitization, regulate response duration. In humans, functional receptor genes number around 400, with the remainder as pseudogenes, reflecting evolutionary reduction in olfactory acuity compared to . Central processing begins in the , where primary afferents synapse with mitral and tufted cells in ~2,000 glomeruli per bulb, enabling and contrast enhancement. These second-order neurons project directly to the , , and limbic structures like the and hippocampus, bypassing the unlike other sensory modalities, which facilitates rapid integration with . This monosynaptic pathway underscores olfaction's archaic evolutionary role in behaviors, with confirming sparse, distributed coding in cortical areas for odor identity and valence. Advances in cryo-electron microscopy have since visualized receptor structures, confirming a conserved ligand-binding pocket across GPCRs, though challenges persist in resolving broad tunings due to weak individual receptor affinities.

References

  1. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/odor
  2. https://www.merriam-webster.com/dictionary/odor
  3. https://www.epa.gov/indoor-air-quality-iaq/what-are-volatile-organic-compounds-vocs
  4. https://www.brainfacts.org/thinking-sensing-and-behaving/smell/2015/making-sense-of-scents-smell-and-the-brain
  5. https://www.ncbi.nlm.nih.gov/books/NBK11032/
  6. https://www.rutgers.edu/news/human-sense-smell-its-stronger-we-think
  7. https://newsinhealth.nih.gov/2016/08/what-your-nose-knows
  8. https://aromaco.co.uk/20-fascinating-facts-about-our-sense-of-smell/
  9. https://www.uclahealth.org/news/article/why-your-sense-of-smell-is-important-to-your-health
  10. https://www.health.ny.gov/publications/6500/
  11. https://pmc.ncbi.nlm.nih.gov/articles/PMC7504400/
  12. https://www.epa.gov/indoor-air-quality-iaq/volatile-organic-compounds-impact-indoor-air-quality
  13. https://www.sciencedirect.com/topics/immunology-and-microbiology/odor
  14. https://www.pnas.org/doi/10.1073/pnas.1804106115
  15. https://pubmed.ncbi.nlm.nih.gov/31040214/
  16. https://www.sciencedirect.com/science/article/pii/S258900422501716X
  17. https://academic.oup.com/chemse/article/doi/10.1093/chemse/bjaf022/8196596
  18. https://www.kenhub.com/en/library/physiology/physiology-of-smell
  19. https://pmc.ncbi.nlm.nih.gov/articles/PMC3990440/
  20. https://pmc.ncbi.nlm.nih.gov/articles/PMC7912024/
  21. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/odor
  22. https://chemistry.stackexchange.com/questions/120153/whats-the-biggest-organic-molecule-that-could-have-a-smell
  23. https://digitalcommons.chapman.edu/cgi/viewcontent.cgi?article=1625&context=sees_articles
  24. https://authors.library.caltech.edu/records/13cyv-fz171
  25. https://www.open.edu/openlearn/ocw/mod/oucontent/view.php?id=83425&section=3.3
  26. https://ehs.utexas.edu/sites/default/files/Odorous-chemicals-guidance.pdf
  27. https://pmc.ncbi.nlm.nih.gov/articles/PMC10208748/
  28. https://pmc.ncbi.nlm.nih.gov/articles/PMC4176544/
  29. https://www.ncbi.nlm.nih.gov/books/NBK542239/
  30. https://pmc.ncbi.nlm.nih.gov/articles/PMC7052838/
  31. https://www.ncbi.nlm.nih.gov/books/NBK11039/
  32. https://pmc.ncbi.nlm.nih.gov/articles/PMC10710919/
  33. https://pmc.ncbi.nlm.nih.gov/articles/PMC3408771/
