Odorant Evaluation: A Study of Ethanethiol and Tetrahydrothiophene as Warning Agents in Propane Marvin L. Whisman", John W. Goetzinger, Faye 0. Cotton, and Dennis W. Brinkman Bartlesville Energy Research Center, Department of Energy, Bartlesville, Okla. 74003 Ethanethiol and tetrahydrothiophene were evaluated as odorants in propane. Human awareness was appraised by use of four testing modes that ranged from full knowledge of the odor project (directed) to deliberate distraction (misdirected). Several odorant concentrations were evaluated for each odorant system in all test modes. In addition to defects of the nasal anatomy and psychological factors, unfamiliarity with a given environment and mental distractions reduced awareness to odorants. Significant efforts were directed toward determination of odorant levels that not only c a n be detected, but also will be detected.
The odorization of gas was started about 1880 in Europe (1) to reduce fatalities resulting from escaping gas. The first reported odorant, ethanethiol (EtSH), still is predominantly used to odorize liquefied petroleum (LP) gas. Two odorant studies by the Bureau of Mines in 1930 ( 2 )and 1931 ( 3 )served as the starting point for the work described. The 1931 work still is a basis for odorization levels used by petroleum producers and regulatory agencies ( 4 ) . The purpose of this investigation was to reexamine the early work, taking advantage of modern analytical instrumentation and improved techniques in sensory perception evaluation to test the present levels of EtSH and tetrahydrothiophene (THT) in commercial propane. A more detailed description of the tests and results is available in the form of a governmental publication (5). Experimental
Odorant Systems. Three odorant systems were studied: ethanethiol (EtSH), tetrahydrothiophene (THT), and an approximately equal mixture of these. All were technical grade chemicals, similar to those used in commercial operations. The propane was odorless and oil free, and purity was certified as greater than 95%. Except when propane level was a variable, propane was added a t a level of 0.43 mol % in the test atmosphere, which is one-fifth the lower limit of flammability. This is generally designated as the level a t which a warning is necessary. As a reference, LP-gas is usually dosed a t 1 lb of odorant per 10 000 gal fuel. Using equilibrium data, the concentration of odorant in propane vapor is calculated. The odorant level in the room is then calculated so that the propane is present a t one-fifth its level of flammability, which is 0.43%in air (mole ratio). Using this method, the normal level of EtSH is 13 ppb a t 0.43%, and T H T is about 1.3 ppb, assuming the containers to be at 25 "C. Testing Facilities. Three experimental facilities were used. A dynamic odor presentation room was used by a trained panel for preliminary evaluations. Previous use of this facility was described by Hurn and Marshall (6). A walk-in test facility could accommodate six or more participants in a room containing mixtures to be evaluated. A small entry cubicle minimized mixing of outside air with the test blend during entry. Large double doors on opposite sides aided in rapid airing of the facility between test runs. Walls and ceiling were covered with polyvinyl fluoride film (Tedlar, Flexcon Co., Spencer, Mass.), and the floor was covered with epoxy-based paint. The third test facility was a small cubicle inside a 22-ft
travel trailer. The walls of the cubicle were covered with Melamine hardboard similar to the material commonly used to cover the walls of showers and kitchens, and the floor and ceiling were covered with the original vinyl coverings. Only one participant was present in this room at any one time. Testing Modes. Four testing modes were used to simulate differing environmental conditions and degrees of awareness. The conventional test mode used in odorization studies (7,8) was labeled as the directed mode. In this situation, each odor panelist was aware of the exact nature of the experiment and had some previous experience or training with the odor(s) involved. This directed mode indicates the level of odorant that c a n be detected, but not necessarily the level that will be detected. This mode was used in two ways. One panel used rating scales just as in studies by previous investigators. A second panel provided only positive-negative responses. The second mode attempted to simulate conditions encountered in familiar surroundings and was labeled the semidirected mode. Participants were told only that it was a sensory perception test and were given a sound, a color, and a temperature to evaluate, in addition to the odor. Each group went into the walk-in facility several times, evaluating different levels, so that a familiarity was gained with the testing room, as well as the character and relative intensities of sounds, colors, and odors. However, no emphasis was placed on odor perceptions, and no reference point for the rating scale was given. Two other test modes were devised, utilizing the mobile lab, to evaluate the effect of distraction during testing upon olfactory responses. The most demanding mode was designated as misdirected because of the deliberate distraction given to odorization participants. In this situation, each participant was tested only one time. Thus, the test cubicle represented a strange environment to each participant, and the attention of the participant was directed away from odor by asking each to read provided material on the wall and to judge the room temperature with no mention of odor evaluation. If no mention of odors was made voluntarily during debriefing, investigators asked if the participant noticed an odor inside the trailer cubicle. An intermediate procedure, designated as undirected, was also