An odor or odour is caused by one or more volatilized chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

s, generally at a very low concentration, that humans or other animals perceive by the sense
Senses are physiological capacities of organisms that provide inputs for perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology , and philosophy of perception...

 of olfaction
Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...

. Odors are also commonly called scents, which can refer to both pleasant and unpleasant odors. The terms fragrance, scent, and aroma are used primarily by the food and cosmetic industry to describe a pleasant odor, and are sometimes used to refer to perfume
Perfume is a mixture of fragrant essential oils and/or aroma compounds, fixatives, and solvents used to give the human body, animals, objects, and living spaces "a pleasant scent"...

s. In contrast, malodor, stench, reek, and stink are used specifically to describe unpleasant odors.


In the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

, odour refers to scents in general. In the United States, odor has a more negative connotation, such as stench or stink, while scent or aroma are used for pleasant smells.


The sense of smell gives rise to the perception of odors, mediated by the olfactory nerve
Olfactory nerve
The olfactory nerve, or cranial nerve I, is the first of twelve cranial nerves. It is instrumental in the sense of smell. Derived from the embryonic nasal placode, the olfactory nerve is capable of regeneration.-Anatomy:...

. The olfactory receptor (OR) cells are neurons present in the olfactory epithelium, a small patch of tissue in back of the nasal cavity
Nasal cavity
The nasal cavity is a large air filled space above and behind the nose in the middle of the face.- Function :The nasal cavity conditions the air to be received by the other areas of the respiratory tract...

. There are millions of olfactory receptor neurons that act as sensory signaling cells. Each neuron has cilia in direct contact with air. The olfactory nerve is considered the smell mediator, the axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

 connects the brain to the external air. Odorous molecules act as a chemical stimulus. Molecules bind to receptor proteins extended from cilia, initiating an electric signal.

The primary sequences of thousands of olfactory receptors are known from the genomes of more than a dozen organisms: they are seven-helix transmembrane proteins, but there are (as of July 2011) no known structures of any OR. There is a highly conserved sequence in roughly three quarters of all ORs that is a tripodal metal ion binding site, and Suslick
Kenneth S. Suslick
Kenneth S. Suslick is the Marvin T. Schmidt Professor of Chemistry, Professor of Materials Science & Engineering, and Professor of the Beckman Institute for Advanced Science & Engineering at the University of Illinois at Urbana-Champaign...

 has proposed that the ORs are in fact metalloproteins (mostly likely with zinc, copper and possibly manganese ions) that serve as a Lewis Acid site for binding of many odorant molecules.

When the signal reaches a threshold, the neuron fires, sending a signal traveling along the axon to the olfactory bulb
Olfactory bulb
The olfactory bulb is a structure of the vertebrate forebrain involved in olfaction, the perception of odors.-Anatomy:In most vertebrates, the olfactory bulb is the most rostral part of the brain. In humans, however, the olfactory bulb is on the inferior side of the brain...

, part of the limbic system
Limbic system
The limbic system is a set of brain structures including the hippocampus, amygdala, anterior thalamic nuclei, septum, limbic cortex and fornix, which seemingly support a variety of functions including emotion, behavior, long term memory, and olfaction. The term "limbic" comes from the Latin...

 of the brain. Interpretation of the smell begins, relating the smell to past experiences and in relation to the substance(s) emitted. The olfactory bulb acts as a relay station connecting the nose to the olfactory cortex in the brain. Olfactory information is further processed and projected through a pathway to the central nervous system (CNS), which controls emotions and behavior as well as basic thought processes.

Odor sensation depends on the concentration (number of molecules) available to the olfactory receptors. A single odorant stimulus type is typically recognized by multiple receptors, and different odorants are recognized by combinations of receptors, the patterns of neuron signals helping to identify the smell. The olfactory system does not interpret a single compound, but instead the whole odorous mix, not necessarily corresponding to concentration or intensity of any single constituent.