  34. https://pmc.ncbi.nlm.nih.gov/articles/PMC12219263/
  35. https://pmc.ncbi.nlm.nih.gov/articles/PMC6381007/
  36. https://www.news-medical.net/news/20230606/Differences-in-olfactory-abilities-in-men-and-women.aspx
  37. https://www.frontiersin.org/journals/psychology/articles/10.3389/fpsyg.2014.00020/full
  38. https://pmc.ncbi.nlm.nih.gov/articles/PMC7662159/
  39. https://academic.oup.com/chemse/article/doi/10.1093/chemse/bjae011/7624027
  40. https://www.ncbi.nlm.nih.gov/books/NBK55977/
  41. https://pubmed.ncbi.nlm.nih.gov/10531048/
  42. https://www.cell.com/current-biology/fulltext/S0960-9822%2820%2931178-7
  43. https://pmc.ncbi.nlm.nih.gov/articles/PMC2596880/
  44. https://www.annualreviews.org/content/journals/10.1146/annurev-neuro-071013-013941
  45. https://academic.oup.com/chemse/article/25/4/473/342750
  46. https://elifesciences.org/articles/43735
  47. https://pmc.ncbi.nlm.nih.gov/articles/PMC9130158/
  48. https://pmc.ncbi.nlm.nih.gov/articles/PMC4140790/
  49. https://www.sciencedirect.com/science/article/pii/S0896627305008949
  50. https://www.cell.com/neuron/pdf/S0896-6273%2805%2900894-9.pdf
  51. https://pmc.ncbi.nlm.nih.gov/articles/PMC8388720/
  52. https://www.depts.ttu.edu/agriculturalsciences/news/posts/2021/06/afs-hall-barnes-test-canine-olfactory-abilities-and-edna-technology.php
  53. https://manoa.hawaii.edu/exploringourfluidearth/chemical/chemistry-and-seawater/elemental-abundance/weird-science-compare-your-sense-smell-shark-s-sense-smell
  54. https://saveourseas.com/worldofsharks/podcast/how-powerful-is-a-sharks-sense-of-smell
  55. https://www.ncbi.nlm.nih.gov/books/NBK200986/
  56. https://royalsocietypublishing.org/doi/10.1098/rsob.200252
  57. https://pmc.ncbi.nlm.nih.gov/articles/PMC2692081/
  58. https://www.science.org/content/article/textbooks-say-most-birds-cant-smell-scientists-are-proving-them-wrong
  59. https://embargoed.www.cell.com/current-biology/fulltext/S0960-9822%2808%2900947-0
  60. https://pmc.ncbi.nlm.nih.gov/articles/PMC9744530/
  61. https://bioone.org/journals/ardea/volume-99/issue-2/078.099.0207/Smelling-Out-Predators-is-Innate-in-Birds/10.5253/078.099.0207.pdf
  62. https://researchers.mq.edu.au/en/publications/does-evolution-in-isolation-from-mammalian-predators-have-behavio/
  63. https://pmc.ncbi.nlm.nih.gov/articles/PMC5104188/
  64. https://pmc.ncbi.nlm.nih.gov/articles/PMC5607904/
  65. https://news.climate.columbia.edu/2011/09/23/your-nose-knows-evolution/
  66. https://blaustein.science.oregonstate.edu/publications/Behavior/SexualSelectionAmNat.pdf
  67. https://onlinelibrary.wiley.com/doi/full/10.1002/ece3.11025
  68. https://pmc.ncbi.nlm.nih.gov/articles/PMC2817421/
  69. https://pmc.ncbi.nlm.nih.gov/articles/PMC2742912/
  70. https://www.sciencedirect.com/science/article/abs/pii/S000334722500003X
  71. https://pubmed.ncbi.nlm.nih.gov/21086527/
  72. https://www.sciencedirect.com/science/article/pii/S0960982216310600
  73. https://www.sciencedirect.com/topics/chemistry/bombykol
  74. https://edu.rsc.org/feature/in-pursuit-of-bombykol/2020169.article
  75. https://www.sciencedirect.com/science/article/pii/S0163782715000272
  76. https://pmc.ncbi.nlm.nih.gov/articles/PMC6921000/
  77. https://www.americanscientist.org/article/how-animals-communicate-via-pheromones
