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An odor, or odour, is caused by one or more volatilized chemical compounds that are generally found in low concentrations that humans and animals can perceive by their sense of smell. An odor is also called a "smell" or a "scent", which can refer to either a pleasant or an Unpleasant odor.

While "scent" 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.

In the United Kingdom, "odour" refers to scents in general; but in the United States, and for many non-native English speakers around the world, "odor" generally has a negative connotation as a synonym for "stink".[1] An unpleasant odor can also be described as "reeking" or called a "malodor", "stench", "pong", or "stink".

Physiology of smell

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.[2]

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.

Most odors consists 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 very personal perceptions, individual reactions are usually related. They relate to things such as gender, age, state of health, and personal history.

The ability to identify odor varies among people and decreases with age.

Pregnant women have increased smell sensitivity, sometimes resulting in abnormal taste and smell perceptions, leading to food cravings or aversions.[6] 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.[7]

For most untrained people, the process of smelling gives little information concerning the specific ingredients of an odor. Their smell perception primarily offers information related to the emotional impact. Experienced people, however, such as flavorists and perfumers, can pick out individual chemicals in complex mixtures through smell alone.

Odor perception is a primal sense.

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.[10] Odors can change due to environmental conditions: for example, odors tend to be more distinguishable in cool dry air.[11]

Habituation affects the ability to distinguish odors after continuous exposure.

The primary gene sequences for thousands of olfactory receptors are known for the genomes of more than a dozen organisms.

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.[16]

Measuring techniques

In Germany, the concentrations of odorants have since the 1870s been defined by Olfaktometrie, which helps to analyze the human sense of smell according to odor substance concentration, intensity of odor, odor quality, and hedonic assessment.

The most accurate smell sensing is when a smell is first encountered, before habituation begins to change perception of odor.

Odor concentration is an odor's pervasiveness.

The measurement of odor concentration is the most widespread method to quantify odors.

To establish the odor concentration, an olfactometer is used which employs a group of human panelists.

In comparing the odor emitted from each port, the panelists are asked to report if they can detect a difference between the ports.

Humans can discriminate between two odorants that differ in concentration by as little as 7%.[20] A human's odor detection threshold is variable.

There are a number of issues which have to be overcome with sampling, these include: – If the source is under vacuum – if the source is at a high temperature – If the source has high humidity

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

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.[23] An odor emission often consists of a complex mixture of many odorous compounds.

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).[25]

Odor intensity is the perceived strength of odor sensation.

The perceived strength of the odor sensation is measured in conjunction with odor concentration.

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.[26]

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

This method is applied by in the laboratory and is done so by a series of suitably trained panelists/observers who have been trained to appropriately define intensity.

Hedonic assessment is the process of rating odors according to a scale ranging from extremely unpleasant to extremely pleasant. It is important to note that 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). Moreover, it is important to note that perception of an odor may change from pleasant to unpleasant with increasing concentration, intensity, time, frequency, and previous experience with a specific odor—all factors in determining a response.[27]

The overall set of qualities are sometimes identified as the "FIDOL factors", (short for F requency, I ntensity, D uration, O ffensiveness and L ocation).[28]

The character of an odor is a critical element in assessing an odor.

Most commonly, a set of standard descriptors is used, which may range from "fragrant" to "sewer odor".[29] Although the method is fairly simplistic, it is important for the FIDOL factors to be understood by the person rating the smell.

Different categorizations of primary odors have been proposed, including the following, which identifies seven primary odors:[17][30][31]

Although recent progress has been made, the idea of primary perceptions is disputed, and more so the concept of primary odors.[31]

In many countries odor modeling is used to determine the extent of an impact from an odor source.

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

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[32] and wind tunnels which include the UNSW wind tunnel.[33] 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 is often referred to as back calculation.

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

The human sense of smell is a primary factor in the sensation of comfort.

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.

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.


Some odors are sought after, such as from perfumes and flowers, some of which command high prices.

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 their spirits, and boost self-confidence. This belief has led to the concept of "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. However, the evidence for the effectiveness of aromatherapy consists mostly of anecdotes and lacks controlled scientific studies to back up its claims.

With some fragrances, such as those found in perfume, scented shampoo, scented deodorant, or similar products, people can be allergic to the ingredients.

Unpleasant odors play various roles in nature, often to warn of danger, though this may not be known to the subject who smells it.[36] 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.[36]

It is commonly thought that those exuding an unpleasant body odor will be 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.[36] This includes smells associated with pollution.[36]

What causes a substance to smell unpleasant may be different from what one perceives.

Unpleasant odors can arise from specific industrial processes, adversely affecting workers and even residents downwind of the source.

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.

The study of odors is a growing field but is a complex and difficult one.

