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An odor or odour (see spelling differences) is caused by one or more volatilized chemical compounds, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction. Odors are also called smells, 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 perfumes. In contrast, malodorous, stench, reek, and stink are used specifically to describe unpleasant odors.


In the United Kingdommarker, "odour" refers to scents in general. In the US, "odor" has a more negative connotation; "scent" or "aroma" are used for pleasant smells.


The widest range of odors consists of organic compounds, although some inorganic substances, 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; the detection of stimuli 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 and analytical measure of odor is impossible. While odor feelings are very personal perceptions, individual reactions are related to gender, age, state of health, and private affectations. Common odors that people are used to, such as their own body odor, are less noticeable to individuals than external or uncommon odors.

For most people, the process of smelling gives little information concerning the ingredients of a substance. It only offers information related to the emotional impact. Experienced people, however, such as flavorists and perfumers, can pick out individual chemicals in complex mixes through smell alone.

Odor analysis

In Germanymarker, the concentrations of odorants have since the 1870’s been defined by the “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 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, which 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 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 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.

Odor concentration

The measurement of odor concentration is the most widespread method to quantify odors. It is standardized in CEN EN 13725:2003. The method is based on dilution of an odor sample to the odor threshold (the point at which the odor is only perceptible 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 panel of test persons. 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 gases emitted from each port, the panelists are asked to report the presence of odor. 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 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 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.

Odor intensity

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 most often applied by having a dilution series tested by a panel of independent observers who have been trained to differentiate intensity.

Hedonic assessment

Hedonic assessment is the process of scaling odors on a scale ranging from extremely unpleasant via neutral up to extremely pleasant. There is no correlation between this method and the method of measuring the odor intensity. However, the hedonic perception of some odors may change from pleasant to unpleasant with increasing concentration and intensity.

Odor type

This is a verbal characterization of the sensed odor by the test person, such as disgusting, caustic, ruffling, etc. There are no more applications needed than a test person to run this method. The evaluation of the odor type could be an emission or an immission method. It has a great impact on evaluating the source of the odor emission.

Emission measurement

The following details have to be differentiated while the emission is measured:

First there is the odor time slice (Result = Part of “odor hours per year” per area).Then there is the olfactory flag scope (Result = Current scope at actual meteorology situation). And last but not least there is the harassment exaltation by questionings (Result = differentiated acquisition harassments).

Sampling technique

There are two main odor sampling techniques, the direct odor sampling and the indirect odor sampling technique.

Direct odor sampling

Air will be sampled at the source and fed straight into the olfactometer for assessment by an odor panel. The following problems can be associated with this technique:

Odor panel members need to be seated in an odor neutral environment, thus they need to be housed in a separate area. This is difficult to achieve when assessing odor released from, for example factories, where the odor can be emitted from a stack on the end of a production line. This means that the odor sample collected needs to be transported from the stack to the unit where the odor panel sits. This can sometimes be on the other side of the factory plant. The sample then must therefore pass through a very long sample line to the olfactometer. This can have influences on the sample quality, can have potential air blockages due to water condensation or other operational procedures. Therefore most odor annoyance assessment companies use the indirect air sampling method.

Indirect odor sampling

Indirect odor sampling is done with the use of odor (air) sampling bags, which are made from an odor neutral material e.g. Teflon. The odor sample bags are connected to an air sampling line which is then, for example, hooked up to a stack. The air stream is then sampled and stored in the odor sample bag and can then be analyzed in a suitable environment (e.g. in an odor laboratory).

The indirect method is used to sample a wide variety of odor sources. From stacks on the end of a factory line, water surfaces or ambient air surroundings.

Each odor source has its own set of problems when sampled; these problems need to be overcome in order to collect a representative sample of the odor source. The following problems can be encountered:

Vacuum: Vacuum can be overcome by placing the odor sample bag in vacuum container which can be placed under vacuum. If the vacuum is higher than the vacuum at the source, the odor sample will collect in the bag.
High temperatures and moisture content: High temperatures and high moisture contents inside the odor source leads to complications when sampled. When the sample leaves the source, it will cool down and produce condensate in the sample line and or odor sample bag. This can lead to growth of bacteria or when drying out release more odor, thus alter the odor concentration of the sample. The same is true when sampling in high moisture conditions. A way round the problem is to use a stack dilution probe through which an inert gas (for example dry nitrogen) can be fed that dries the sample stream. This prevents the moisture condensing in the sample line and or the odor sample bag.
High lethal gas concentrations: Sometimes odor sources emit a high concentration of gases that are lethal to man. These samples must be diluted to a safe level, before being presented to the odor panel. This pre-dilution can be done in a stack-dilution probe, by the addition of an inert gas or on a dilution device for example an extra olfactometer.
Odor concentrations: More often than not, odor sampled at the source is higher then the ambient odor concentration. In a few cases the odor concentration can be so high that panelists will make a positive identification even if the olfactometer is diluting the odor sample in its upper dilution range. The sample must then be pre-diluted to make a sensible reading, this pre-dilution can again be done with a stack-dilution probe, by the addition of an inert gas or on a dilution device for example an extra olfactometer.
Large odor emitting surface (land or water): When a large surface is emitting odor, for example a sewage treatment plant, a fixed dimension “hood” can be used. In one end of the hood, clean air is blown in at a known rate, and on the other end, a sample is collected via the indirect method. If a large land surface is emitting odor, for example a bio filter (a big concrete basin filled with wood chip through which the factories waste air is pumped), a section can be cornered off with plastic (e.g. Teflon) (of which the dimensions are known). The air from the factory will inflate the plastic (lift it up) and an odor sample can be taken from under the plastic via the direct air method.