  78. https://pmc.ncbi.nlm.nih.gov/articles/PMC4310675/
  79. https://www.jneurosci.org/content/26/28/7375.short
  80. https://www.pnas.org/doi/10.1073/pnas.1214249110
  81. https://pmc.ncbi.nlm.nih.gov/articles/PMC6674185/
  82. https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/mammalian-pheromones
  83. https://anatomypubs.onlinelibrary.wiley.com/doi/full/10.1002/ar.a.20125
  84. https://www.u-tokyo.ac.jp/focus/en/articles/a_00238.html
  85. https://www.ncbi.nlm.nih.gov/books/NBK55973/
  86. https://pmc.ncbi.nlm.nih.gov/articles/PMC10039832/
  87. https://pubmed.ncbi.nlm.nih.gov/8836161/
  88. https://www.sciencedirect.com/science/article/pii/0960076096000623
  89. https://academic.oup.com/chemse/article-abstract/26/4/433/266106
  90. https://pmc.ncbi.nlm.nih.gov/articles/PMC4375873/
  91. https://www.sciencedirect.com/science/article/abs/pii/S0031938424002816
  92. https://pmc.ncbi.nlm.nih.gov/articles/PMC7209928/
  93. https://royalsocietypublishing.org/doi/10.1098/rsos.160831
  94. https://journals.sagepub.com/doi/full/10.1177/17456916231188147
  95. https://www.sciencedirect.com/science/article/pii/S1090513823000156
  96. https://scienceline.org/2025/02/quest-to-identify-human-pheromones/
  97. https://onlinelibrary.wiley.com/doi/abs/10.1002/9781118971758.ch11
  98. https://pubmed.ncbi.nlm.nih.gov/11767935/
  99. https://www.sciencedirect.com/science/article/pii/S1879729615001039
  100. https://www.frontiersin.org/journals/neuroscience/articles/10.3389/fnins.2021.646956/full
  101. https://www.mdpi.com/2073-4433/11/1/92
  102. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/olfactometry
  103. https://pubmed.ncbi.nlm.nih.gov/40796200/
  104. https://academic.oup.com/chemse/article/doi/10.1093/chemse/bjaf029/8232092
  105. https://pmc.ncbi.nlm.nih.gov/articles/PMC9688190/
  106. https://pubs.acs.org/doi/10.1021/acs.jafc.3c08129
  107. https://pubmed.ncbi.nlm.nih.gov/38640191/
  108. https://www.sciencedirect.com/science/article/pii/S2589004221013420
  109. https://pmc.ncbi.nlm.nih.gov/articles/PMC11540037/
  110. https://www.sciencedirect.com/science/article/pii/S2405844024071342
  111. https://www.epa.gov/scram/air-quality-dispersion-modeling-preferred-and-recommended-models
  112. https://www.apsi.tech/material/modeling/OdorModeling.pdf
  113. https://scholarworks.uni.edu/cgi/viewcontent.cgi?article=1871&context=etd
  114. https://www.olores.org/en/techniques/dispersion-modelling
  115. https://www.sciencedirect.com/science/article/pii/0273122396005951
  116. https://www.cell.com/iscience/fulltext/S2589-0042%2825%2901716-X
  117. https://onlinelibrary.wiley.com/doi/full/10.1002/lio2.1160
  118. https://pubmed.ncbi.nlm.nih.gov/23387344/
  119. https://www.iowadnr.gov/media/2318/download?inline
  120. https://www.azosensors.com/article.aspx?ArticleID=2960
  121. https://pmc.ncbi.nlm.nih.gov/articles/PMC2215629/
  122. https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=9101LKY9.TXT
  123. https://www.sciencedirect.com/science/article/pii/S1352231024001845
  124. https://pmc.ncbi.nlm.nih.gov/articles/PMC3231359/
  125. https://www.nature.com/articles/s41598-022-23068-1
  126. https://pmc.ncbi.nlm.nih.gov/articles/PMC10436339/
  127. https://www.epa.gov/sites/default/files/2020-08/documents/odorguide1992.pdf