The widest range of odors consists of organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s, although some simple compounds not containing carbon, such as hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

 and ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

, are also odorants. The perception of an odor effect is a two-step process. First, there is the physiological part; the detection of stimuli
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....

 by receptors in the nose. The stimuli are processed by the region of the human brain which is responsible for olfaction. Because of this, an objective
Objectivity (science)
Objectivity in science is a value that informs how science is practiced and how scientific truths are created. It is the idea that scientists, in attempting to uncover truths about the natural world, must aspire to eliminate personal biases, a priori commitments, emotional involvement, etc...

 and analytical
Analytical technique
An analytical technique is a method that is used to determine the concentration of a chemical compound or chemical element. There are a wide variety of techniques used for analysis, from simple weighing to titrations to very advanced techniques using highly specialized instrumentation...

 measure of odor is impossible. While odor feelings are very personal perceptions, individual reactions are related to gender
Gender is a range of characteristics used to distinguish between males and females, particularly in the cases of men and women and the masculine and feminine attributes assigned to them. Depending on the context, the discriminating characteristics vary from sex to social role to gender identity...

, age, state of health, and personal history.

Common odors that people are used to, such as their own body odor
Body odor
Body odor or body odour, sometimes colloquially abbreviated as B.O., is the smell of bacteria growing on the body. The bacteria multiply rapidly in the presence of sweat, but sweat itself is almost completely odorless to humans....

, are less noticeable to individuals than external or uncommon odors. This is due to habituation; after continuous odor exposure, the sense of smell fatigues quickly, but recovers rapidly after the stimulus is removed. Odors can change due to environmental conditions, for example odors tend to be more distinguishable in cool dry air.

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, the conditioned odorant is blocked out because of habituation. This depends on the strength of the odorants in the mixture which can change perception and processing of an odor. This process helps classify similar odors as well as adjust sensitivity to differences in complex stimuli.

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 flavorist
A flavorist, also known as flavor chemist, is someone who uses chemistry to engineer artificial and natural flavors. The tools and materials used by flavorists are almost the same as that used by perfumers with the exception that flavorists seek to mimic or modify both the olfactory and gustation...

s and perfumer
A perfumer is a term used for an expert on creating perfume compositions, sometimes referred to affectionately as a Nose due to their fine sense of smell and skill in producing olfactory compositions...

s, can pick out individual chemicals in complex mixes through smell alone.

Odor perception is a primal sense. The sense of smell enables pleasure, can subconsciously warn of danger, help locate mates, find food, or detect predators. Humans have a surprisingly good sense of smell (even though they only have 350 functional olfactory receptor genes compared to the 1,300 found in mice) correlated to an evolutionary decline in sense of smell. Humans remarkable sense of smell is just as good as many animals, and can distinguish a diversity of odors- approximately 10,000 scents. This is because of the retro nasal route in humans to increase sensation. However, animals such as dogs show a greater sensitivity to odors than humans especially in studies using short-chained compounds. Higher cognitive brain mechanisms and more olfactory brain regions enable humans to discriminate odors better than other mammals despite fewer olfactory receptor genes.

It has been proposed that there are seven primary odors: (with examples)
  1. Musk
    Musk is a class of aromatic substances commonly used as base notes in perfumery. They include glandular secretions from animals such as the musk deer, numerous plants emitting similar fragrances, and artificial substances with similar odors. Musk was a name originally given to a substance with a...

    y- perfumes/aftershave
  2. Putrid- rotten eggs
  3. Pungent- vinegar
  4. Camphoraceous- mothballs
  5. Ethereal
    Essential oil
    An essential oil is a concentrated hydrophobic liquid containing volatile aroma compounds from plants. Essential oils are also known as volatile oils, ethereal oils or aetherolea, or simply as the "oil of" the plant from which they were extracted, such as oil of clove...

    - dry cleaning fluid
  6. Floral- roses
  7. Peppermint
    Peppermint is a hybrid mint, a cross between the watermint and spearmint . The plant, indigenous to Europe, is now widespread in cultivation throughout all regions of the world...

    y- mint gum

Odor analysis

The ability to identify odors varies among people and decreases with age. Studies show there are sex differences in odor discrimination; women usually outperform males. Pregnant women also have increased smell sensitivity, sometimes resulting in abnormal taste and smell perceptions, leading to food cravings or aversions. Deficits in smell also increase with age as well as a prevalence of taste problems (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 to over 70 years of age.

In Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

, the concentrations of odorants have since the 1870s been defined by “Olfaktometrie”, which helps to analyze the human sense of smell using the following parameters: odor substance concentration, intensity of odor, and hedonic assessment.

