Olfactory threshold

Odors are characterized by quaUty and intensity. Descriptive quaUties such as sour, sweet, pungent, fishy, and spicy are commonly used. Intensity is deterrnined by how much the concentration of the odoriferous substance exceeds its detection threshold (the concentration at which most people can detect an odor). Odor intensity is approximately proportional to the logarithm of the concentration. However, several factors affect the abiUty of an individual to detect an odor the sensitivity of a subject s olfactory system, the presence of other masking odors, and olfactory fatigue (ie, reduced olfactory sensitivity during continued exposure to the odorous substance). In addition, the average person s sensitivity to odor decreases with age. 

M. Devos, F. Patte, J. Rouault, P. Laffort, and L. J. Van Gemert, eds., Standardi edHuman Olfactory Thresholds, IRE Press, New York, 1990. 

The odor threshold of carbon disulfide is about 1 ppm in air but varies widely depending on individual sensitivity and purity of the carbon disulfide. However, using the sense of smell to detect excessive concentrations of carbon disulfide is unreHable because of the frequent co-presence of hydrogen sulfide that dulls the olfactory sense. 

AHyl chloride has a disagreeable, pungent odor. The odor threshold has been estimated at approximately 3—6 ppm (51). Olfactory detection of odor is thus not an adequate warning of overexposure. 

Residual monomers exhibit a characteristic sharp odor even in subtoxic concentration, due to the very low olfactory threshold. Modem requirements in terms of environmental safeguard have led to significant improvements in the control of polymerization effluents, driving off gases, and residual monomer in the raw polymer. Consequendy, the acryUc elastomers of the 1990s are practically odor-free, and represent a significant improvement over the products of the past. 

Some materials with low odour thresholds may paralyse the olfactory nerves and cause the sense of smelt to be lost within minutes (e.g. hydrogen sulphide). 

With regard to the odor threshold for hydrogen sulfide, it should be noted that although odor can be perceived at 0.5 ppb in air, olfactory fatigue can occur at concentrations of 100 ppm or greater causing a loss of odor perception (Leonardos et al. 1969). 

A good example is l-p-mcnthene-8-thiol the (R) enantiomer is an important flavor component of grapefruit juice, having a very low olfactory threshold of... 

This pathway provides several amplification steps between odorant binding and signal generation. Due to the electrically compact structure of the cell, it is possible for the activation of only a few tens of channels to drive the membrane to the threshold for action-potential generation. Thus, it is theoretically possible that the limit of olfactory detection is a single molecule, although this has not yet been conclusively demonstrated. 

Hydrogen cyanide has frequently been associated with the odor of bitter almonds (Ballantyne 1983 Gee 1987). The threshold odor for olfactory detection of atmospheric HCN is 1 mg/L, but the odor may not be detected for various reasons, including the presence of other odors and the fact that only 20 to 40% of those tested could detect a cyanide odor. 

Olfactory sensitivity for one individual varies about factor three due to climatological, physiological, environmental reaons etc. The sensory sensitivity also varies from odorant to odorant. So it is difficult to select a panel with a sensitivity distribution similar to that of the population. The preferred method in the United Kingdom for screening panelists uses the actual odor to be tested as a key component. In France selection is carried out on the basis of the threshold for five standard odorants. In Germany a normal sense of smell is requested of persons between the age of 18 and 50 years, in the Netherlands no exact specifications are given. Anyway, an extreme clustering around the mean or towards the extremes has to be avoided. 

PUNTER, P.H. (1983) Measurement of human olfactory thresholds for several groups of structurally related compounds. Chemical Senses, vol 7, nr 3/4, 215-235. 

P.H.Punter, Measurement of Human Olfactory Threshold for several Groups of Structural related Compounds, Chemical Senses, 7(3/4), 1983, 215-235. 

When comparing the dust-borne concentrations of butyric acid and p-cresol with the odour thresholds it seems that the concentrations are too small to be relevant for an odour nuisance. However, if the dust is removed from the gas phase of the air from animal houses the odour disappears (39), (40), (14). This supports the opinion of HAMMOND et al. (40) that the odor is concentrated on the dust particles. The authors conclude from their data that the concentration of the two odorants butyric acid and p-cresol is about 4TO7 greater on an aerosol particle than it is in an equal volume of air. Thus, an aerosol particle deposited on the olfactory organ carries odour equivalent to a much greater volume of air (40). These considerations indicate that dust from animal houses should be taken into account in connection with odour emission/immission measurements not only by chemical analysis but by sensory evaluations using olfactometers without dustfilters, as well. 

Jones, F.N. Olfactory absolute thresholds and their implications for the nature of the receptor process, J. Psychol, 40 223-227, 1955. 

D. Krestel, D. Passe, J.C. Smith and L. Jonsson, Behavioural Determination of Olfactory Thresholds to Amyl Acetate in Dogs , Neurosci. Biobehav. Rev. 8 (1984) 169—174. 