devised. The participant entered the test facility once so that the trailer cubicle environment was unfamiliar, but no distractions, misleading instructions, or additional duties were required. The participant was asked to describe the room conditions upon exiting and, if no mention of odor was made, a general inquiry was posed by the interviewer. Care was taken in both of the latter modes to keep past participants from biasing future subjects. Test sites were changed daily covering cities in Kansas, Missouri, Arkansas, and Oklahoma. Analytical Instrumentation. The level of EtSH and/or T H T in air-propane mixtures was measured by gas chromatographic techniques utilizing a Tracor Model 27HA atmospheric sulfur analyzer with a flame photometric detector. A Teflon column packed with deactivated silica gel was used for EtSH determinations, providing a 2-min retention time under the conditions available from the authors. The second column was a Teflon tube packed with 50% Carbowax 4000 on 80/100 mesh Chromosorb 750. T H T was determined on this column and had a retention time of about 1 min. The flame photometric detector was calibrated daily with
This article not subject to US. Copyright. Published 1978 American Chemical Society
Volume 12, Number 12, November 1978
1285
permeation tubes. Output of these tubes was certified by the manufacturer, but was recalibrated in-house using standard blends prepared in polyvinyl fluoride bags. Procedures for making standard odorant blends are presented in a related paper (9). The tubes were maintained a t 25 "C f 0.02 in a constant temperature bath, while the airflow into the sample chamber was measured with a soap-bubble flow meter. Vapors were drawn from the trailer cubicle or walk-in room through Teflon tubing directly into the sample loop for the odorant to be determined. Intensity Rating Scales. Two relative rating scales were used. The trained odor panel used the Turk (10) system in which ratings from 0 to 12 were given, each step representing a factor of two change in concentration from the previous number. Some of the directed tests and all semidirected tests used the rating system developed by Fieldner and coworkers ( 3 )for the Bureau of Mines (BOM). That system consists of numbers from 0 to 5 having descriptive words related to each. Zero represents no odor and is followed by very faint, faint, moderate, strong, and very strong. The level corresponding to an average rating of 2 (faint) is generally regarded as the appropriate dosing level. Other directed studies employed yes-no responses as a substitute for intensity rating scales.
Results and Discussion In evaluating these data, it must be recognized that it is essentially impossible to protect 100%of the population with odorants. No matter how strong the odor, factors such as disease, age, specific odor "blindness" ( I I ) , or total distraction will prevent some from detecting an odor a t any level. On the other hand, with careful selection, an odor evaluation panel can be assembled with extraordinary olfactory responses. Neither of these extremes is of much relevance to either industry or consumers. Rating scales, such as the Turk and BOM systems used in part of the tests in this study, are totally subjective unless the evaluator has been specifically and thoroughly trained. Depending on the reference point(s) given, or the order of odor levels presented, differing results can be obtained. While the Turk scale begins near the threshold level and progresses a t regular concentration intervals, the BOM scale is not so well defined. A rating of two on a scale whose maximum is 10 ppb differs greatly from a rating of two on a scale from 0 to 100 ppm. With both scales, considerable training is required for proper orientation. Moreover, in attaining the goal of determining optimum odorant levels to protect the public, a rating system does not necessarily provide the information required. An average rating of two does not indicate how many people are protected. A rather sizable fraction could have rated the odorant system a t one, or even zero. Thus, the undirected and misdirected results, in which only yes-no responses were obtained, tend to yield more valid conclusions, both because no numerical rating system was used and because the environmental conditions might be more relevant to real life situations. However, each participant may have been sensitized by the fact that they were involved in some kind of experiment. Trained Panel-Directed Mode. Two trained odor panels were selected from local residents. In initial sessions the first panel became able to distinguish common household odors, such as those of orange, vanilla, almond, clove, and cinnamon, singly and in groups. They were taught to rate both the quality and intensity of odors before being asked to rate EtSH and T H T on the Turk scale. The second panel was repetitively exposed to various levels of odorant(s) in the walk-in facility until they were familiar with the odorants, the facilities, and the procedures. Subsequently, trained panelists were used for four specific 1286
