The Science of Smell Part 1: Odor perception and physiological response Olfaction, the sense of smell, is the least Although the detection threshold concen trations understood of the five senses. This, among other of sub stanc es that evoke a smell are slight (table factors, makes the task of reducing livestock odors 1), a concentration only 10 to 50 times above the a considerable challenge. detection threshold value often is the maximum intensity that can be detected by humans. This, Odor terminology and perception however, is in contrast to other sensory systems An odorant is a substance capable of eliciting an where maximum intensities are many more olfactory response whereas odor is the sensation multiples of threshold intensi ties.The maximum resulting from stimulation of the olfactory organs. intensity of sight, for instance, is about 500,000 Odors play an important part in our everyday life, times that of the threshold intensity and a factor of from appetite stimulation to serving as warning 1 trillion is observed for hearing. For this reason, signals for disease detection. A number of diseases smell often identifies the presence or absence have charac teristic odors including gangrene, of odor rather than quantifies its intensi ty or diabetes, leukemia, and schizophre nia. Odors have concentration. been impli cated in depression and nausea as well. The ability to perceive an odor varies widely Detectable odors can have a significant impact among individuals. More than a thousand- on people by affect ing moods as well as having fold difference between the least and the most physiologi cal impacts on the olfactory system. sensi tive individuals in acuity have been observed. People associate odors with past experiences and, Differences between individuals are, in part, from those experiences, involun tarily assess the attributable to age, smoking habits, gender, nasal odor as likable, dislikable or indifferent. Effects aller gies, or head colds. Non smokers over the age on individuals, however, vary from one person to of 15 show greater acuity than smokers in general. another. Furthermore, females tend to have a keener sense of smell than males, a finding that has Odor threshold is a term used to identify the been substantiated in recent work at Iowa State concentration at which animals respond 50 University. Generally, the olfactory sensory nerves percent of the time to repeated presentations of an atrophy from the time of birth to the extent that odorant. This term is reserved, primarily, for use only 82 percent of the acuity remains at the age of in research with animals. Most often, however, 20; 38 percent at the age of 60 and 28 percent at odor threshold is used to mean detection threshold, the age of 80. Consequently, olfactory acuity and which identifies the concen tra tion at which 50 like or dislike of an odor decrease with age. percent of a human panel can identify the presence of an odor or odorant without characterizing the Infants appear to like all classes of odorous stimulus. Detection threshold is the term most materi als, perhaps because the lack previous frequently used when discussing odor research experience and because of their innate curiosity. results associated with livestock operations. The Children younger than five years old rated sweat recogni tion threshold is the concentration at which and feces as pleasant but above that age, as 50 percent of the human panel can identify the unpleasant. Like and dislike of a particu lar odor odorant or odor, such as the smell of ammonia or can change with odor concentration or intensity. peppermint. Generally, humans can distinguish between more PM 1963a May 2004 Table 1. Examples of varying threshold measurements of odorous substances (odorants). Odorant Formula Characteristic Odor Odor Detection Recognition Threshold Threshold Threshold (ppm) (ppm) (ppm) Acetaldehyde CH3•CHO Pungent, fruity .004 -- .21 Allyl mercaptan CH2•CH•CH2•SH Strong garlic, coffee .00005 .016 -- Ammonia NH3 Sharp, pungent .037 -- 46.8 Amyl mercaptan CH3•(CH2)3•CH2•SH Unpleasant, putrid .0003 -- -- Benzyl mercaptan C6H5•CH2•SH Unpleasant, strong .00019 -- -- Butylamine C2H5•CH2•CH2•NH Sour, ammnia-like -- -- .24 Cadaverine H2N•(CH2)5•NH2 Putrid, decaying flesh -- -- -- Chlorine Cl2 Pungent, suffocating .01 .01 .314 Chlorophenol ClC6H5O Medicinal, phenolic .00018 -- -- Crotyl mercaptan CH3•CH:CH•CH2•SH Skunk-like .000029 .0077 -- Dibutylamine (C4H9)2NH Fishy .016 -- -- Disopropylamine (C3H7)2NH Fishy .0035 -- .085 Dimethylamine (CH3)2NH Putrid, fishy .047 -- .047 Dimethylsulfide (CH3)2S Decayed vegetables .001 -- .001 Diphenylsulfide (C6H5)2S Unpleasant .000048 -- .0021 Ethylamine C2H5NH2 Ammoniacal .83 -- .83 Ethyl mercaptan C2H5•SH Decayed cabbage .00019 .0026 .001 Hydrogen sulfide H2S Rotten eggs .00047 -- .0047 Indole C2H6NH Fecal nauseating -- -- -- Methylamine CH3NH2 Putrid, fishy .021 -- .021 Methyl mercaptan CH3SH Decayed cabbage .0011 -- .0021 Ozone O3 Irritating above 2 ppm .001 .5 -- Proply mercaptan CH3•CH2•CH2•SH Unpleasant .001 .5 -- Putrescine NH2(CH2)4NH2 Putrid, nauseating -- -- -- Pyridine