Measuring Absolute Olfactory Sensitivity (Threshold Values)

Establishing absolute threshold concentrations for odorants is important in understanding how people perceive indoor air, as prevailing levels o f airborne chemicals need to be at this level or greater before they can be regarded as a potential for discomfort. Also, odorants, such as butanol and acetone, have been used as yardsticks for comparing the quality or intensity o f indoor air both in chamber studies and in field studies. There is a need therefore, to present the judges making these comparisons, with a concentration o f odorant which is in the perceptible range i.e. above the threshold value. Research into threshold values has also established that human are extremely variable in their responses.

Measuring the sensitivity o f the human olfactory system requires a reliable procedures for presenting the stimuli to subjects. To this end a range o f olfactometers have been developed with one of the most famous being attributed to Zwaardemaker (1857-1930) (Cain 1978a). Figure 2.4 shows four different methods used in olfactory studies for presenting a subject with varying concentrations o f odorants, the Zwaardemaker’s olfactometer is shown in the top-

C h a p t e r 2 - H u m a n P ereeptiun of Indoor A ir Q u a lit y and O d o u r

To nose

Outer lube moved to expose more of its inner surface which has been impregnated with o d o ro u ^ iaterial I n n e r t u b e Handle A i r t a k e n in v i a th e e x p o s e d in te r n a l s u r f a c e o f t h e o u t e r tub e LSI L2J Kr) Odorant

Figure 2.4: Different methods used to present subjects with odorants. Source. Doty 1991

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left diagram. The different methods clearly expose the subjects to different concentrations depending on how they are administered. This is particularly important when studying odours at their threshold levels where concentrations are usually very low indeed. As a result o f studies into olfactory sensitivity important insights into how people perceive odours have been established. For example, it is now realised that at very low concentrations, subjects rarely perceive a qualitative characteristic to an odorant (e.g. ‘rose-like’ etc.) instead they observe only a faint presence o f something (Doty 1991). The lowest concentration where this faint presence is detected is referred to as the odour detection threshold and where the odour quality is recognised the term odour recognition threshold is used. Lindvall (1974) reports that an individual’s detection threshold is not fixed but rather it exists as a gradual transition from total absence to a definitely confirmed odour impression. The range o f odour detection thresholds for several substances are shown in Table 2.1 (Cain 1988). The odour recognition threshold is normally several orders o f magnitude larger than the detection threshold (AIHA 1993).

Compound Odour detection threshold eoncentration in air

(mg/nP) (ppm. v/v) Ethane 1.5 X 10^ 120000 M ethanol 6 . 6 X 1 0^ 500 Chloroform 3.2 X 10^ 65 Benzene 1.7 X 10' 5.2 Cam phor 1.1 X 10° 0.17 Furfural 2.3 X 10 ' 0.059 Isoamyl Acetate 3.8 X 10 - 0.0071

5 a-A n d ro st-16-en-3 -one 2 .1 X 1 0^ 0.00019

2-M ethoxyl-3-isobutylpryrazine 3.6 X 10 ° 0.00000054

Table 2.1: Odour detection threshold values for several substances (Source: Cain 1988)

Techniques for measuring human thresholds for various physical stimuli (e.g. sight, hearing, taste as well as olfaction) were established in the mid nineteenth century by Fechner (Sekuler and Blake 1994). He identified three basic

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techniques; (1) the method o f constant stimuli; (2) the method o f limits; (3) the

method o f adjustment.

The method o f constant stimuli involves exposing a subject to a fixed set o f odorants o f various intensity in random order. The subject states whether the odour could be sensed or not. The technique requires that each intensity o f odorant is presented several times to establish a reliable threshold value. Data can be plotted on a graph o f percentage o f detections against intensity. An ideal response would be as shown in figure 2.5(A). However, data from these test are more likely to produce the ‘ogive’ curve also shown in Figure 2.5(B). The threshold value is then taken as the detection level at 50% o f the trials.

The method o f limits also uses a range of odorants with varying intensity but instead o f these being presented in a random order they are, generally, presented in increasing intensity until the observer states that the odour can be detected. This method is quicker to administer than the method o f constant stimuli as it involves fewer presentations.

The method o f adjustment is similar to the method of limits except that the subject is allowed to vary the intensity o f the stimulus until it can be perceived instead o f being presented with a range o f intensities.

The above methods have developed from the study o f the relationship between various stimuli perceptions (e.g. sound, light, odour etc.) and measures o f the physical quantity (e.g. loudness, brightness, vapour concentrations etc.) known as psychophysics. However, two important modifications to the above methods can

be identified within the study o f olfactory thresholds; (1) the staircase method; (2)

the forced choice objective method.

