is used to locate the source of odors and perhaps most directly related to odor nuisance. the 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, where 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 intensity can be expressed using an odor intensity scale, which is a verbal description of an odor sensation to which a numerical value is assigned. 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 odor intensity is determined in a laboratory by specialists who have been trained to accurately define intensity. hedonic tone assessment hedonic assessment is the process of rating odors according to a scale ranging from extremely unpleasant to extremely pleasant. intensity and hedonic tone, whilst similar, refer to different things: that is, the strength of the odor (intensity) and the pleasantness of an odor (hedonic tone). the perception of an odor may change from pleasant to unpleasant with increasing concentration, intensity, time, frequency, or previous experience with a specific odor—all factors in determining a response. fidol factors the overall set of qualities are sometimes identified as the "fidol (frequency, intensity, duration, offensiveness, location) factors". 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 rating the smell. 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. categorization different categorizations of primary odors have been proposed, including the following, which identifies 7 primary odors: musky – perfumes putrid – rotten eggs pungent – vinegar camphoraceous – mothballs ethereal – dry cleaning fluid floral – roses (see also floral scent) pepperminty – mint gum though the concept of primary odors is not universally accepted. interpretive dispersion modeling in many countries odor modeling is used to determine the extent of an impact from an odor source. these are a function of modeled 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: direct and indirect odor sampling techniques. direct sampling direct refers to the placement of an enclosure on or over 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 such as the one at the university of new south wales (unsw). 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, that have a vertical velocity component. for such sources, consideration must 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 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 downwind concentrations, stability class (or other similar factor), wind speed, and wind direction. health risks the human sense of smell is a primary factor in the sensation of comfort. olfaction as a sensory system brings awareness of 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 of odor and of irritation is unique to each person, and varies 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 exposure to a volatile chemical elicits a different response based on sensory and physiological signals, and interpretation of these signals is influenced by experience, expectations, personality, or situational factors. volatile organic compounds (vocs) may have higher concentrations in confined indoor environments, due to restricted infiltration of fresh air, as compared to the outdoor environment, leading to greater potential for toxic health exposures from a variety of chemical compounds. health effects of odor are traced to the sensation of an odor or the odorant itself. health effects and symptoms vary—including 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, stress-induced illness, or hypersensitivity. the ability to perform tasks may decrease, and other social/behavioral changes may occur. 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 occupational exposure limits (oels) to ensure the health and safety of 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. workforce populations vary in terms of discomfort from odors because of exposure history or habituation, and they may not realize possible risks of exposure to chemicals that produce specific odors. types some odors are sought after, such as from perfumes and flowers, some of which command high prices. whole industries have developed around products that remove or mask unpleasant odors, such as deodorant. 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 spirits, and boost self-confidence. this belief has led to "aromatherapy", wherein fragrances are claimed to cure a wide range of psychological and physical problems. aromatherapy claims that fragrances can positively affect sleep, stress, alertness, social interaction, and general feelings of well-being. evidence for the effectiveness of aromatherapy is mostly anecdotal and controlled scientific studies to substantiate its claims are lacking. some people are allergic to the fragrances found in perfume, scented shampoo, scented deodorant, or similar products. reactions, as with other chemical allergies, can range from slight headaches to anaphylactic shock, which can result in death. unpleasant odors play various roles in nature, often to warn of danger, though this may not be known to the subject who smells it. the natural gas industry uses odor to enable consumers to identify leaks. natural gas in its native state is colorless and almost odorless. to help users detect leaks, an odorizer with the scent of rotten eggs, tert-butylthiol (t-butyl mercaptan), is added. sometimes a related compound, thiophane, may be used in the mixture. an odor that is viewed as unpleasant by some people or cultures can be viewed as attractive by others where it is more familiar or has a better reputation. it is commonly thought that those exuding an unpleasant body odor are unattractive to others. but studies have shown that a person who is exposed to a particular unpleasant odor can be attracted to others who have been exposed to the same unpleasant odor. this includes odors associated with pollution. what causes a substance to smell unpleasant may be different from what one perceives. for example, perspiration is often viewed as having an unpleasant odor, but it is odorless. it is bacteria in the perspiration that causes the odor. unpleasant odors can arise from specific industrial processes, adversely affecting workers and even residents downwind of the source. the most common sources of industrial odor arise from sewage treatment plants, refineries, animal rendering factories, and industries processing chemicals (such as sulfur) which have odorous characteristics. sometimes industrial odor sources are the subject of community controversy and scientific analysis. body odor is present both in animals and humans and its intensity can be influenced by many factors (behavioral patterns, survival strategies). body odor has a strong genetic basis both in animals and humans, but it can be also strongly influenced by various diseases and psychological conditions. study 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 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 is complicated by the complex chemistry taking place at the moment of a smell sensation. for example, iron-containing metallic objects are perceived to have a distinctive odor when touched, although iron's vapor pressure is negligible. according to a 2006 study, this smell is the result of aldehydes (for example, nonanal) and ketones: 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, and are sometimes called "airborne hormones". a female moth may release a pheromone that can entice a male moth that is several kilometers downwind. honeybee queens constantly release pheromones that regulate the activity of the hive. worker bees can release such smells to call other bees into an appropriate cavity when a swarm moves into new quarters, or to "sound" an alarm when the hive is threatened. advanced technology most artificial or electronic nose instruments work by combining output from an array of non-specific chemical sensors to produce a fingerprint of whatever volatile chemicals in the local environment. most electronic noses need to be "trained" to recognize chemicals of interest before it can be used. many current electronic-nose instruments suffer from problems with reproducibility subject to 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. behavioral cues odor perception is a complex process involving the central nervous system and 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 an 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 in 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 and are