To establish the odor concentration, an olfactometer test is used, which employs a panel of human noses as sensors. In the olfactometry testing procedure, a diluted odorous mixture and an odor-free gas (as a reference) are presented separately from sniffing ports to a group of panelists, who are housed in an odor-neutral room. They are asked to compare the gases emitted from each sniffing port, after which the panelists are asked to report the presence of odor together with a confidence level such as guessing, inkling, or certainty of their assessment. The gas-diluting ratio is then decreased by a factor of two (i.e. chemical concentration is increased by a factor of two). The panelists are then asked to repeat their judgment. This continues for a number of dilution levels. The responses of the panelists over a range of dilution settings are used to calculate the concentration of the odor in terms of European Odor Units (ouE/m³). The main panel calibration gas used is Butan-1-ol, which at a certain diluting gives 1 ouE/m³.

General survey

The analytic methods could 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 conducted by olfactometry using an olfactometer
An olfactometer is an instrument typically used to detect and measure ambient odor dilution. Olfactometers are used in conjunction with human subjects in laboratory settings, most often in market research, to quantify and qualify human olfaction. Olfactometers are used to gauge the odor detection...

 to dilute the odor sample. On the contrary, olfactometry is rarely used for immission measurement because of the low odor concentrations. The same measuring principals are used, but the judgment of the air assay happens without diluting the samples.


Different aspects of odor can be measured through a number of quantitative methods, such as assessing concentration or apparent intensity.

Initial entry into a room provides the most accurate sensing of smell, before habituation begins to change perception of odor.

Sensation of odor has 4 properties related to threshold and tolerance: Odor concentration, odor intensity, odor quality, and hedonic tone

Measuring odor concentration

Odor concentration is an odor's pervasiveness. To measure odor sensation, an odor is diluted to certain amounts to reach 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 blank. The recognition threshold is the concentration of an odor in air in which 50% of a population can discern from an odorous sample and odor free blank.The recognition odor threshold is usually a factor of 2 to 5 times higher than the detection threshold.

The measurement of odor concentration is the most widespread method to quantify odors. It is standardized in CEN EN
European Committee for Standardization
The European Committee for Standardization or Comité Européen de Normalisation , is a non-profit organisation whose mission is to foster the European economy in global trading, the welfare of European citizens and the environment by providing an efficient infrastructure to interested parties for...

 13725:2003. The method is based on dilution of an odor sample to the odor threshold (the point at which the odor is only just detectable to 50 % of the test panel). 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 Odor Unit, OUE.
Therefore, the odor concentration at the odor threshold is 1 OUE by definition.

To establish the odor concentration, an olfactometer is used which employs a group of panelists. A diluted odorous mixture and an odor-free gas (as a reference) are presented from sniffing ports to a group of panelists. 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 their judgment. This continues until the panelists respond certain and correct twice in a row. These responses used to calculate the concentration of the odor in terms of European Odor Units (OUE/m3).

The test persons must fulfill certain requirements, for example regarding their sensitivity of odor perception. The main panel calibration gas to verify this requirement used is n-Butanol
n-Butanol or n-butyl alcohol or normal butanol is a primary alcohol with a 4-carbon structure and the molecular formula C4H9OH. Its isomers include isobutanol, 2-butanol, and tert-butanol...

 (as 1 OUE/m3≡40 ppb/v n-butanol).

To collect an odor sample, the samples must be 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, and a vacuum is placed on the drum, which fills the sample bag as the bag expands, and draws the sample from the source into the bag. Critically, all components which touch the odor sample, must be odor free, which includes sample lines and fittings.

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. It was found in a study that humans that were completely unable to detect the odor of androstenone developed the ability to detect it after repeated exposure.

Humans can discriminate between two odorants that differ in concentration by as little as 7%.

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.

Odor intensity

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

Perceived strength of the odor sensation is measured in conjunction with odor concentration. This can be modeled by the Weber-Fechner law:
I= a * log(c)+b

I is the perceived psychological intensity at the dilution step on the butanol scale, a is the Weber-Fechner coefficient, C is the chemical concentrations, and b is the intercept constant (0.5 by definition)

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.