The staircase method is a further modification o f the method o f limits. A subject is usually presented with an odorant well below its threshold intensity. The intensity o f the odorant is gradually increased until the subject can detect an odour sensation. The intensity is then decreased until the subject can no longer detect it.

C h a p t e r 2 - H u m a n P erception o f In door A ir Q u a l it y and O d o u r IDEAL B V3 E C/Î bO Strong Weal ACTUAL 1 r 1 I r GO pH Strong Detection Level Weak / # Odour detected

# Odour not detected —

I I I I j L I I I

1 2 3 4 5 6 7 9 10 11 12 13 14 15 16 Trial

Figure 2.5 - Top (A) \ Idealised relationship between the percentage of'yes' responses as

an odorant's intensity increases indicating a precise threshold value.

Middle (B)\ A generalised graph shown how actual responses occur when an odorant's intensity is increased.

Bottom (C): A possible result using the staircase method to determine an odorant's threshold value

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Once again the intensity is increased. A generalised graph o f intensity for each trail is shown in Figure 2.5(C). The threshold is found from the average at which the subject’s responses changed.

The forced choice objective method evolved from the need for a more objective approach to whether or not a subject had actually sensed the stimuli rather than guess that it was present. In the study o f olfactory thresholds forced choice procedures have been incorporated into the method o f constant stimuli, the method o f limits and the staircase method (Doty 1991).

The forced choice method of constant stimuli presents subjects with two odorants; one blank and the other with an intensity which lies between ‘imperceptible’ and ‘clearly-perceptible’. The subjects are offered these samples in a random order and they attempt to judge which is the strongest when presented alongside a blank odorant. As the subject is required to make a choice between two odorants this technique is often referred to as the force-choice two- alternative technique. As the threshold concentration is not an absolute level many individuals will give incorrect responses. The percentage o f correct answers is then plotted against concentration. As 75% o f correct answers lie midway between the 50% level (chance) and the 100% (perfect) performance level, this is taken as the detection threshold concentration. A variant o f this method uses a three-alternative force-choice strategy where two blanks and an odorant is

offered. The chance level now becomes 33.3% with the mid-point being 6 6.6%.

The method o f constant stimuli has the disadvantages o f requiring a large number of trials and that subjects suffer boredom and fatigue, and influences due to adaptation, habituation and facilitation (see below). Cain et al (1979) suggest that the maximum number o f trials is limited to 1 or 2 per minute. Figure 2.6 shows the results from a constant stimuli test. This shows the large variations that can occur with this method and that the method can produce more than one threshold value.

The above general techniques have been applied, with different degrees o f success to the study o f odour and indoor air quality in buildings. The different methods

C h a p t e r 2 - H u m a n P erception o f Indoor A ir Q u a l it y and O d o u r

Odour Detection Trials

o fc₀ U

1

_ë Oh 100 _ 1 1 1 1 1 1 1 r 1 1 90 Threshold level j — 80 . ^ ... .... / 70 — y \ / — 60 Chance L e v ^ l_ _ - ^ -" ^ \ / — 50 __ # ... \ / ... ... 0 1 1 - 0 0.6 1.0 1.8 3.2 5.6 10.0 17.8 31.6 56.2 100.0 Concentration (ppm)

Figure 2.6; Odour detection trials using a forced choice Method of Constant Stimuli (two-choice test, odorant versus blank) with Methyl Ethyl Ketone ( MEK) for 100 trials plotted using a logarithmic axis. * These points are shown on the original as >100% but they are not explained in the text. Source: Doty 1991

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can however produce different measurements o f thresholds and so it is important to identify which method is being used for a particular study.

The method o f limits where a subject is presented normally with an ascending series o f odorants has been favoured for use in field and chamber studies where the relationship between perceptions o f indoor air quality or odour has been measured against varying ventilation rates (Cain et al 1979, Duffee and Jan 1981). The concentration o f an air sample’s odour is established by progressively diluting the air and the threshold point being measured in the number o f dilutions required for a certain percentage o f panelists to no longer detect the presence o f an odour.

This number has often been set at 50% and the threshold referred to as ED50

(Effective Dose at the 50% level). The ED50 value is defined as that odour

concentration at which half the panelists would begin to detect odour in a dynamic test (Dravnieks and Prokop 1975).

