1、12.1CHAPTER 12ODORSOdor Sources 12.1Sense of Smell 12.1Factors Affecting Odor Perception. 12.2Odor Sensation Attributes 12.3Dilution of Odors by Ventilation 12.5Odor Concentration . 12.5Olf Units. 12.6ARIOUS factors make odor control an important considerationVin ventilation engineering: (1) contemp
2、orary construction meth-ods result in buildings that allow less air infiltration through the build-ing envelope; (2) indoor sources of odors associated with modernbuilding materials, furnishings, and office equipment have increased;(3) outdoor air is often polluted; and (4) energy costs encourage lo
3、werventilation rates at a time when requirements for a relatively odor-freeenvironment are greater than ever.Since Yaglou et al.s (1936) classic studies, the philosophy behindventilation of nonindustrial buildings has mainly been to provide in-door air that is acceptable to occupants. Air is evaluat
4、ed by the ol-factory sense, although the general chemical sense, which issensitive to irritants in the air, also plays a role.This chapter reviews how odoriferous substances are perceived.Chapter 46 of the 2011 ASHRAE HandbookHVAC Applicationscovers control methods. Chapter 10 of this volume has mor
5、e infor-mation on indoor environmental health.ODOR SOURCESOutdoor sources of odors include automotive and diesel exhausts,hazardous waste sites, sewage treatment plants, compost piles,refuse facilities, printing plants, refineries, chemical plants, andmany other stationary and mobile sources. These
6、sources produceboth inorganic compounds (e.g., ammonia and hydrogen sulfide)and volatile organic compounds (VOCs), including some that evap-orate from solid or liquid particulate matter. Odors emitted by out-door sources eventually enter the indoor environment.Indoor sources also emit odors. Sources
7、 include tobacco prod-ucts, bathrooms and toilets, building materials (e.g., adhesives,paints, caulks, processed wood, carpets, plastic sheeting, insulationboard), consumer products (e.g., food, toiletries, cleaning materials,polishes), hobby materials, fabrics, and foam cushions. In offices,offset
8、printing processes, copiers, and computer printers may pro-duce odors. Electrostatic processes may emit ozone. Humans emit awide range of odorants, including acetaldehyde, ammonia, ethanol,hydrogen sulfide, and mercaptans.Mildew and other decay processes often produce odors in occu-pied spaces (home
9、 and office), damp basements, and ventilationsystems (e.g., from wetted air-conditioning coils and spray dehu-midifiers).Chapter 46 of the 2011 ASHRAE HandbookHVAC Applica-tions gives further information on contaminant sources and genera-tion rates.SENSE OF SMELLOlfactory StimuliOrganic substances w
10、ith relative molecular masses greater than300 are generally odorless. Some substances with relative molecularmasses less than 300 are such potent olfactory stimuli that they can beperceived at concentrations too low to be detected with direct-readinginstruments. Trimethylamine, for example, can be r
11、ecognized as afishy odor by a human at a concentration of about 104ppm.Table 1 shows odor detection threshold concentrations forselected compounds. The threshold limit value (TLV) is the con-centration of a compound that should have no adverse health conse-quences if a worker is regularly exposed fo
12、r 8 h periods (ACGIH,revised annually). Table 1 also includes the ratio of the TLV to theodor threshold for each compound. For ratios greater than 1, mostoccupants can detect the odor and leave the area long before thecompound becomes a health risk. As the ratio increases, the safetyfactor provided
13、by the odor also increases. Table 1 is not a compre-hensive list of the chemicals found in indoor air. AIHA (1989) andEPA (1992) list odor thresholds for selected chemicals.Olfactory sensitivity often makes it possible to detect potentiallyharmful substances at concentrations below dangerous levels
14、so thatthey can be eliminated. Foul-smelling air is often assumed to beunhealthy. In reality, however, there is little correlation between odorperception and toxicity, and there is considerable individual varia-tion in the perception of pleasantness/unpleasantness of odors.When symptoms such as naus
15、ea, headache, and loss of appetite arecaused by an unpleasant odor, it may not matter whether the air istoxic but whether the odor is perceived to be unpleasant, associatedThe preparation of this chapter is assigned to TC 2.3, Gaseous Air Contam-inants and Gas Contaminant Removal Equipment.Table 1 O
16、dor Thresholds, ACGIH TLVs, and TLV/Threshold Ratios of Selected Gaseous Air PollutantsCompoundOdor Threshold,appmvTLV,bppmv RatioAcetaldehyde 0.067 25-C 360Acetone 62 500 8.1Acetonitrile 1600 20 0.013Acrolein 1.8 0.1-C 0.06Ammonia 17 25 1.5Benzene 61 0.5 0.01Benzyl chloride 0.041 1 24Carbon tetrach
17、loride 250 5 0.02Chlorine 0.08 0.5 6Chloroform 192 10 0.05Dioxane 12 20 1.7Ethylene dichloride 26 10 0.4Hydrogen sulfide 0.0094 10 1064Methanol 160 200 1.25Methylene chloride 160 50 0.3Methyl ethyl ketone 16 200 12.5Phenol 0.06 5 83Sulfur dioxide 2.7 2 0.74Tetrachloroethane 7.3 1 0.14Tetrachloroethy
