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Odor receptors on the antennae of a Luna moth

An odor or odour is a chemical dissolved in air, generally at a very low concentration, which we perceive by the sense of olfaction. Odors are also called smells, which can refer to both pleasant and unpleasant odors. In contrast, stench and stink are used specifically to describe an unpleasant odor. The terms fragrance, scent, or aroma are used primarily by the food and cosmetic industry to describe a pleasant odor, and is sometimes used to refer to perfumes.

Body odor


Odor is a sensation caused by odorant molecules dissolved in air. The widest range of odors consist of organic compounds although some inorganic substances, such as hydrogen sulphide and ammonia, are also odorants.

In order for something to give off a smell it must be volatile - that is, it must easily evaporate at normal temperatures so that molecules can be passed into the person's nose.

In order for us to smell the odor it must be water soluble - so that molecules of the substance pass through the mucus that coats the inner surface of the nasal cavity and reach the olfactory cells.

it must also be lipid soluble - because the olfactory hairs are composed primarily of lipids and the surface of the olfactory cells are also lipid containing

The perception of an odour effect is a two step process. First, there is the physiological part; the sense of the stimulus by receptors in the nose. After that the psychological part follows. The stimuli are processed by the region of the human brain which is responsible for smelling. Because of this a 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. However, experienced people, such as flavorists and perfumers can pick out individual chemicals in complex mixes through smell alone.

Odour analysis

Odorimetry is the analysis of the characteristtics of odours

The concentrations of odorant in Germany are defined by the “Olfaktometrie” since the 70’s of the last century. In this connexion it’s about the standard method to define the sense barrier of odours on basis the thinner of concentrated odour loaded assays. Following parameters are defined: odour substance concentration, intensity of odour and hedonism assessment.

General survey

The analytic methods could be subdivided into the physically, the gas chromatographically and the chemosensory method. It differs between the emission and immission measurement while measurement of odour. During the emission measurement the odour concentration in the air is so high, that the so called “Olfaktometer” is needed to thinner the assay. Because of this all measurement methods based on thinning assays are called “olfaktometrical methods”. An “Olfaktormeter” is seldom used during the immission measurement. The same measure principals are used but the judgement of the air assay happens without thinning the assay.

Odorant concentration

This is the oldest and most important method to define odour emission. Ledger of this method is the concentration of odour substrate at the odour barrier. This barrier is also called apperception barrier. The barrier has got an odorant concentration of 1 GEE/m³ and is subscribed with cod. To define the odoront concentration, it is necessary to thinner the air essay to the odour barrier with the help of the “Olfaktormeter”, and the thinner number Z at the odour barrier, is the same number like the odour substance concentration.

Because our sense of smell is so sensitive the amount of the substance necessary to create a noticable smell is sometimes very small. There is a substance called methyl mercaptan which is noticable in concentrations of only 1/25,000,000,000 milligram per milliliter of air.It is added to natural gas so that leaks will be noticed.

Odour intensity

This size scales an special air array according to the intensity scale. The scale is differentiated into the following steps:

0 no odour 1 very weak ( odour barrier ) 2 weak 3 obvious 4 strong 5 very strong 6 intolerable

If it is an emission measurement (thinned by the olfaktometer) than the evaluation of odour intensity got to be ranked to the olfactometry methods. A direct evaluation is used, when the array is measured from the immission side.

Hedonism assessment

The hedonism assessment is the process of scaling odours beginning with extremely unpleasant followed by neutral up to extremely pleasant. There is no difference between this process and the method of measuring the odour intensity. But the method of emission measurement is seldom used and that of immission measurement is not used.

Odour type

This is a verbal characterisation of the sensed odour by the test person, like disgusting, caustic, ruffling… etc. There are no more applications needed than a test person to run this method. The evaluation of the odour type could be an emission or an immission method. It has an great impact on evaluating the source of the odour emission. Typically it begins with an acrid odor or rotten egg odor like and progresses to a sweet odor like .

Immission measurement

You have to differentiate into the following details, while measuring the immission:

First there is the odour time slice ( Result = Part of “odour 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

It is possible to measure directly where it is needed, while olfactory measurement. But if a nearly unsophisticated result should be reached, then this is unusual. To be not affected by other odours than the odours in the specific air sample, test person should value the air sample in a nearly unloaded environment. That is the reason why air samples usually are taken within a sampling bag, so that the underlying valuation could happen in a suitable environment. 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, standardised, meaningful and reproducible. This is needed to make different measurements comparable. Odorant concentrations scaled in GG/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 odours

In the beginning of legislating the environmental protection the question of evaluating different odours appeared. Since that time following laws had been made:

1. “refinery guideline” early 70’s 2. federal immission protection law (1974) 3. technical guideline to keep the air clean 4. olfactory immission guideline (early 80’s till 1998)

Sanctions at the place of the immission, like plurally vitrification against aircraft noise, drop out. Terms of transmission could be marginal changed by establishing ramparts, plantings and so on, but the objective efficiency of those sanctions is more likely small. But the subjective efficiency of a sealed planting is remarkable. The choice of the location is the most important sanction, that means to keep an adequate distance to the next development and to take care of the meteorology terms, like the main wind directions. The reduction of the immission, in cases of big air flow volumes with a small emission concentration, could be a effective and economic alternative, instead of reducing the emission with different sanctions.