Patients who were irradiated for 5 weeks with photons of 8 MV of energy at 3 Gy/min, for nasopharyngeal carcinoma or pituitary adenoma, became less sensitive to odors. Their thresholds for amylacetate and eugenol were determined before and several times after radiotherapy. One week after irradiation, the thresholds had increased from 10 to 2 dilution steps. Sensitivity recovered over the subsequent weeks, reaching 6-8 dilution steps 6 months after treatment. The dose received by the olfactory area was estimated as 2Gy/day (Ophir etal, 1988). 

It is important to identify and measure the concentrations of a number of compounds in a mixture simultaneously for several reasons. First, among related compounds there may exist precursors of active ones, and pathways of pheromone synthesis may be elucidated. This is true for steroids in the human axilla. Nixon etal. (1988) determined the concentration of five steroids extracted from axillary hair of adult men aged 18 to 40 years. The relationships in concentrations between the two ketones 5Q -androst-16-en-3-one and 4,16-androstadien-3-one suggest that axillary bacteria reduce the former to the latter with the aid of the enzyme 4-ene-5a-reductase. Humans have a low olfactory threshold for several 16-androstenes, and the fact that some men have large quantities of 16-androstenes (Nixon etal., 1988) is biologically suggestive. 

The main olfactory system is designed with considerable redundancy. Rats that have been bulbectomized on one side still have the same absolute threshold and intensity difference threshold as intact rats (Slotnick and Schoonover, 1984). 

The long-held dichotomy of macrosmatic and microsmatic vertebrates is no longer tenable. Neuroanatomists had assumed that taxa with relatively few olfactory receptor cells and small olfactory bulbs would also be inferior in olfactoiy performance (threshold and number of compounds detected) to those with more receptors and larger bulbs. However, we now know from single-cell recordings that a particular receptor cell type can respond to a wide range of odor compounds that share certain features. Keverne (1983) pointed out that the olfactory bulbs act as a filter, while more complex pattern analysis takes place in the neocortex. The more evolved the neocortex, as in primates, the... 

Eels, Anguilla anguilla, are extremely sensitive to odors. Their detection threshold for /3-phenylethyl alcohol lies at the unbelievable concentration of 3 X 10 mol/l. This corresponds to 1ml of this alcohol diluted in 58 times the volume of Lake Constance (80km long) in southern Germany where these experiments were performed. It has been calculated that only three molecules are in the olfactory sac at any one time (Teichmann, 1957, 1959). Coho salmon... 

Thresholds of the olfactory receptors of male goldfish are 35 pg/ml water for the prostaglandin F2a, and 100 times less for its 15-keto-derivative. The males receptor threshold for 17,20-progesterone (from females) is a tiny fraction of 1 pg/ml water. Three grams (one teaspoonful) would provide an abovethreshold stimulus when diluted in 500 x 500 x 500 m water (Bjerselius and Olsen, 1993). In lampreys, testosterone from males attracts females at a concentration of 29 pg/ml water but urine with a testosterone concentration of 29 X 10 pg/ml is active (Adams etal., 1987). 

The most celebrated mammalian olfactory detector is the dog s nose. As a predator, the dog locates its prey by air scenting (following a gradient of airborne odors) and tracking. Practitioners are familiar with the extreme olfactory sensitivity of the dog. The detection threshold for butyric acid has been determined as 9 x 10 molecules/cm air (Neuhaus, 1953). Considering that... 

Thresholds can vary between and within individuals, with the estrus cycle, and with the chemical background. Some benchmarks will be given here. In laboratory (Wistar) rats, the olfactory detection threshold for ethyl acetate was measured as 7.3 x 10 vol% in subadults, while adult rats were more sensitive (1.4 X 10 vol%) (Apfelbach etah, 1990). (By comparison, among insects the gypsy moth is 10 times as sensitive. The threshold for bombykol, defined as... 

Odor thresholds in humans appear to be more influenced by the environment than genetics. The detection thresholds for acetic acid, isobutyric acid, and cyclohexanone varied as much in monozygotic twins as in fraternal twins. Instead, smoking and diabetes were related to lower olfactory sensitivity, and body fatness and alcohol consumption to greater sensitivity to the latter two compounds (Hubert eta/., 1980). 

Olfactoiy thresholds also vary with the ovarian cycle. Women are most sensitive to odors around the time of ovulation, when estrogen levels are highest, and less sensitive to odors during menstruation. This may have sensory-physiological reasons. During menstruation the mucus layer on the olfactory epithelium is thicker and more likely to trap molecules, while the thin mucus layer at the time of ovulation renders the receptors more accessible. The thickness of the mucus layer, in turn, is controlled by testosterone and estrogen (Mair etal, 1978). 

The apocrine glands in the axilla can secrete enormous amounts of steroids such as dihydrotestosterone and pregnenolone (Brooksbank, 1970). Three single steroid compounds have also been tested on T-shirts. Surprisingly, both sexes attributed androstenol to females. Two other synthetic compounds were attributed to one or the other sex, depending on concentration, and one was perceived as very negative. Androstenol and the two synthetic compounds have very low olfactory thresholds for humans. 

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