odor odor an odor (american english) or odour (commonwealth english; see spelling differences) is caused by one or more volatilized chemical compounds that are generally found in low concentrations that humans and many animals can perceive via their sense of smell. an odor is also called a "smell" or a "scent", which can refer to either a pleasant or an unpleasant odor. "smell", from allegory of the senses by jan brueghel the elder, museo del prado while "odor" and "smell" can refer to pleasant and unpleasant odors, the terms "scent", "aroma", and "fragrance" are usually reserved for pleasant-smelling odors and are frequently used in the food and cosmetic industry to describe floral scents or to refer to perfumes. physiology of smell odor control covers at a sewage treatment plant: under these covers, grit and gravel are settled out of the wastewater. sense of smell the perception of odors, or sense of smell, is mediated by the olfactory nerve. the olfactory receptor (or) cells are neurons present in the olfactory epithelium, which is a small patch of tissue at the back of the nasal cavity. there are millions of olfactory receptor neurons that act as sensory signaling cells. each neuron has cilia in direct contact with the air. odorous molecules bind to receptor proteins extending from cilia and act as a chemical stimulus, initiating electric signals that travel along the olfactory nerve's axons to the brain. when an electrical signal reaches a threshold, the neuron fires, which sends a signal traveling along the axon to the olfactory bulb, a part of the limbic system of the brain. interpretation of the smell begins there, relating the smell to past experiences and in relation to the substance(s) inhaled. the olfactory bulb acts as a relay station connecting the nose to the olfactory cortex in the brain. olfactory information is further processed and forwarded to the central nervous system (cns), which controls emotions and behavior as well as basic thought processes. odor sensation usually depends on the concentration (number of molecules) available to the olfactory receptors. a single odorant is usually recognized by many receptors. different odorants are recognized by combinations of receptors. the patterns of neuron signals help to identify the smell. the olfactory system does not interpret a single compound, but instead the whole odorous mix. this does not correspond to the concentration or intensity of any single constituent. most odors consist of organic compounds, although some simple compounds not containing carbon, 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. this is the detection of stimuli by receptors in the nose. the stimuli are recognized by the region of the human brain which handles olfaction. because of this, an objective and analytical measure of odor is impossible. while odor feelings are personal perceptions, individual reactions are usually related. they relate to things such as gender, age, state of health, and personal history. smell acuity by age and sex the ability to identify odor varies among people and decreases with age. studies claim that there are sex differences in odor discrimination, and that women usually outperform men. conversely, there are some studies claiming a male advantage. a 2019 meta-analysis claimed that the differences in olfaction are extremely small, but confirmed a small advantage for women. pregnant women have increased smell sensitivity, sometimes resulting in abnormal taste and smell perceptions, leading to food cravings or aversions. the ability to taste also decreases with age as 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, especially once over 70 years of age. smell acuity compared to other animals for most untrained individuals, the act of smelling acquires little information concerning the specific ingredients of an odor. their smell perception primarily offers information that elicits an emotional response. experienced individuals, however, such as flavorists and perfumers, can identify discrete chemicals in complex mixtures using only their sense of smell. odor perception is a primary evolutionary sense. the sense of smell can induce pleasure or subconsciously warn of danger, which may, for example, help to locate mates, find food, or detect predators. humans have an unusually good sense of smell considering they have only 350 functional olfactory receptor genes compared to the 1,300 found in mice, for example. this is despite an apparent evolutionary decline in the sense of smell. the human sense of smell is comparable with many animals, able to distinguish between a diverse range of odors. studies have reported that humans can distinguish in the region of one trillion unique aromas. habituation or adaptation odors that a person is used to, such as their own body odor, are less noticeable than uncommon odors. this is due to "habituation." after continuous odor exposure, the sense of smell is fatigued, but recovers if the stimulus is removed for a time. 