  128. https://www.mdpi.com/2073-4433/10/9/524
  129. https://pubmed.ncbi.nlm.nih.gov/17932684/
  130. https://academic.oup.com/chemse/article/24/2/191/312277
  131. https://www.jneurosci.org/content/25/39/8903
  132. https://www.researchgate.net/publication/8231456_Odour_intensity_and_hedonic_tone_-_Important_parameters_to_describe_odour_annoyance_to_residents
  133. https://pmc.ncbi.nlm.nih.gov/articles/PMC7209930/
  134. https://pmc.ncbi.nlm.nih.gov/articles/PMC11859731/
  135. https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2019.00972/full
  136. https://asm.org/articles/2021/december/microbial-origins-of-body-odor
  137. https://www.mdpi.com/2073-4441/17/8/1236
  138. https://www.sciencedirect.com/science/article/abs/pii/S0959652619339459
  139. https://www.sciencedirect.com/science/article/pii/S0160412025003472
  140. https://www.nature.com/articles/s41598-022-18086-y
  141. https://www.nature.com/articles/s41598-020-68860-z
  142. https://www.mdpi.com/2304-8158/10/5/1051
  143. https://academic.oup.com/chemse/article/38/3/189/320944
  144. https://academic.oup.com/chemse/article-abstract/25/4/487/342753
  145. https://www.pnas.org/doi/10.1073/pnas.1915252117
  146. https://www.researchgate.net/publication/346026452_Odor_habituation_can_modulate_very_early_olfactory_event-related_potential
  147. https://pubmed.ncbi.nlm.nih.gov/28408237/
  148. https://pmc.ncbi.nlm.nih.gov/articles/PMC9405088/
  149. https://royalsocietypublishing.org/doi/10.1098/rspb.1995.0087
  150. https://www.sciencedirect.com/science/article/abs/pii/S0306453008002667
  151. https://pmc.ncbi.nlm.nih.gov/articles/PMC7209936/
  152. https://royalsocietypublishing.org/doi/10.1098/rspb.2017.1830
  153. https://www.news-medical.net/news/20250609/Study-explores-whether-female-odor-based-mating-preferences-could-predict-gamete-level-compatibility.aspx
  154. https://royalsocietypublishing.org/doi/10.1098/rspb.1997.0204
  155. https://www.reddit.com/r/science/comments/vlcjzo/study_people_click_better_when_their_body_odor/
  156. https://journals.sagepub.com/doi/10.1177/147470490600400106
  157. https://academic.oup.com/chemse/article/32/6/603/317723
  158. https://cognitiveresearchjournal.springeropen.com/articles/10.1186/s41235-021-00311-3
  159. https://pmc.ncbi.nlm.nih.gov/articles/PMC2717541/
  160. https://www.sciencedirect.com/science/article/pii/S1878929320301080
  161. https://www.science.org/doi/10.1126/sciadv.abg6867
  162. https://academic.oup.com/scan/article/19/1/nsae038/7690002
  163. https://royalsocietypublishing.org/doi/10.1098/rstb.2019.0266
  164. https://pubmed.ncbi.nlm.nih.gov/15653193/
  165. https://academic.oup.com/scan/article/18/1/nsad006/7039764
  166. https://www.medicalnewstoday.com/articles/232635
  167. https://www.sciencedirect.com/topics/engineering/odorization
  168. https://energyeducation.ca/encyclopedia/Odorant
  169. https://www.gdscorp.com/blog/natural-gas-monitoring/natural-gas-odorization-explained/
  170. https://www.epa.gov/biosolids/fact-sheet-odor-control-biosolids-management
  171. https://www.mdpi.com/2073-4441/13/24/3503
  172. https://www.sciencedirect.com/science/article/abs/pii/S0013935123004693
  173. https://www.mdpi.com/1424-8220/23/12/5716
  174. https://pmc.ncbi.nlm.nih.gov/articles/PMC8004717/