Even the capsuling 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 capsuling. Below the capsuling a damp and aggressive atmosphere arises, so that the inner materials of the capsuling are set out a high degree of mechanical stress. Not to let the explosion hazard slide. In terms of capsuling you have to think about to suck away the exhausted air. When emission is avoided through capsuling, than odorants remain inside the medium and try to leakage at the next suitable spot. By the way, capsuling is never really gas- proof, at some spots considerable higher concentrated substances could leak out.

There are three different types of proceeding principles with respectively different proceeding characteristics for treating exhausted air:

  • chemical treatment
  • physical treatment
  • biological treatment

Adsorption as separating process

Adsorption subscribes a thermo separation process, which is characterised by taking up molecules out of a fluid phase at a solid surface. Molecules of a gas- or fluidness mixture are taken up by a solid matter with more porous and interface active surface. The solid matter is called “adsorbens”, the adsorbed fluid is called “adsorptive”. While adsorbing you have to differ between “Physisorption” and “Chemisorption” because of different types of bonds.


Main article: Physisorption

A special type of adsorption is physisorption. The difference between physisorption and chemisorption is that, the adsorb molecule is tied up with the substrate by physical strengths. Physical strengths are defined as strengths which doesn’t cause chemical bonds. Such interactions are mostly unfocused in contrast to chemical bonds. “Van-Der-Waals” – strengths are a special type of such physical strengths. Electrostatic interactions between induced, fluctuated dipoles, characterise them. To be more specific you have to call those strengths “London’s Dispersal strengths”. A so called dipole moment accrues because of fluctuations in the distribution of electrons caused by the lack of fixations between those electrons. The temporary mean value is however zero. Even though it’s only a more transient dipole moment, this moment can cause an nonparallel dipole moment in a adjacent molecule. Operating strengths there are in inverse proportion to the sixth potency of the distance between those molecules. Generally there is to say, that those strengths have a relative big scope at their disposal, but these strengths are relatively weak. Physisorption is a 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.


Main article: Chemisorption

Now the mediation of the chemisorption is up to come. Most comment the physisorption is pre-amplifier to chemisorption. Compared to the physisorption, chemisorption is not reversible and requires a big activation energy. Usually the bond energy is about 800 kJ/mol. At the physisorption the bond energy is only about 80 kJ/mol. A monomolecular layer could be maximal 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 used at the heterogeneous catalysis. The substrate is then called catalytic converter. You have to divide between physissorption and chemisorption not only because of the bond energy. An important criteria for chemisorption is the chemical mutation of the absorbat respectively the adsorben. 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 an other 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 - adsorptions heat – is released, which has to be apprehended as difference of the enthalpy of the fluid or gaseous phase and the enthalpy inside the adsorbat. With an increase of the loading on the surface of the adsorben the bond energy decreases in the area of the monomolecular covering. For higher loading this value approaches zero. This should mean, that there is a limit for the loading of an absorben. 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 eg air fresheners and deodorants). The perception of odors is also very much dependent upon circumstance and culture. The odor of cooking processes may be agreeable while cooking but not necessarily after the meal.

The odor molecules send 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 certain 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, certain animal rendering plants and industries processing chemicals (such as sulphur) which have odorous characteristics. Sometimes industrial odor sources are the subject of community controversy and scientific analysis.

Main article: Classification of odors

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 [1] this smell is the result of aldehydes (for example nonanal) and ketones (exampe: 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 deliberately 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 odors 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 smelling machines could do everything from test perfumes to help detect cancer or explosives by detecting certain 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

References & Bibliography

Key texts


  • Amoore, J.E. (1970) Molecular Basis of Odor. Charles C. Thomas, Springfield, IL.


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Additional material



External links


  1. 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
    1. REDIRECT Template:Doi

Zwaardemaker, H.C. (1895) Classifying smells. Cited in: A.J. Haagen-Smit (1952) Smell and taste, Scientific American 186(3): March.

da:Lugt de:Geruch es:Olfato fr:Odeur he:ריח nl:Geur pt:Odor ru:Запах

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