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


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 sulfide and ammonia, are also odorants. The perception of an odor 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 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. However, experienced people, such as flavorists and perfumers can pick out individual chemicals in complex mixes through smell alone.

Odor Analysis

The concentrations of odorant in Germany are defined by the "Olfaktometrie" since the 1870"s. In this connection it"s about the standard method to define the sense barrier of odors on basis the thinner of concentrated odor loaded assays. Following parameters are defined: odor substance concentration, intensity of odor 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 odor. During the emission measurement the odor 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 judgment of the air assay happens without thinning the assay.


While no generally acceptable method for measuring odor exists, measurement of different aspects of odor can be attempted through a number of quantitative methods, for example:

Odorant concentration

This is the oldest method to define odor emission. Ledger of this method is the concentration of odor substrate at the odor threshold. This threshold is also called apperception barrier. The threshold has got an odorant concentration of 1 GEE/m and is subscribed with cod. To define the odorant concentration, it is necessary to dilute the air assay to the odor threshold with the help of the "Olfaktormeter". The dilution factor Z, at the odor barrier, is the same number as the odor substance concentration.

Odor Intensity

This applies a scale to a special air array according to intensity. The scale is differentiated into the following steps:

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

If it is an emission measurement (diluted by the olfactometer), then the evaluation of odor intensity must be ranked to the olfactometry methods. A direct evaluation is used, when the array is measured from the emission side.

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

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

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

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

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 odors than the odors 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, standardized, meaningful and reproducible. This is needed to make different measurements comparable. Odorant concentrations scaled in GG/m 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

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

1. "refinery guideline" (early 1970s) 2. federal emission protection law (1974) 3. technical guideline to keep the air clean 4. olfactory emission guideline (early 1980s until 1998)

Controls at the point of the emission, 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 controls is likely minimal. But the subjective efficiency of a plantings is remarkable.

The choice of the location is the most important control, that means keeping an adequate distance to the nearest receptor and paying attention to the meteorology conditions, such as the prevailing wind direction. Reduction of the emission, by way of dilution of a small emission concentration with large air flow volumes, could be an effective and economic alternative, instead of 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.

In terms of encapsulation, you have to think about exhaust the spent air. When emission is avoided through capsuling, then odorants remain inside the medium and try to leak 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 ways for treating exhausted air:

• chemical treatment • physical treatment • biological treatment

Adsorption as separating process

Adsorption subscribes a thermo separation process, which is characterized 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.


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, characterize 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 a nonparallel dipole moment in an 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 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.


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 adsorbent

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 adsorbent 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 absorbent. 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 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 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[1] 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 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 smells to call other bees into an appropriate cavity when a swarm moves in or to "sound" an alarm when the hive is threatened.

In Mammals some pathway of Pheromones identification lay in vomeronasal organ and some - in odor receptors.

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.