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

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 Tone Assessment

Hedonic assessment is the process of scaling odors on a scale ranging from extremely unpleasant via neutral up 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 determining a response. The overall set of qualities are sometimes identified as the "FIDOL factors", short for Frequency, Intensity, Duration, Offensiveness and Location.

Odor character

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.

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

Interpreting dispersion modeling

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

There are two main odor sampling techniques, the direct odor sampling and the indirect odor sampling technique. Indirect refers to collecting samples from the air stream which has already passed over the emitting surface.

Direct sampling

Direct refers to the placement of a 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 and wind tunnels which include the UNSW Wind tunnel. 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, which have a vertical velocity component. For such sources, consideration needs to 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 odor sampling

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 down wind concentrations
- Stability class (or other similar factor)
- Wind speed and direction

Odor in the Indoor Environment

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
Throat irritation
Throat irritation is a common complaint in individuals of all ages. Throat irritation means different things to different people. Some people may mean a dry cough; others describe it as a scratchy feeling at the back of the throat and others simply mean a sensation of something stuck at the back of...

. Perception of odor and of irritation is unique to each person, and varies over 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 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. 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 or 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. Work force populations vary in levels 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.

Legislative provisions associated with odors

While developing environmental legislation in Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

, it was noted that there was a need for a method to accurately measure odor. Since that time, the following laws have been established:
  1. “refinery guideline” (early 1970s)
  2. federal emission protection law (1974)
  3. technical guideline to keep the air fresh
  4. olfactory emission guideline (early 1980s until 1998)

Barriers around the sources of the odors have turned out to be badly feasible. Ramparts
Defensive wall
A defensive wall is a fortification used to protect a city or settlement from potential aggressors. In ancient to modern times, they were used to enclose settlements...

, plantings etc. are little effective, whereas plantations are, after all, often at least perceived as a remarkable factor.

The only workable principle is, so far, to place the sources of the odors far enough from anybody who could feel disturbed, and to pay attention to the prevailing wind direction. Also diluting the emissions with a large air flow promises so be an effective way to avoid complicated and expensive technical measures.

Encapsulating of olfactorily relevant asset areas is only theoretically the ideal method to reduce the emission. Within an enclosure, damp and an oppressive atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

 can arise, so that the inner materials of the capsule produce a strong mechanical stress. Encapsulating even brings about a noteworthy explosion hazard.

For encapsulation to be viable, there must be some way to exhaust the spent air. Odorants remain inside the medium and tend to leak at the next suitable spot. The encapsulated space is never really gas-proof, and at some spots substances may leak out at considerably higher concentrations than they would do without the made efforts to hold them back.

There are three different ways exhausted air may be treated:
  • chemical treatment
  • physical treatment
  • biological treatment

Adsorption as separating process

Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...

 is a thermodynamic separation process, which is characterized by the removal of molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s out of a fluid phase at a solid surface. Molecules of a gaseous or fluid mixture are selectively taken up by a solid with a porous interface surface. The solid matter is called the adsorbant, the adsorbed fluid is called the adsorbate. There are two types of adsorption, physisorption
Physisorption, also called physical adsorption, is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption...

 and chemisorption
Chemisorption is a sub-class of adsorption, driven by a chemical reaction occurring at the exposed surface. A new chemical species is generated at the adsorbant surface...

. The type of force driving the adsorption process is different between the two.


A special type of adsorption is physisorption. The difference between physisorption and chemisorption is that the adsorbed molecule is tied up with the substrate by physical forces, defined here as forces which do not cause chemical bonds. Such interactions are mostly unfocused in contrast to chemical bonds. “Van-Der-Waals” – forces are a special type of such physical forces. These forces are characterized by electrostatic interactions between induced, fluctuating dipoles. To be more specific you have to call those forces “London's Dispersal forces.” A so called dipole moment occurs because of fluctuations
Statistical fluctuations
Statistical fluctuations are fluctuations in quantities derived from many identical random processes. They are fundamental and unavoidable. It can be proved that the relative fluctuations reduce as the square root of the number of identical processes....

 in the distribution of electrons around individual atoms. The temporary mean value of this force is however zero. Even though it’s only a mere transient dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...

 moment, this moment can cause a nonparallel dipole moment in an adjacent molecule. Operating forces of this nature are in inverse proportion to the sixth power of the distance between those molecules. These forces occur in almost every chemical system, but are relatively weak.

