The Science of Smell Part 3:Odor detection and measurementAs perceived by humans, odors have ﬁve basic recognition threshold is the concentration at which 50properties that can be quantiﬁed: 1) intensity, 2) degree percent of the panel can identify the odorant or odor.of offensiveness, 3) character, 4) frequency, and 5)duration, all of which contribute to the neighbor’s Although the detection threshold concentrations ofattitude towards the odor as well as the business substances that evoke a smell are low, often times in thegenerating the odor. It is generally accepted that the parts per billion (ppb) or parts per trillion (ppt) range,extent of objection and reaction to odor by neighbors a concentration only 10 to 50 times above the detectionis highly variable. The reaction can be based on threshold value often is the maximum intensity thatprevious experience, relationship to the odor-producing can be detected by humans. This is in contrast to otherenterprise and the sensitivity of the individual. Weather sensory systems where maximum intensities are many(temperature, humidity, wind direction) affects the more multiples of threshold intensities. For example,volatility of compounds, preventing or enhancing the maximum intensity of sight is about 500,000 timesmovement into the gaseous phase where an odor can be that of the threshold intensity and a factor of 1 trilliondispersed downwind. is observed for hearing. For this reason, smell is often concerned with identifying the presence or absenceMost of us will accept even a strong odor for a short of odor rather than with quantifying intensity orperiod of time, provided we don’t have to smell it often. concentration.But we have a threshold for the frequency and durationof the odor, above which our tolerance is exceeded Perception of a mixture of odorants, such as thoseand we view the odor as a nuisance. These thresholds, in livestock odor, is very different from how eachhowever, are person-speciﬁc. While it is the frequency chemical would be perceived independently. Odorantsand duration of an odor that often triggers a nuisance can act as additive agents, counteractants, maskingcomplaint, odor measurement procedures typically focus agents, or be synergistic in nature. The combinationon the ﬁrst three traits (intensity, offensiveness, and of two odorants can have an odor equal to that ofcharacter). From a human health standpoint, exposure either one of the components, have an odor less thantime is an essential measure in predicting any negative that of one of the components, have an odor equaleffects that may occur and this encompasses frequency to the sum of the components, or even have an odorand duration as well as concentration (intensity). greater than the sum of the components. This makesAs a result, regulatory procedures often include odor quantiﬁcation and characterization a challengingconcentration, frequency, and duration as part of the process.compliance protocol. Odor can be evaluated subjectively in terms of intensityDeﬁning odor (strength) or in terms of quality (i.e., offensiveness).An odorant is a substance capable of eliciting an olfactory Odor quality is evaluated by describing the odorresponse whereas odor is the sensation resulting from or comparing the sample odor to familiar odors.stimulation of the olfactory organs. Odor threshold is a Evaluation of odor quality is difﬁcult because of theterm used to identify the concentration at which animals challenges that come with trying to describe odors.respond 50 percent of the time to repeated presentationsof an odorant being tested. Most often, however, odor Odor measurement techniques“threshold” is used to describe the detection threshold, Dilution-to-threshold methodswhich identiﬁes the concentration at which 50 percent Dilution-to-threshold techniques dilute an odorof a human panel can identify the presence of an odor sample with odorless air at a number of levels and theor odorant without characterizing the stimulus. The dilution series is presented in ascending order of odor concentration. From one level to the next, the dilution PM 1963c October 2004
decreases and the amount of odorous air increases.The ﬁrst few levels include the sample diluted with alarge amount of odorless air so evaluation can beginbelow the threshold of detection. Preferably, multiplepresentations (two odorless air samples and the dilutedodor sample) are made at each level of dilution.When a forced-choice method is used, a panelist,typically trained to conduct these evaluations, mustidentify the presentation that is different from theothers at each level, even if it is a guess. This permitsuse of all the data. The threshold of detection is thedilution level at which the panelist can determinea difference between the diluted and the odorlesssamples. After the detection threshold is reached, the Photo 1. Using a Nasal Ranger® to detectpanelist continues the evaluation at the next level or odors.two to be certain the identiﬁcation was not made bychance. Examples of the dilution-to-threshold methods International Olfactometer® (St. Croix Sensory, St.include use of scentometery and olfactometery. Elmo, Minn.), for example, allows samples to be presented at 14 dilutions that represent a range inScentometry dilution-to-threshold of 8 to 66,667. These units areOne method of odor concentration evaluation that is often used in a laboratory setting by 7 to 10 panelists toavailable on-site employs the use of a Scentometer® evaluate each sample