18、lene 47 25 0.5Toluene 1.6 20 13Trichloroethylene 82 10 0.1Xylene (isomers) 20 100 5Sources: ACGIH (updated annually), AIHA (1989).aAll thresholds are detection thresholds (ED50).bAll TLVs are 8 h time-weighted averages, except those shown with -C, which are 15 min ceiling values.12.2 2013 ASHRAE Han
19、dbookFundamentals (SI)with an unpleasant experience, or simply felt to be out of appropri-ate context. The magnitude of the symptoms is related to the magni-tude of the odor, but even a room with a low but recognizable odorcan make occupants uneasy. Several papers review sensory irritationand its re
20、lation to indoor air pollution (Cain and Cometto-Muiz1995; Cometto-Muiz and Cain 1992; Cometto-Muiz et al. 1997;Shams Esfandabad 1993).Anatomy and PhysiologyThe olfactory receptors lie in the olfactory cleft, which is high inthe nasal cavity. About five million olfactory neurons (a small clusterof n
21、erve cells inside the nasal cavity above the bridge of the nose)each send an axon (an extension of the neuron) into the olfactory bulbof the brain. Information received from the receptors is passed tovarious central structures of the brain (e.g., olfactory cortex, hippo-campus, amygdala). One sniff
22、of an odorant can often evoke a com-plex, emotion-laden memory, such as a scene from childhood.The surrounding nasal tissue contains other diffusely distributednerve endings of the trigeminal nerve that also respond to airbornevapors. These receptors mediate the chemosensory responses suchas ticklin
23、g, burning, cooling, and, occasionally, painful sensationsthat accompany olfactory sensations. Most odorous substances atsufficient concentration also stimulate these nerve endings.Olfactory AcuityThe olfactory acuity of the population is normally distributed.Most people have an average ability to s
24、mell substances or torespond to odoriferous stimuli, a few people are very sensitive orhypersensitive, and a few others are insensitive, including some whoare totally unable to smell (anosmic). The olfactory acuity of anindividual varies with the odorant.Hormonal factors, which often influence emoti
25、onal states, canmodulate olfactory sensitivity. Although the evidence is not uni-formly compelling, research has found that (1) the sensitivity offemales varies during the menstrual cycle, reaching a peak justbefore and during ovulation (Schneider 1974); (2) females are gen-erally more sensitive tha
26、n males, but this difference only emergesaround the time of sexual maturity (Koelega and Koster 1974);(3) sensitivity is altered by some diseases (Schneider 1974); and(4) various hormones and drugs (e.g., estrogen, alcohol) alter sensi-tivity (Engen et al. 1975; Schneider 1974).Other factors that ma
27、y affect olfactory perception include theindividuals olfactory acuity, the magnitude of flow rate towardolfactory receptors, temperature, and relative humidity. Olfactoryacuity can also vary with age (Stevens et al. 1989; Wysocki and Gil-bert 1989), genetics (Wysocki and Beauchamp 1984), exposure hi
28、s-tory (Dalton and Wysocki 1996; Wysocki et al. 1997), and diseaseor injury (Cowart et al. 1993, 1997). Humans are able to perceive alarge number of odors, yet untrained individuals are able to nameonly a few (Ruth 1986).Individuals who are totally unable to detect odors are relativelyrare (Cowart e
29、t al. 1997). A more common occurrence is an inabilityto detect one or a very limited number of odors, a condition knownas specific anosmia. Although the huge number of possible chem-icals makes for an untestable hypothesis, it has been posited thatmost, if not all, individuals have a specific anosmi
30、a to one or morecompounds (Wysocki and Beauchamp 1984). The fact that individ-uals with specific anosmias have normal olfactory acuity for allother odors suggests that such anosmias may be caused by geneticdifferences.In olfactory science, adaptation refers to decreased sensitivity orresponsiveness
31、to an odor after prolonged exposure. This exposurecan selectively impair the perception of the exposure odorant, butthere are also examples of cross-adaptation, where exposure to oneodorant can result in adaptation to other odors as well. Adaptationcan occur in the short term, where perception of a
32、rooms odor beginsto fade within seconds of entering the room (Cometto-Muiz andCain 1995; Pierce et al. 1996). With long-term adaptation, an indi-vidual who habitually returns to the same environment does notsmell odors that are quite discernible to a naive observer. This effectappears to shift both
33、the threshold and the suprathreshold (stimuliabove the threshold level) response to the odor (Dalton and Wysocki1996). This is an important phenomenon for indoor air quality (IAQ)personnel to be aware of because it is often one of the biggest reasonsfor variations in detectability or response in rea