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, a habitual odorant is blocked. this depends on the strength of the odorants in the mixture, which can change the perception and processing of an odor. this process helps classify similar odors as well as adjust sensitivity to differences in complex stimuli. genetic component the primary gene sequences for thousands of olfactory receptors are known for the genomes of more than a dozen organisms. they are seven-helix-turn transmembrane proteins. but there are no known structures for any olfactory receptor. there is a conserved sequence in roughly three quarters of all ors. this is a tripodal metal-ion binding site, and suslick has proposed that the ors are in fact metalloproteins (most likely with zinc, copper, and manganese ions) that serve as a lewis acid site for the binding of many odorant molecules. in 1978, crabtree suggested that cu(i) is "the most likely candidate for a metallo-receptor site in olfaction" of strong-smelling volatiles. these are also good metal-coordinating ligands, such as thiols. in 2012, zhuang, matsunami, and block confirmed the crabtree/suslick proposal for the specific case of a mouse or, mor244-3, showing that copper is essential for detection of certain thiols and other sulfur-containing compounds. thus, by using a chemical that binds to copper in the mouse nose, so that copper wasn't available to the receptors, the authors showed that the mice couldn't detect the thiols without the copper. however, these authors also found that mor244-3 lacks the specific metal ion binding site suggested by suslick, instead showing a different motif in the ec2 domain. evolutionary impact gordon shepherd proposed that the retro-nasal route of olfaction (odorants introduced to the olfactory mucosa through the oral cavity often as food) was partially responsible for the development of human olfactory acuity. he suggested the evolutionary pressure of diversification of food sources and increased complexity of food preparation presented humans with a broader range of odorants, ultimately leading to a "richer repertoire of smells". animals such as dogs show a greater sensitivity to odors than humans, especially in studies using short-chain compounds. higher cognitive brain mechanisms and more olfactory brain regions enable humans to discriminate odors better than other mammals despite fewer olfactory receptor genes. measuring techniques olfaktometrie in germany, concentrations of odorants have since the 1870s been defined by olfaktometrie, which helps to analyze the human sense of smell according to odor substance concentration, intensity of odor, odor quality, and hedonic assessment. the most accurate smell sensing is when a smell is first encountered, before habituation begins to change perception of odor. concentration odor concentration refers to an odor's pervasiveness. to measure odor sensation, an odor is diluted to 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 reference sample. the recognition odor threshold is usually a factor of two to five 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 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 odour unit", oue. therefore, the odor concentration at the odor threshold is 1 oue by definition. olfactometer to establish odor concentration, an olfactometer is used which employs a group of human panelists. a diluted odorous mixture and an odor-free gas—n-butanol—as a reference are presented from sniffing ports to a group of panelists who are sensitive in odor perception. to collect an odor sample, the samples are 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, where a vacuum is created outside the bag, which fills under expansion, and draws into itself the sample from the source. critically, all components which touch the odor sample, must be odor free, which includes lines and fittings. 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 the test. this continues until the panelists respond with certainty and correctly twice in a row. these responses are used to calculate the concentration of the odor in terms of european odor units (oue/m3, where 1 oue/m3≡40 ppb/v n-butanol). humans can discriminate between two odorants that differ in concentration by as little as 7%. 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 who were unable to detect the odor of androstenone developed the ability to detect it after repeated exposure. people who cannot smell are said to be anosmic. 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. other analytical methods other analytic methods can 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 taken by olfactometry using an olfactometer to dilute the odor sample. olfactometry is rarely used for immission measurement because of low odor concentrations involved. the same measuring principles are used, but the judgment of the air-assay happens without diluting the samples. odor measurement is essential for odor regulation and control. an odor emission often consists of a complex mixture of many odorous compounds. analytical monitoring of individual chemical compounds present in such an odor is usually not practical. as a result, odor sensory methods, instead of instrumental methods, are normally used to measure such odor. odor sensory methods are available to monitor odor both from source emissions and in the ambient air. these two contexts require different approaches for measuring odor. the collection of odor samples is more easily accomplished for a source emission than for odor in the ambient air. field measurement with portable field olfactometers can seem more effective, but olfactometer use is not regulated in europe, while it is popular in the u.s. and canada, where several states set limits at the receptor sites or along the perimeter of odor-emitting plants, expressed in units of dilution-to-threshold (d/t). intensity odor intensity is the perceived strength of odor sensation. this intensity property