rather than the small number of(Barneby and Cheney, Columbus, OH) or a Nasal evaluators that are used in the ﬁeld measurements (SeeRanger® (St. Croix Sensory, St. Elmo, MN). The Photo 2). Efforts to establish the relationship betweenScentometer® is a plastic box with a number of air olfactometer readings and that from the portable unitsinlets and two snifﬁng ports. Two of the air inlets are currently underway at Iowa State University.have activated charcoal ﬁlters to remove odors andprovide clean air. The remaining inlets are of varying Ranking methodsdiameter to permit a range of dilutions of odorous air Odor can be evaluated using panelists to rank samples,to be sampled. An observer begins by opening the port a procedure in which an arbitrary scale is used toof smallest diameter to start with the largest dilution describe either the intensity or offensiveness of an(lowest concentration) of the odor. odor. Typically, a scale of 0 to 10 is used, with 0 indicating no odor or not offensive and 10 representingAs successively larger ports are opened, the dilution of a very intense or offensive odor. Such methods usethe odorous air decreases and the odor concentration either odor adsorbed onto cotton or a liquid sampleincreases. When the evaluator can ﬁrst detect the that has been diluted. Manure can be diluted withodor, the odor threshold has been reached. Odor water to a range of concentrations and then evaluatedconcentrations are expressed as dilutions to threshold. by a panel.The range of dilutions to threshold possible for theScentometer includes 1.5, 2, 7, 15, 31, 170, and 350. One study, for example, diluted stored dairy manureThe Nasal Ranger® operates on the same principles with water to create ﬁve dilution levels. For eachand has selectable dilution ratios of 2, 4, 7, 15, 30, and level, two blank samples of water and one diluted60. Inhalation or airﬂow rate is controlled on the Nasal manure sample were presented in ﬂasks that had beenRanger®. For both instruments, an individual observer painted black to avoid bias based on appearance ofor a couple of people rather than a larger panel of the diluted manure. Panelists evaluated the samples inevaluators frequently conducts measurements. an ascending series; the dilution decreased and odor increased from one level to the next. At each dilutionOlfactometry level, panelists identiﬁed the ﬂask in each set of threeOlfactometers operate much like the Scentometer® that contained the odorous sample (forced-choice).and the Nasal Ranger®. The primary differences are A separate study analyzed panelist variability whenthat olfactometers are not portable and an operator this procedure was used and observed that each panelclosely controls sample delivery. Larger dilution- member had a distinct and repeatable odor probabilityto-threshold ranges are available. The AS’CENT distribution.
variation, the difference in sense of smell from one person is another consideration in human assessment methods. The measurement of odor concentration by dilution is more direct and objective than that of odor quality or intensity. However, each of the above procedures requires the use of the human nose as a detector, so not one is completely objective. The imprecision that results from the large difference between the dilution levels has been identiﬁed by researchers as a concern as well. Use of a forced-choice method, such as that used with dynamic olfactometers, in which a panelist must simply identify the presence or absence of an odor isPhoto 2. The AS’CENT International generally a better method than ranking, as the humanOlfactometer®. nose cannot distinguish small differences between levels of intensity.Referencing methodsThis method uses different amounts of 1-butanol as a Emerging methodsstandard to which sample odor intensity is compared, Efforts are underway across the United States toagain using a human panel. The range of 1-butanol develop evaluation methods that can be used onsiteconcentrations is often from 0 to 80 ppm. As the and without the inﬂuence of human subjectivity withconcentration of butanol is changed, the sample odor the goal of providing an objective and affordable meansis compared to the butanol to determine at what of quantifying odors.concentration of butanol the sample’s intensity isequivalent. The use of butanol as a reference standard Surrogate compoundsis widely accepted as common practice in Europe and Odors from livestock facilities contain hundredshas been incorporated into portable and laboratory of different compounds, all interacting with eachscale instrumentation. Most of the methods currently other and their environment in additive and non-used in the United States employ butanol as a means additive (counteractant, masking) manners. From theof assessing panelist suitability rather than as the standpoint of odor control, it is desirable to knowsole means of determining an odor’s strength or which compounds are most important in deﬁning anacceptability. odor, so that those few compounds can be targeted with control strategies.Challenges with current methodsChallenges with current methodology include the use Compounds that have been well-correlated to odorof humans for assessment. Work has shown that the measures in studies led by Iowa State Universitysame panelist’s response from one day to the next can and elsewhere, and might be useful as surrogates invary by as much as three-fold, possibly due to health determining odor, include volatile fatty acids (acetate,or