The study of odors is complicated by the complex chemistry taking place at the moment of a smell sensation.

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.

There are hopes that advanced technology could do everything from testing perfumes to helping detect cancer or explosives by detecting specific scents, but artificial noses are still problematic.

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 it is exposed to. Most electronic noses need to be "trained" to recognize whatever chemicals are of interest for the application in question before it can be used. The training involves exposure to chemicals with the response being recorded and statistically analyzed, often using multivariate analysis and neural network techniques, to "learn" the chemicals. 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.[39][40][41][42][43][44]

Behavioral cues

Odor perception is a complex process involving the central nervous system and can evoke psychological and physiological responses.

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." 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.[50] 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.[51]

Body odor is a sensory cue critical for mate selection in humans because it is a signal of immunological health.

Studies have suggested that people might be using odor cues associated with the immune system to select mates.

An odor can cue recall of a distant memory.

The sense of smell is not overlooked as a way of marketing products.

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

Scent preferences

Both men and women use perfume to boost their sexual attractiveness to members of the opposite, or same, sex.

In 2001, a study found that the major histocompatibility complex (MHC) (a polymorphic set of genes which is important for immune-function in humans) is correlated with the ingredients found in perfume. This suggests that humans do, in fact, choose perfumes that complement or enhance their natural scents (their pheromones). This evidence offers support for the hypothesis that perfume is chosen by individuals to advertise their physical health. Research suggests that this advertisement of good health will, in fact, enhance females’ attractiveness to the opposite sex as health markers have been shown to do.[58] While strong evidence has been found to support the hypothesis that wearing perfume enhances females’ attractiveness to males, little research has been done into the effect of fragrance on males’ attractiveness to females. Considerably more research has covered the effect of males’ natural odor and females’ ratings of attractiveness. Many studies (e.g.[59]) found that odor predicted attractiveness when female raters were not on any form of contraceptive pill. For those who were, there was no relation between attractiveness and body odor.

A person's odor can increase or decrease ratings of attractiveness because the olfactory receptors in the brain are directly linked with the limbic system, the part of the brain that is thought to be most involved with emotion.

Major histocompatibility complex (MHC) is a genotype found in vertebrates, including humans. MHC is thought to contribute to mate choice in animals and humans. In sexual selection, females opt for mates with an MHC which differs from their own, optimising genes for their offspring.[62] The "heterozygote advantage" and "Red Queen" explanations for these findings fall under the "pathogen hypothesis". Due to differences in MHC alleles' resistance to pathogens, a preference for mates with a dissimilar MHC composition has been argued to act as a mechanism to avoid infectious diseases. According to the heterozygotes-advantage hypothesis, diversity within the MHC genotype is beneficial for the immune system due to a greater range of antigens available to the host. Therefore, the hypothesis proposes that MHC heterozygotes will be superior to MHC homozygotes in fighting off pathogens. Experimental research has shown mixed findings for this idea.[63] The "Red Queen" or "rare-allele" hypothesis suggests that diversity in the MHC gene provides a moving target for pathogens, making it more difficult for them to adapt to MHC genotypes in the host.[64] Another hypothesis suggests that preferences for MHC-dissimilar mates could serve to avoid inbreeding.[65]

Body odor can provide MHC information.

Women’s preferences for body odor change with their menstrual cycles.[72] The ovulatory-shift hypothesis argues that women experience elevated immediate sexual attraction, relative to low-fertile days of the cycle, to men with characteristics that reflect good genetic quality.[73] Body odor may provide significant cues about a potential sexual partner's genetic quality, reproductive status, and health, with a woman's preferences for particular body odors becoming heightened during her most fertile days.[74] As certain body odors can reflect good genetic quality, woman are more likely to prefer these scents when they are fertile, as this is when they are most likely to produce offspring with any potential mates, with conception-risk being related to a preference for the scent of male symmetry.[72] Men also prefer the scent of women at their fertile cycle points.[75]

There are several scents that reflect good genetic quality that females prefer during the most fertile phase of their cycles.

If women are taking a contraceptive pill the changes in mate scent preferences over the menstrual cycle are not expressed.[81] If odor plays a role in human mate choice then the contraceptive pill could disrupt disassortative mate preferences.[82] Those taking a contraceptive pill show no significant preference for the scent of either symmetrical or asymmetrical men, whereas normally cycling women prefer the scent of shirts worn by symmetrical men.[83] Males' preferences for women's scent may also change if the woman is taking oral contraceptives. When women take a contraceptive pill, this has been found to demolish the cycle attractiveness of odors that men find attractive in normally ovulating women.[84] Therefore, a contraceptive pill affects both women's preferences for scent and also affects their own scents, making their scents less attractive to males than the scent of normally cycling women.

See also

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