C6H5N Disagreeable, irritating .0037 -- -- Skatole C9H9N Fecal, nauseating .0012 .223 .47 Sulfur dioxide SO2 Pungent, irritating .009 -- -- Tert-butyl mercaptan (CH3)3C•SH Skunk, unpleasant .00008 -- -- Thiocresol CH3•C6H4•SH Skunk, rancid .0001 .019 -- Thiophenol C6H5SH Putrid, garlic-like .000062 .014 .28 Triethylamine (C2H5)3N Ammoniacal, fishy .08 -- -- Figure 1. Nasal cavity and detail of nerve fibers from olfactory cells. Olfactory Tract Mitral Cell Second Neuron Glomerulus Olfactory Bulb Mucus Gland Olfactory Nerves Basal Cell Cell Neuron Olfactory Epithelium Olfactory Receptor Cell Olfactory Hairs Mucus Layer Supporting Cell Goblet Cell than 5,000 odors but some individuals Figure 2. Olfactory system. experi ence anosmia (smell blind ness) for one or more odors. Rhinencephalon - Limbic Center -Taste and Smell In this situation, the individual Approximate Olfactory Bulb apparently has a normal sense of smell, Olfactory Cleft - Olfactory Epithelium but is unable to detect one partic ular odor regardless of its intensity. For Superior Concha example, because methyl mercaptan has Middle Concha an odor recognition threshold of only Inferior Concha 0.0021 ppm (Table 1), it is often mixed Nares - Vestibule (Nostril) with natural gas as an indicator of leaks; however, approximately one in one thou sand persons is unable to detect the strong odor of this mercap tan. An estimated 30 percent of the elderly have lost impulses to the olfactory bulb located at the base the ability to per ceive the minute amount of this of the front brain (Fig. 2). At the bulb, fibers from mercaptan used in natural gas. the nose contact with other nerves, which travel on to various parts of the brain. Odor physiology Olfaction depends upon the interaction between An estimated 100 million receptor cells are present the odor stimulus and the olfactory epithelium. in humans. For a substance to be detected as an The olfactory membrane is a sensitive area, odor by the receptor cells, several criteria must be covering 4 to 6 square cm in each nostril (Fig. met: 1). Beneath the mem brane is a mucous layer. 1) the substance must be volatile enough to The nerve cells or peripheral receptor cells that permeate the air near the sensory area; primarily sense odors and fragrances are located in 2) the substance must be at least slightly the epithelium. Cilia extend from the nerve cells water-soluble to pass through the mucous into the mucous layer, which greatly increases the layer and to the olfactory cells; potential receptor area. The cilia are thought to 3) the substance must be lipid-soluble contain the ultimate olfactory receptors, which are because olfactory cilia are composed specialized protein molecules. Specific anosmia primarily of lipid material; and finally, may result from the inability to synthesize the 4) a minimum number of odorous particles appropriate protein. The receptor cells transmit must be in contact with the recep tors for a minimum length of time. Many theories have been proposed to describe including municipal sewage treatment systems, the mechanism of smelling odors. Most can be coal burning, industries and factories, and classified into one of two groups: a physical theory livestock operations. or a chemical theory. The physical theory proposes that the shape of the odorant molecule determines Both ammonia and hydrogen sulfide can cause which olfactory cells will be stimulated and, olfactory losses as a result of chronic or prolonged therefore, what kind of odor will be perceived. exposure. Ammonia also can affect the central Each receptor cell has several different types ner vous system. A number of other chemical of molecular receptor sites, and selection and pollutants, including some insecticides result in proportion of the various sites differ from cell to losses in olfaction by damaging olfactory receptors. cell. The use of medications may exacerbate chemosensory disorders. The chemical theory, which is more widely accepted, as sumes that the odorant molecules On average, olfactory receptors renew themselves bind chemically to protein receptors in the every thirty days. Pollutants may alter this mem branes of the olfactory cilia. The type of turnover rate or disrupt the integrity of the lipid receptor in each olfactory cell determines the type membranes of olfactory receptors. Threshold of stimulant that will excite the cell. Binding to levels have been identified for a number of the receptor indirectly creates a receptor potential pollut ants, above which odor or irritation occur. in the olfactory cell that generates impulses in Unfortunately, however, knowledge of the exact the olfactory nerve fibers. Receptor sensitivity mechanisms by which pollutants alter olfaction is may explain some of the variation in detection limited. thresh olds exhibited by different compounds. For exam ple, ammo nia has an odor threshold of Resources 0.037 ppm whereas the corresponding values for This publication along with PM 1963b, Science of hydrogen sulfide and sulfur dioxide are 0.00047 Smell Part 2: Odor chemistry; and 0.009 ppm, respectively (Table 1).