mood of the individual. Variability in the sensitivity butyrate, propionate) and phenolics (phenol, cresol,of the individual conducting the evaluation and indole, skatole).odor fatigue are further concerns that are commonlyaddressed in procedural protocol. In order to identify and quantify the constituents of odor, gas chromatography-mass spectrometryOdor fatigue is a temporary condition where a person (GC/MS) is most frequently employed. Samples arebecomes acclimated to an odorant or odor to the point commonly trapped (adsorbed) onto some type ofthat they are no longer aware that the odor is present. sorbent material that concentrates compounds ofAn example would be when you walk into a barbeque interest then quantiﬁed by GC/MS. Concentrationsrestaurant and by the time you leave, you are unaware of identiﬁed compounds and the interactions of theof the aroma that attracted you in the door. Onsite identiﬁed compounds are mathematically correlated tomethods are complicated by the inﬂuence that visual odor measurements made using traditional methods,perception might have in an evaluation (smelling most commonly the dilution-to-threshold methods.with your eyes, so to speak). Each of us has a unique Interpretation of the results is complicated becauseodor acuity. While methods try to minimize panelist odors that are equal in concentration may not be equal
in offensiveness or intensity. Furthermore, two odors certainly great. Recent work demonstrated that anof equal concentration may be perceived as having electronic nose can distinguish between pig anddifferent intensities. chicken slurry and between emissions from swine and dairy facilities because the sensor response patternsWhile gas chromatography coupled to mass between the comparisons were different. At the currentspectrometry (GC/MS) is frequently used to identify point of development, the electronic nose appears toand quantify odorous compounds and the use of be less sensitive than olfactometry measures, thoughsurrogate compounds is an objective method, this sensor improvements occur routinely. Sensor selectionapproach does not represent the experience of odor is critical from both the standpoint of sensitivity tosensation as perceived by humans. Efforts to combine compounds that contribute to the offensive odorsboth instrumental and human methods are under (malodor) as well as response and durability of thedevelopment. sensors in humid environments.Electronic nose ConclusionsElectronic nose analysis with a sensor array is a Odor measurement is a complicated task. While apotential technology for odor evaluation. To date, number of methods are available, none are withoutrelatively little research has been conducted with drawbacks. However, dilution-to-threshold methodselectronic noses in the area of agricultural manure are the most widely accepted methods at the currentodors. The electronic nose has been developed in an time.attempt to mimic the human sense of smell and isfrequently used in the food, beverage, and perfume Resourcesindustries for product development and quality Additional information regarding measurement of odorcontrol. can be found in PM 1990 Instruments for Measuring Concentrations and Emission Rates of Aerial PollutantsThe sensor array of an electronic nose detects the from AFOs available on the Air Quality and Animalchemicals that humans perceive as odors and records Agriculture Web page at:numerical results. The instrument will generate a http://www.extension.iastate.edu/airqualitydifferent pattern of response for different types ofsamples. Commercially available electronic noses This publication along with PM 1963a, Science of Smellhave 32, 64, or 128 sensors. Each sensor has an Part 1: Odor perception and physiological response; PMindividual characteristic response, and some of the 1963b, Science of Smell Part 2 Odor Chemistry; andsensors overlap and are sensitive to similar chemicals, PM 1963d, Science of Smell Part 4 Principles of Odoras are the receptors in the human nose. A single Control can be found on the Air Quality and Animalsensor is partially responsive to a broad range of Agriculture Web page at:chemicals and more responsive to a narrow range of http://www.extension.iastate.edu/airqualitycompounds. Multiple sensors in a single instrumentprovide for responsive to a great number and many Prepared by Wendy Powers, extension environmentaltypes of chemicals, with certain sensors that mix specialist, Department of Animal Science. Edited by Jeanbeing moderately to extremely sensitive to speciﬁc McGuire, extension communications specialist, and Mattcompounds. Carlson, extension communications intern, Communication Services, Iowa State University.The technology is relatively new to the agriculturalindustry, although the potential for application is File: Environmental Quality 4-1. . . and justice for allThe U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin,gender, religion, age, disability, political beliefs, sexual orientation, and marital or family status. (Not all prohibited bases apply to all programs.) Manymaterials can be made available in alternative formats for ADA clients. To ﬁle a complaint of discrimination, write USDA, Ofﬁce of Civil Rights, Room326-W, Whitten Building, 14th and Independence Avenue, SW, Washington, DC 20250-9410 or call 202-720-5964.Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Stan-ley R. Johnson, director, Cooperative Extension Service, Iowa State University of Science and Technology, Ames, Iowa.