Odour perceptions have been studied at levels above their threshold values (suprathreshold sensitivity) and a number o f methodologies have evolved to measure them. Doty (1991) has identified three general classes o f measurement;
1. those that require subjects to judge the relative amount o f one or more
attributes o f a set o f odorants, such as intensity or pleasantness, referred to as Attribute Scaling:
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
2. those in which difference thresholds are determined so that the minimum
increase in concentration required to make odorant (A) perceptibly more intense than concentration (B), referred to as Difference Threshold Measurement: and
3. those which specifically test the ability o f subjects to detect, recognise, identify, classify or remember stimuli above the threshold value, referred to as Discrete Stimulus Measures.
2.321 Attribute Scaling
Odours may be measured according to a number o f psychological attributes including intensity, pleasantness, and quality. The preferred method for clinical and research purposes is to measure odour intensity. However, Parducci (1963) points out that the intensity o f odours are relative and can be influenced by contextual factors, such that a moderately strong odour is judged as more intense when placed within the context o f a set of weak odours, than within the context o f a set o f strong odours (the stimulus range effect (Poulton 1968)). This factor may be particularly important in the building context where different sources o f odour may exist, for example, occupants, services, fabric and fittings have all been implicated as odour-emitters, reducing one element may simply increase the sensitivity to the others.
Perceived odour intensity has been measured using category scales, ratio scales and by magnitude matching.
1. Categorv scales are often used in field study situations and generally uses a scale o f numbered phrases;
a. No odour b. Slight Odour c. Moderate Odour d. Strong Odour
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e. Very Strong Odour f. Overpowering Odour
However values on this scale do not reflect a ratio relationship i.e. a value o f 2 does not represent a perceived magnitude twice as great as a value o f one (ASHRAE 1993). The category scale has been the preferred method for measuring human perception to odours both in field studies and chamber studies (see Chapter 3). The number of categories in these scales depends both on the ability o f the subject to discriminate small changes in the intensity (or other attributes) and the range of the stimuli encountered.
2. Ratio scaling require judges to assign numbers proportional to the perceived magnitude or intensity. A subject may be asked to assign the number 10 to one concentration and the subsequent sample is perceived as twice as strong the subject should vote at 20. This method o f ratio scaling is referred to as magnitude estimation (Cain and Moskowitz 1974). Intensity magnitude estimation data are most commonly analysed by plotting the log magnitude estimates as a fimction o f the log o f the odorant concentration and the best-fit line established from the graph. Doty (1991) reports three possible biases with this technique; (1) it requires that subjects are able to accurately remember the level o f the prior stimuli, (2) that round number bias can occur where subjects over-select certain number (e.g. 2, 5, 10, 20, 50, 100 etc.) and (3) non-sensory factors such as the numbers used in the instruction, the range in quality and complexity o f the odorant, the subjects experience with numbers etc. This technique is therefore more suitable for trained observers.
3. A third method used to gauge an unknown odorant’s intensity, is to match its intensity with a selection from a range of concentrations using a standard odorant such as butanol. This approach is termed magnitude matching. A judge is presented with a series o f concentrations o f a standard odorant (often 1- butanol) and required to match the intensity o f an unknown odorant with one o f the standard concentrations (Cain 1978b). M ost chamber and field studies
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have employed a selected team o f people to measure the odour concentration using magnitude matching techniques.
Human response to odour intensity has been established using the above psycho physical scales and has been found to obey Stevens’ power law where the perceived intensity (S) increases as the power o f the concentration level (C):
S=kC" [2.1]
Where k is a constant and the exponent n will depend on the odorant, ranging from 0.2 to 0.7 but always less than 1 (ASHRAE 1985) i.e. doubling the concentration results in less than double the perceived intensity. The consequence o f the exponent being less than one is that progressive increases in an odorant’s intensity will result in a decreasing level o f magnitude perception.
Figures 2.11 shows the Dravnieks binary dilution olfactometer which has been used in field and chamber studies to determine the ambient air’s odour intensity by the method o f magnitude matching. A judge chooses a port, from a selection o f eight, that matches any given stimulus (air from a test chamber or a study building). If the stimulus appears to fall between ports then the subject should indicate this. The olfactometer presents butanol vapours ranging from very weak to strong. The concentration o f butanol doubles between successive ports ranging from 16 ppm to 2000 ppm. However, the perceived intensity will not double as the exponent for butanol is 0.66. The perceived intensity will therefore increase by
20.66 ^ i 5g Qj. ^g% stronger in adjacent ports. ASHRAE (1993) have shown the
relationship between butanol concentration and perceived intensity (Figure 2.12). The graph also indicates concentrations where the qualitative response changes from ‘neutral’ to ‘unpleasant’. This shows that the qualitative characteristics o f an odorant can change with intensity. ASHRAE (1993) also reveal the relationship between butanol concentrations and several other vapours (Figure 2.13) using the binary dilution olfactometer. The Figure indicates that the butanol olfactometer has provided a reasonable relationship between the intensities o f the two odorants despite the qualitative parameters o f these odorant being significantly different.
C lia p t c r 2 - llii in a n P erception o f Indoor A ir Q n a l it ) and O d o u r M A K E U P AIR D IS T R IB U T O R m a k e u p AIR.—• 1-BUTANOL b u t a n o l v a p o r DISTRIBUTOR AIR TO VESSEL UNKNOWN ODOR
Figure 2.11: Dravniek's binary dilution olfactometer Source; A S H R A E 1993
C h a p t e r 2 - H u m a n P ercep tio n o f In door A ir Q u a lit y and O d o u r
1000
BUTANOL REFERENCE SCALE
c <D G 100 3 o O u UNl’LEASANT 10 NEUTRAL (D Dh "THRESHOLD' 1 10 100 1000 10000 Concentration o f Butanol (ppm)
Figure 2.12: Relationship between perceived butanol intensity and butanol concentration Source: ASHRAE 1993
C h a p t e r 2 - H u m a n P ereep tiu n o f Indoor A ir Q u a lit y and O d o u r 10000
I
1 - lODOBUTANE ACETONE I 1000 PYRADINE 3 CÙ (4-, O c 01
G u o G o u 1 - HEAXANO. UX) b u t y l e t h e: 1 10 100 1(X)0 10000 lOOOtX) g 3 -ë 01
Concentration o f Different Odorants (ppm)
Figure 2.13: Results using a binary dilution olfactometer to compare perceived butanol intensity and perceived intensity of other odorants. Source: ASHRAE 1993
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The Dravnieks binary dilution olfactometer has successfully demonstrated the use o f a standard odorant (butanol) as a basic measure o f another odorant’s perceived intensity i.e. a concentration o f 1000 ppm o f acetone is equivalent to approximately 90 ppm o f butanol (Figure 2.13). This ability to use a standard odorant as a yardstick has been adopted to measure the suprathreshold odour intensity o f indoor air. However, it has also been used along with more subjective measures such as category scales.