All involved sampling parts have to be made out of olfactory neutral materials. Principally every sampling has to meet the logically requirements, has to be defined, standardized, meaningful and reproducible. This is needed to make different measurements comparable. Odorant concentrations scaled in either GG/m³ or ouE/m³ aren’t convincing while comparing different emissions of different plants. Because of this instead of comparing different concentrations, different emission mass currents of the emitted freight are compared.

Legislative provisions associated with odors

While developing environmental legislation in Germanymarker, it was noted that there was a need for a method with which to accurately measure odor. Since that time, the following laws have been made:

  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)

Controls at the point of the emission, like plural vitrification against aircraft noise, drop out. Terms of transmission could be marginally changed by establishing ramparts, plantings and so on, but the objective efficiency of those controls is likely minimal. But the subjective efficiency of a plantings is remarkable.

The choice of the location is the most important control. This involves keeping an adequate distance from the nearest receptor and paying attention to the meteorological conditions; e.g., prevailing wind direction. Reducing the concentraion of an odor (emission), by diluting a small emission with a large air flow, could be an effective and economic alternative to reducing the emission with different controls.

Encapsulating of olfactory relevant asset areas is the best known method to reduce the emission, but it is not the most suitable one. Different matters need to be considered by encapsulation. Within an enclosure a damp and oppressive atmosphere can arise, so that the inner materials of the capsule produce a high degree of mechanical stress. Not to let the explosion hazard slide.

For encapsulation to be viable, there must be some way to exhaust the spent air. When emission is avoided through capsuling, odorants remain inside the medium and tend to leak at the next suitable spot. In any case, capsuling is never really gas-proof, and at some spots substances may leak out at considerably higher concentrations.

There are three different ways exhausted air may be treated:

  • chemical treatment
  • physical treatment
  • biological treatment

Adsorption as separating process

Adsorption is a thermo separation process, which is characterized by the removal of molecules out of a fluid phase at a solid surface. Molecules of a gas- or fluid mixture are 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 and chemisorption. 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 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 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 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 and requires a larger activation energy. 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. 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 adsorben

During the adsorption of a molecule, energy - the heat of adsorption – is released. This energy is the difference of the enthalpy of the adsorben in the fluid or gaseous phase and the its corresponding enthalpy on the surface of the adsorbant. With an increase of the 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 adsorbant.The procedure of turning back that process is called desorption.Adsorption as a separating process is a challenging process, in the case of finding the eligible adsorbents, which could link as multilateral as possible.

Types of odors

Some odors such as perfumes and flowers are sought after, elite varieties commanding high prices. Whole industries have developed products to remove unpleasant odors (see deodorant). The perception of odors is also very much dependent upon circumstance and culture. The odor of cooking processes may be pleasurable while cooking but not necessarily after the meal.

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

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 plants, refineries, specific animal rendering plants and industries processing chemicals (such as sulfur) which have odorous characteristics. Sometimes industrial odor sources are the subject of community controversy and scientific analysis.

The study of odors

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 very 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 can also get complicated because of the complex chemistry taking place at the moment of a smell sensation. For example iron metal objects are perceived to have an odor when touched although iron vapor pressure is negligible. According to a 2006 study this smell is the result of aldehydes (for example nonanal) and ketones (example: 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.


Pheromones are odors that are used for communication. A female moth may release a pheromone that can entice a male moth that is several kilometers away. Honeybee queens constantly release pheromones that regulate the activity of the hive. Workers can release such smells to call other bees into an appropriate cavity when a swarm moves in or to "sound" an alarm when the hive is threatened.

Advanced technology

There are hopes that advanced technology could do everything from test perfumes to help 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 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 and neural network techniques, to "learn" the chemicals. Many current electronic nose instruments suffer from problems with reproducibility with varying ambient temperature and humidity.

See also


  1. CEN EN 13725:2003, Air quality - Determination of odour concentration by dynamic olfactometry.
  2. Communication The Two Odors of Iron when Touched or Pickled: (Skin) Carbonyl Compounds and Organophosphines Dietmar Glindemann, Andrea Dietrich, Hans-Joachim Staerk, Peter Kuschk Angewandte Chemie International Edition web release 2006

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