A dynamic forced-choice triangular olfactometer (Figure 2.7) is used with three outlets, one emitting odorous air and two blanks. The outlets are arranged in a circular manner to prevent positional bias. The experimenter and the subjects are blind to which sample contains the odorous air. A subject chooses by pushing a switch adjacent to the selected output which is evaluated as emitting an odour. Once the subject has chosen one o f the three outlets the procedure is repeated but with a higher concentration in the non-blank outlet. Dravnieks (1975) recommends that one ascending run per person be employed and the average

point o f detection be calculated from a panel o f 8 to 10 judges. This technique has

been adopted in field trials designed to measure prevailing odour levels in buildings (Cain et al 1979, Turiel et al 1981, Duffee and Jann 1981).

Figure 2.8 shows a typical panellist’s response form (Duffee and Jann 1981). The

odorous air was presented from the most-diluted (x81; dilution level number 1) to

the least-diluted (x l, dilution level number 5) where a ‘O ’ represents an incorrect response and an ‘X ’ represents a correct response. After an individual has reacted to all the dilutions, the experimenter examines each response profile back from dilution level number 5 (x l) and circles the maximum dilution (lowest

C h a p t e r 2 - H u m a n P ercep tion o f Indoor A ir Q u a l it y and O d o u r

/

^ V Pushbutton Switch C Ny Ports aiTajiged in a circular manner

Figure 2.7: A dynamic forced-choice triangular olfactometer. Source! Dravnieks and Prokop 1975

C h a p t e r 2 - H um an P erception of Indoor A ir Q u a l it y and O d o u r

EDc;n E v a l u a c t o n Form f o r ^ b i e n t Odor P-y’-naaiic T r l y i g l e O l f a c t o m e t e r

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t o Che t n d i v l d u a l I^aximug LiVeLihood Oaor Thresholcts D i l u t i o n F a c t o r s

Mx ) 27x ' Tx r~ 3 r r i x i "i/v;

01 t a c come t e r L e v e l s ' " 6" Log of Ind i v i d u a l Maximum L i k e l i h o o d T h r e s h o l d - 0 . 2 i ' a l s a a t No. 5 l e v e l * +0. 24 No. 5 c o r r e c t ; mi as No. 4 4-0.72 Nos . 4 , 5 c o r r e c t ; m i s s No. 3 +1.19 Nos . 3 , 4 , 5 c o r r e c t , m i s s No. 2 + 1.67 o n l v misa a t No. 1 l e v e l +2. 15 a l l l e v e l s c o r r e c t

*assumed chan would d e c e c n c o r r a c n l y an 3x c o n c e n t r a t i o n from a m b i en t

P a n e l L e a d e r

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Figure 2.8: An odour panel's response form using a dynamic forced-choice triangular olfactometer Sourcei Duffee and Jann 1981

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concentration) achieved before a wrong response was encountered. This maximum dilution is then converted to a Tog threshold’ by finding the geometric mean between this maximum and the next successive dilution. The form shows that ‘Helen’ has achieved a maximum dilution level number o f 4 (x3) and therefore the log geometric mean between 3 and 9 is found by (log 3 + log 9)/2 = 0.72 (or x5.3). This value is shown in the ‘log threshold’ column. Once all panel members have established their thresholds the group’s log threshold values are summed, and the arithmetic mean value calculated. This is then converted from a

log value to the ED50 value which in Figure 2.8 has been found to be 8.1. Which is

the number o f dilutions that the odorous air required before 50% o f the panel cannot detect odour.

The staircase method is a popular laboratory based method as it requires less trials and provides more reliable data. It is a variant o f the method o f limits but it uses only odorant concentrations near the threshold value. Doty (1991) reports a procedure where subjects are presented with two bottles (one odorant and one blank) and they are asked to report which bottle contains the strongest odour. If no difference is perceived, a guess is required. If a subject is incorrect in their response before five correct successive trials at that concentration have been completed, a stronger intensity is presented. Once five correct responses have been achieved at any given concentration level, the concentration is decreased to a lower value equal to half the previous increase. However, from this point on, only one or two trials are presented at each step. The geometric mean o f the last four staircase reversal points (from a total o f seven) is used as the threshold estimate.

The American Industrial Hygiene Association (AIHA 1989) has reviewed the published literature regarding odour detection and odour recognition thresholds. They have only accepted data from tests that have adopted the following three criteria:

(1) a concentration presentation procedure designed to eliminate olfactory

fatigue

C h a p t e r 2 - H u m a n P e r c e p t i o n o f I n d o o r A i r Q u a l i t y a n d O d o u r

In document The human response to indoor air quality and the significance of the olf and decipol units (Page 33-46)