Physisorption is an exothermic and reversible reaction. Obviously stronger strengths accrue through the interaction between solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...

 dipoles at polar surfaces or reflexive loadings, appearing in electric conductive surfaces. Such interactions could be defined as a chemisorption because of their strength.


In many reactions, physisorption is a pre-cursor to chemisorption. Compared to physisorption, chemisorption is not reversible
Reversible reaction
A reversible reaction is a chemical reaction that results in an equilibrium mixture of reactants and products. For a reaction involving two reactants and two products this can be expressed symbolically as...

 and requires a larger activation energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

. Usually the bond energy is about 800 kJ/mol. For physisorption the bond energy is only about 80 kJ/mol. A monomolecular layer could be maximally adsorbed. Strong bonds between the adsorbative molecules and the substrate could lead to the point that their intermolecular bonds partly or completely detach. In such a case you have to call this a dissociation. Those molecules are in a highly reactive state. This is the basis of heterogeneous catalysis
Heterogeneous catalysis
In chemistry, heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. Phase here refers not only to solid, liquid, vs gas, but also immiscible liquids, e.g. oil and water. The great majority of practical heterogeneous catalysts...

. The substrate is then called catalytic converter.
The differences between Chemisorption and Physisorption extends beyond an increased activation energy. An important criteria for chemisorption is the chemical mutation of the absorbent. Thereby it is possible that you have to deal with a chemisorption in a few combinations with a relatively low bond energy, for example 80 kJ/mol, as a physisorption could be another combination with a bond energy even by 100 kJ/mol.
The interaction with different adsorbative molecules is very different. The surface could be taken by substances, which point out a very high bond energy with the substrate, and as a consequence of this the wanted reaction is impossible. Because of that feature those substances are called catalytic converter venom. Heat is released during that process too.

Loading of the adsorbate

During the adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...

 of a molecule, energy — the heat of adsorption — is released. This energy is the difference of the enthalpy
Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.Enthalpy is a...

 of the adsorbate in the fluid or gaseous phase and the its corresponding enthalpy on the surface of the adsorbant. With an increase of adsorbate loading on the surface of the adsorbant, the bond energy decreases in the area of the monomolecular covering. For higher loading, this value approaches zero. This implies that there is a limit for the loading of an adsorbate. (The procedure of reversing that process is called desorption). Adsorption as a separating process is a challenging process, in the case of finding the eligible adsorbates, which could link as multilateral as possible.

Types of odors

Some odors such as perfumes and flowers are sought after, with elite varieties commanding high prices. Whole industries have developed around products to remove unpleasant odors (see deodorant
Deodorants are substances applied to the body to affect body odor caused by bacterial growth and the smell associated with bacterial breakdown of perspiration in armpits, feet and other areas of the body. A subgroup of deodorants, antiperspirants, affect odor as well as prevent sweating by...

). The perception of odors is also very much dependent upon circumstance and culture. The odor of cooking processes may be pleasurable while one is cooking, but not necessarily after the meal.

The odor molecules transmit messages to the limbic system
Limbic system
The limbic system is a set of brain structures including the hippocampus, amygdala, anterior thalamic nuclei, septum, limbic cortex and fornix, which seemingly support a variety of functions including emotion, behavior, long term memory, and olfaction. The term "limbic" comes from the Latin...

, 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
Aromatherapy is a form of alternative medicine that uses volatile plant materials, known as essential oils, and other aromatic compounds for the purpose of altering a person's mind, mood, cognitive function or health....

” 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
Anecdotal evidence
The expression anecdotal evidence refers to evidence from anecdotes. Because of the small sample, there is a larger chance that it may be true but unreliable due to cherry-picked or otherwise unrepresentative of typical cases....

 and lacks controlled scientific
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...

 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. The reaction, as with other chemical allergies, can be anywhere from a slight headache to anaphylactic shock, which can result in death.

Unpleasant odors play various roles in nature, often to warn of danger, though this may not be known to the subject who smells it. An odor that is viewed as unpleasant by some people or cultures can be viewed as attractive by others where there is more familiarity or a better reputation.