  175. https://www.sciencedirect.com/science/article/abs/pii/S0048969721039346
  176. https://www.science.org/doi/10.1126/sciadv.adp1764
  177. https://www.nature.com/articles/s41467-024-45430-9
  178. https://advanced.onlinelibrary.wiley.com/doi/10.1002/advs.202412669
  179. https://phys.org/news/2025-05-ai-powered-electronic-nose-diverse.html
  180. https://envirotecmagazine.com/2025/05/18/ai-bridges-the-olfactory-gap/
  181. https://ainos.com/news_view/84
  182. https://www.sciencedirect.com/science/article/pii/S0026265X25027110
  183. https://www.sciencedirect.com/science/article/abs/pii/S0026265X25009920
  184. https://iopscience.iop.org/article/10.1088/2634-4386/add0dc
  185. https://arxiv.org/html/2510.19660v1
  186. https://www.nature.com/articles/s41467-023-37678-4
  187. https://pmc.ncbi.nlm.nih.gov/articles/PMC7668391/
  188. https://www.frontiersin.org/journals/human-neuroscience/articles/10.3389/fnhum.2024.1457527/full
  189. https://www.frontiersin.org/journals/behavioral-neuroscience/articles/10.3389/fnbeh.2023.1122849/full
  190. https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2793773
  191. https://pmc.ncbi.nlm.nih.gov/articles/PMC11128017/
  192. https://www.sciencedirect.com/science/article/pii/S0149763422003426
  193. https://odh.ohio.gov/wps/wcm/connect/gov/da83d1c4-b5fc-485c-8263-b6f09fd30ca5/VOC%2BFactsheet_2021.01.pdf?MOD=AJPERES&CONVERT_TO=url&CACHEID=ROOTWORKSPACE.Z18_K9I401S01H7F40QBNJU3SO1F56-da83d1c4-b5fc-485c-8263-b6f09fd30ca5-nt04MxI
  194. https://www.sciencedirect.com/science/article/abs/pii/S0013935121014225
  195. https://www.ncbi.nlm.nih.gov/books/NBK201325/
  196. https://asgmt.com/wp-content/uploads/2018/05/002.pdf
  197. https://pmc.ncbi.nlm.nih.gov/articles/PMC10504204/
  198. https://pubmed.ncbi.nlm.nih.gov/37491689/
  199. https://pubmed.ncbi.nlm.nih.gov/35622720/
  200. https://pubs.acs.org/doi/10.1021/acs.jafc.3c00592
  201. https://www.sciencedirect.com/science/article/pii/S1352231024005211
  202. https://pmc.ncbi.nlm.nih.gov/articles/PMC1310916/
  203. https://www.researchgate.net/publication/396306713_Industrial_Odour_and_Psychosocial_Wellbeing_A_Systematic_Review
  204. https://ehjournal.biomedcentral.com/articles/10.1186/s12940-021-00774-3
  205. https://www.mcgill.ca/oss/article/health/zeroing-cause-multiple-chemical-sensitivity
  206. https://www.verywellhealth.com/multiple-chemical-sensitivity-7560848
  207. https://www.sciencedirect.com/science/article/pii/S0149763423001963
  208. https://link.springer.com/article/10.1057/rlp.2008.20
  209. https://www.epa.gov/sites/default/files/2014-08/documents/sick_building_factsheet.pdf
  210. https://theconversation.com/the-scent-of-the-ancient-egyptian-afterlife-has-been-recreated-heres-what-it-smelled-like-213173
  211. https://pmc.ncbi.nlm.nih.gov/articles/PMC12203381/
  212. https://pubmed.ncbi.nlm.nih.gov/35262483/
  213. https://philarchive.org/rec/JOHAOO-9
  214. https://theconversation.com/what-did-ancient-rome-smell-like-honestly-often-pretty-rank-257111
  215. https://www.nobelprize.org/prizes/medicine/2004/summary/
  216. https://www.ncbi.nlm.nih.gov/books/NBK10982/
  217. https://www.rockefeller.edu/news/30838-study-reveals-smell-receptors-work/
Add your contribution
Related Hubs
User Avatar
No comments yet.