34、l-world environmentsand makes the choice of test population or panelists for air qualityevaluations a critical one.FACTORS AFFECTING ODOR PERCEPTIONHumidity and TemperatureTemperature and humidity can both affect the perception ofodors. Cain et al. (1983) reported that a combination of high tem-pera
35、ture (25.5C) and high humidity exacerbates odor problems.Berglund and Cain (1989) found that air was generally perceived tobe fresher and less stuffy with decreasing temperature and humidity.Fang et al. (1998a, 1998b) and Toftum et al. (1998) found little or noincrease in odor intensity with increas
36、ing enthalpy (temperature andhumidity), but reported a very significant decrease in odor accept-ability with increasing enthalpy.Not all researchers have supported these findings. Kerka andHumphreys (1956) reported a decrease in odor intensity with increas-ing humidity. Berg-Munch and Fanger (1982)
37、found no increase inodor intensity with increasing temperature (23 to 32C). Clausen etal. (1985) found no significant change in odor intensity with increas-ing relative humidity (30% to 80%).Although the findings are not homogeneous, they do show thattemperature and humidity can act together to affe
38、ct ones perceptionof odors. Air that is cooler and drier is generally perceived to befresher and more acceptable even if odor intensity is not affected.Sorption and Release of OdorsBecause furnishings and interior surfaces absorb (and laterdesorb) odors during occupancy, spaces frequently retain nor
39、maloccupancy odor levels long after occupancy has ceased. This isobserved when furnaces or radiators, after a long shutdown, areheated at winter start-up and when evaporator coils warm up. Therate of desorption can be decreased by decreasing temperature andrelative humidity, and increased (as for cl
40、eaning) by the reverse.Environmental tobacco smoke may desorb from surfaces longafter smoking has taken place. This phenomenon has caused manyhotels to establish nonsmoking rooms.Where the odor source is intrinsic to the materials (as in lino-leum, paint, rubber, and upholstery), reducing the relati
41、ve humiditydecreases the rate of odor release. Quantitative values should not beused without considering the sorption/desorption phenomenon.Emotional Responses to OdorsThere can be considerable variation between individuals regard-ing the perceived pleasantness or unpleasantness of a given odor.Resp
42、onses to odors may be determined by prior experiences andcan include strong emotional reactions. This is because one of thebrain structures involved in the sense of smell is the amygdala, aregulator of emotional behaviors (Frey 1995). Some IAQ com-plaints can involve emotional responses completely o
43、ut of propor-tion to the concentration of the odorant or the intensity of the odorit produces.Two theories describe physiological reasons for these strongresponses. One of these is kindling, in which repeated, intermittentstimuli amplify nerve responses. The other is response facilitation,in which a
44、n initial stimulus perceived as strong is facilitated(becomes greater) rather than adapted to (Frey 1995).Odors 12.3Because of this emotional aspect, IAQ complaints involvingodors can be very difficult to solve, especially if they are comingfrom a few sensitized individuals. It is important to respo
45、nd quicklyto complaints to minimize the risk of kindling or response facilita-tion.ODOR SENSATION ATTRIBUTESOdor sensation has four components or attributes: detectability,intensity, character, and hedonic tone.Detectability (or threshold) is the minimum concentration of anodorant that provokes dete
46、ction by some predetermined segment ofthe population. Two types of thresholds exist: detection and recog-nition.The detection threshold is the lowest level that elicits responseby a segment of the population. If that segment is 50%, the detectionthreshold is denoted by ED50. Recognition threshold is
47、 the lowestlevel at which a segment of the population can recognize a givenodor. Thresholds can be attributed to 100%, which includes allolfactory sensitivities, or to 10%, which includes only the most sen-sitive segment of the population. Threshold values are not physicalconstants, but statistical
48、measurements of best estimates.Intensity is a quantitative aspect of a descriptive analysis, statingthe degree or magnitude of the perception elicited. Intensity of theperceived odor is, therefore, the strength of the odoriferous sensa-tion. Detection threshold values and, most often, odor intensity
49、determine the need for indoor odor controls.Character defines the odor as similar to some familiar smell(e.g., fishy, sour, flowery). Hedonics, or the hedonic tone of an odor,is the degree to which an odor is perceived as pleasant or unpleasant.Hedonic judgments include both a category judgment (pleasant,neutral, unpleasant) and a magnitude judgment (very unpleasant,slightly pleasant).Important questions areWhat is the minimum concentration of odorant that can bedetected?How does perceived odor magnitude grow with concentrationabove the threshold?No universal method has been a