It is commonly viewed that those holding an unpleasant body odor
Body odor
Body odor or body odour, sometimes colloquially abbreviated as B.O., is the smell of bacteria growing on the body. The bacteria multiply rapidly in the presence of sweat, but sweat itself is almost completely odorless to humans....

 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. This includes smells associated with pollution.

What actually 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 actually odorless. It is the bacteria in the perspiration that cause the odor.

Unpleasant odors can arise from specific industrial processes, adversely affecting workers and even residents downwind of the industry. The most common sources of industrial odor arise from sewage treatment
Sewage treatment
Sewage treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater and household sewage, both runoff and domestic. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants...

 plants, refineries
A refinery is a production facility composed of a group of chemical engineering unit processes and unit operations refining certain materials or converting raw material into products of value.-Types of refineries:Different types of refineries are as follows:...

, 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.

Study of odors

The study of odors is a growing field but is a complex and difficult one. The human olfactory system
Olfactory system
The olfactory system is the sensory system used for olfaction, or the sense of smell. Most mammals and reptiles have two distinct parts to their olfactory system: a main olfactory system and an accessory olfactory system. The main olfactory system detects volatile, airborne substances, while the...

 can detect many thousands of scents based on only very minute airborne concentrations of a chemical. The sense of smell of many animals is even better. Some fragrant flowers give off odor plumes
Bee learning and communication
Honey bees learn and communicate in order to find food sources and for other means.-Learning:Learning is essential for efficient foraging. Honey bees are unlikely to make many repeat visits if a plant provides little in the way of reward...

that move downwind and are detectable by bee
Bees are flying insects closely related to wasps and ants, and are known for their role in pollination and for producing honey and beeswax. Bees are a monophyletic lineage within the superfamily Apoidea, presently classified by the unranked taxon name Anthophila...

s more than a kilometer away.

The study of odors can also get complicated because of the complex chemistry taking place at the moment of a smell sensation. For example iron-containing metallic objects are perceived to have an distinctive odor when touched, although iron's vapor pressure
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...

 is negligible. According to a 2006 study this smell is the result of aldehyde
An aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....

s (for example nonanal
Nonanal, also called nonanaldehyde or pelargonaldehyde, is an alkyl aldehyde. It has a strong fruity or floral odor and is used in flavors and perfume. It is also produced by the human body.- Mosquitoes :...

) and ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

s (example: 1-octen-3-one) released from the human skin on contact with ferrous
Ferrous , in chemistry, indicates a divalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....

 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.


A pheromone is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual...

s are odors that are used for communication, and are sometimes called "airborne hormones". A female moth
A moth is an insect closely related to the butterfly, both being of the order Lepidoptera. Moths form the majority of this order; there are thought to be 150,000 to 250,000 different species of moth , with thousands of species yet to be described...

 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
A beehive is a structure in which bees live and raise their young.Beehive may also refer to:Buildings and locations:* Bee Hive, Alabama, a neighborhood in Alabama* Beehive , a wing of the New Zealand Parliament Buildings...

. Workers 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

There are hopes that advanced technology could do everything from testing perfumes to helping detect cancer or explosives by detecting specific scents, but as of yet artificial noses are still problematic. The complex nature of the human nose, its ability to detect even the most subtle of scents, is at the present moment difficult to replicate.

Most artificial or electronic nose
Electronic nose
An electronic nose is a device intended to detect odors or flavors.Over the last decade, “electronic sensing” or “e-sensing” technologies have undergone important developments from a technical and commercial point of view...

 instruments work by combining output from an array of non-specific chemical sensors to produce a finger print 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
Multivariate analysis
Multivariate analysis is based on the statistical principle of multivariate statistics, which involves observation and analysis of more than one statistical variable at a time...

 and neural network
Neural network
The term neural network was traditionally used to refer to a network or circuit of biological neurons. The modern usage of the term often refers to artificial neural networks, which are composed of artificial neurons or nodes...

 techniques, to "learn" the chemicals. Many current electronic nose instruments suffer from problems with reproducibility subject to varying ambient temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 and humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...

. An example of this type of technology is the colorimetric sensor array
Kenneth S. Suslick
Kenneth S. Suslick is the Marvin T. Schmidt Professor of Chemistry, Professor of Materials Science & Engineering, and Professor of the Beckman Institute for Advanced Science & Engineering at the University of Illinois at Urbana-Champaign...

