Odor

An odor or odour 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 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 perfumes. In contrast, malodor, stench, reek, and stink are used specifically to describe unpleasant odors.

Terminology

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

Basics

Sense of smell gives rise to the perception of odors. Smell is mediated by the olfactory nerve. The olfactory receptor cells are neurons present in the olfactory epithelium- a small patch of tissue in back of nasal cavity. 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 connects the brain to the external air. Odorous molecules act as a chemical stimulus. Molecules bind to receptor proteins extended from cilia initiating a electric signal. When the signal reaches a threshold it fires, traveling along the axon to the olfactory bulb, part or the limbic system right outside of brain. Interpretation of the smell begins, relating the smell to past experiences and in relation to the substance(s) emitted. The olfactory bulb is relay station connecting nose to olfactory cortex. Olfactory information is further processed and projected through a pathway to the 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 is typically recognized by multiple receptors, and different odorants are recognized by combinations of receptors, the neuron fire helps to identify the smell. The olfactory system does not interpret a single compound but instead the whole odorous mix not corresponding to concentration or intensity.

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. This is due to habituation, after continuous odor exposure the sense of smell fatigues quickly, but recovers rapidly. Odors can change due to environmental conditions, for example odors tend to be more distinguishable in cool dry air.

Habituation is the the ability to generalize odors after continuous exposure. The sensitivity and ability to discriminate odors diminishes with exposure to ignore continuous stimulus and focus on a particular sense. 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 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 perception is a primal sense. Sense of smell enables both pleasure, can non consciously warn animals 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 most 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 less olfactory receptor genes. It has been proposed that there are seven primary odors: ( with examples) 1.) musky- perfumes/aftershave 2.) putrid- rotten eggs 3.) pungent- vinegar 4.) camphoraceous- mothballs 5.) ethereal- dry cleaning fluid 6.) floral- roses 7.) pepperminty- 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 leading to 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. 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 increased to over 70 years of age.

In Germany, the concentrations of odorants have since the 1870s 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.

It has been proposed that there are seven primary odors: ( with examples) 1.) musky- perfumes/aftershave 2.) putrid- rotten eggs 3.) pungent- vinegar 4.) camphoraceous- mothballs 5.) ethereal- dry cleaning fluid 6.) floral- roses 7.) pepperminty- mint gum

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 and 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.

Measurement

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.

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 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, OU_E. Therefore, the odor concentration at the odor threshold is 1 OU_E 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 (OU_E/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 (as 1 OU_E/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 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 Modelling

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

These are a function of modelled 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 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

Human sense of smell is a primary factor in the sensation of comfort. Olfaction as a sensory system brings awareness to 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. Perception or 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 response 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. Indoor environments have a higher range of chemical compounds that disrupt health. Health effects of odor are traced to the sensation of an odor or the odorant itself. Health effects and symptoms vary- 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, stressed induced illness, or hypersensitivity. Ability to perform tasks may decrease, ability to perform tasks, and other social/behavioral changes. 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 OELS or occupational exposure limits 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, habituation, or may not realize chemicals possible risks.

Legislative provisions associated with odors

While developing environmental legislation in Germany, 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:

#“refinery guideline” (early 1970s) #federal emission protection law (1974) #technical guideline to keep the air fresh #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.

Physisorption

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.

Chemisorption

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.

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

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. An example of this type of technology is the colorimetric sensor array, which visualizes odor through color change and creates a picture of it.

Odor 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 then 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 and straight 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.

An odor can cue a memory. Most memories that pertain to odor come from the first decade of life compared to verbal and photo memories which usually come from 10-30 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 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. "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 intentional by the manufacturer but is the smell “of number of harmful chemicals, including 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

Olfaction

Aroma compound

Olfactory fatigue

Machine olfaction

Scented water

Olfactometer

References

External links

Answers to several questions from curious kids about smells and odors

American Assoc. for Chemoreception Sciences society

Structure-Odor Relationships

Category:Olfaction Odor