, which visualizes odor through color change and creates a "picture" of it.

Behavioral cues

Odor perception is a complex process involving the central nervous system that 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. Odors can disturb our concentration, diminish productivity, evoke symptoms, and, in general, increase a dislike for a particular environment. Odors can impact the liking for a person, place, food, or product as a form of conditioning. Memories recalled by odors are significantly more emotional and evocative than those recalled by the same cue presented visually or auditorily.
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 with the presence of the same odor.
Nonhuman animals communicate their emotional states through changes in body odor and human body odors are indicative of emotional state.

Human body odors influence interpersonal relationships. Human body odors 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 bringing feelings of security. Reassurance created by familiar parental body odors may contribute significantly to the attachment process.

How a man smells is critical for woman to find a lover. Body odor is a sensory cue critical for mate selection because it is a signal of immunological health. Women prefer men with major histocompatibility complex (MHC) genotypes and odor different than themselves especially during ovulation. Different MHC alleles are favorable because different allele combination's 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."

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
Homosexuality is romantic or sexual attraction or behavior between members of the same sex or gender. As a sexual orientation, homosexuality refers to "an enduring pattern of or disposition to experience sexual, affectional, or romantic attractions" primarily or exclusively to people of the same...

 and straight
Heterosexuality is romantic or sexual attraction or behavior between members of the opposite sex or gender. As a sexual orientation, heterosexuality refers to "an enduring pattern of or disposition to experience sexual, affectional, physical or romantic attractions to persons of the opposite sex";...

 males' brains respond differently 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 meaning that lesbian women were not as responsive to male identified odors, while their response to female cues was similar to straight males. According to the researchers, this research suggests a possible role for human pheromones in the biological basis of sexual orientation
Sexual orientation
Sexual orientation describes a pattern of emotional, romantic, or sexual attractions to the opposite sex, the same sex, both, or neither, and the genders that accompany them. By the convention of organized researchers, these attractions are subsumed under heterosexuality, homosexuality,...


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. 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.

Use of scent in design

The sense of smell is often 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."

The "new car smell" is not intentionally produced by the manufacturer, but is the smell “of number of harmful chemicals, including [the elements] antimony, bromine, chlorine, and lead. Repeated and concentrated exposure to any of these chemicals may contribute to a variety of acute and long-term health issues such as birth defects, impaired learning, liver toxicity, and cancer.”

Currently there is no regulation of the use of scent in product design. The FDA has no say as to what items are used to create a scent. If ingredients are listed on a product, the term "fragrance" can be used in a general sense. The National Academy of Sciences (NAS) reports that “95 percent of the ingredients used to create fragrances today are synthetic compounds derived from petroleum, including benzene derivatives, aldehydes, and many other known toxins and sensitizers. Many of these substances have been linked to cancer, birth defects, central nervous system disorders, and allergic reactions.”

See also

  • Aroma compound
    Aroma compound
    An aroma compound, also known as odorant, aroma, fragrance or flavor, is a chemical compound that has a smell or odor...

  • Machine olfaction
    Machine olfaction
    Machine olfaction is the automated simulation of the sense of smell. It is an emerging requirement of modern robotics where robots or other automated systems are needed to measure the existence of a particular chemical concentration in air...

  • Olfaction
    Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...

  • Olfactometer
    An olfactometer is an instrument typically used to detect and measure ambient odor dilution. Olfactometers are used in conjunction with human subjects in laboratory settings, most often in market research, to quantify and qualify human olfaction. Olfactometers are used to gauge the odor detection...

  • Olfactory fatigue
    Olfactory fatigue
    Olfactory fatigue or adaptation is the temporary, normal inability to distinguish a particular odor after a prolonged exposure to that airborne compound. For example, when entering a restaurant initially the odor of food is often perceived as being very strong, but after time the awareness of the...

  • Scented water
    Scented water
    Scented water, odoriferous water or sweet water, is a water with a sweet aromatic smell. It is made of flowers or herbs and is the precursor of the modern day perfume...

Further reading

— A survey of current olfactory knowledge, experimental investigation of computer-based olfactory interfaces. Includes extensive reference list, partially annotated.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.