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purposes: to establish a threshold detection concentration for each of the odorant systems; to estimate the extent of odor persistence on clothing and hair of participants using the walk-in facility; to estimate the effect of temperature upon olfactory responses; and to establish a minimum detectable concentration based upon yes-no responses. In the first ,of these investigations, the panel utilized the dynamic odor presentation room. The concentration corresponding to a rating of one, the lowest positive rating, was determined by linear regression analysis of the data. The odor threshold for EtSH was 0.2 ppb (mol/mol), while T H T yielded a value of 0.5 ppb. The same procedure, with the same participants, was applied using the walk-in facility to evaluate differences that might exist between these environments. The odor threshold for EtSH in the walk-in facility was 0.1 ppb, and 0.4 ppb corresponded to a rating of 1for THT. The values are considered in good agreement for the two facilities and are not significantly different from values reported by Wilby (12) of 0.32 ppb for EtSH and 0.75 ppb for THT. When mixtures of the two odorants were present, the panel was asked only to determine which was the predominant odor. The panelists were consistently successful in making this differentiation when the predominant odorant level was 2 ppb and above. A similar study of the effect of propane on the odors gave a positive identification correlation. However, panelists described the effect as a reduction of the harshness of the odor without a change in intensity. In subsequent tests, propane was present in all test mixtures a t a level of 0.43%, which is one-fifth the lower limit of flammability. The persistence of odors on subjects exposed to high concentrations (100 ppb) could cause memory effects during testing but was found by the panel to be undetectable. Both males and females, wearing a variety of clothing materials, were checked for lingering odor with similarly negative results. Thirdly, odor ratings by the trained panel were checked at two different temperatures, 20.5 and 8.3 "C. Rating differences at the two temperatures were not large enough to be statistically significant. The fourth study in the trained-panel phase of the directed model investigations was to determine the minimum odorant level producing a positively identifiable, distinct odor which was interpreted as the lowest level at which essentially 100% of the participants responded positively. Figure 1 presents some of the data obtained and shows graphically minimum distinct levels. These were 0.6 ppb (mol/mol in air) for EtSH and 1.3 ppb for THT. By using known equilibrium values ( 9 ) , these levels were translated into LP-gas dosages of 0.07 lb/ 10 000 gal propane for EtSH and 3.3 lb/10 000 gal propane for THT, assuming a minimum storage temperature of -23 "C (-10 O F ) . Threshold concentrations, corresponding to 50% positive response, were 0.3 ppb for EtSH and 0.6 ppb for THT, which compare favorably with data from other modes. Untrained Participants-Directed Mode. Untrained participants used the BOM scale of 0-5 to rate odors in the walk-in test facility. The concentration corresponding to a rating of 1,after regression analysis of the raw data, was considered the threshold, while a value of 2 was taken as the appropriate dosing level. Threshold levels were 0.05 ppb for EtSH and 0.5 ppb for THT. These are reasonably close to the trained panel results, considering the fact that these data are much more relative and subjective than those from trained subjects. Concentrations yielding an average response of 2 are 0.2 ppb for EtSH and 2.6 ppb for THT. Semidirected Mode. Data were obtained from 100 participants, each of whom provided 17 evaluations of visual, tactile, aural, and nasal stimuli. Odorant concentrations from 0 to 75 ppb (mol/mol) were used. Rather than attempt to obtain average concentration corresponding to a given rating,
I
I
1
THT
1‘
Table 1. Semidirected Mode. Positive Response Taken as a Rating of 2 or Higher
/ /
/ / / /
odorant
level (ppb)
EtSH
5 10 20 40 5 10 20 40 5 10 20 40
/ / THT
EtSH:THT
(1:l)
positive responses ( % ) corrected unc orre ct e d
66 80 89 95 78 87 92 94 68 82 89 93
72 83 92 96 82 89 93 95 73 85 92 94
Table II. Undirected and Misdirected Modes test mode
odorant
undirected
EtSH
THT O
f
I ”
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Odorant concentrotion ( p p b )
misdirected
EtSH
Figure 1. Minimum distinct levels as determined by regression analyses of yes-no directed responses THT
the data were interpreted as the percentage of positive responses a t each level, using both a rating of 11 and 2 2 as a positive response. These responses were corrected, by use of Abbot’s formula (13),for false responses at the 0 (no odor) level. When a one or above was the criteria, more than 95% gave positive responses for all concentrations of both odorants down to 5 ppb (mol/mol). However, when a rating of 2 or higher was taken to be a positive response, considerably lower percentages were obtained, as shown in Table I. The 1:l mixture values more nearly correspond to EtSH results. Undirected and Misdirected Modes. Over 1600 participants volunteered at sites throughout Kansas, Missouri, Arkansas, and Oklahoma to participate in an unknown research project without direction, as described previously. More than 1700 participated in the misdirected mode of the study. The results are presented in Table 11. Since the participants were asked only whether or not they smelled an odor, no interpretation of a rating was necessary. Correction for positive responses a t 0 concentration was carried out as before. Almost no response variation with concentration was noticed with the 1:l mixture. Between 20 and 160 ppb, the uncorrected percentage increased from 90 to 92%, while the corrected results increased from 85 to 87%. These values held true for both the misdirected and the undirected participants. As expected, the percentage of participants detecting any one level of odorant in the undirected mode was higher than in the misdirected mode. However, in both situations the data suggest that about 10%of those tested did not notice an odor a t the 100 ppb range, which is well above the levels normally used in commercial propane. The number of undirected and misdirected participants
level (ppb)
20 40 80 160 20 40 80 16020 40 80 160 20 40 80 160
positive responses ( % ) corrected uncorrected
77 87 92 93 80 86 87 88 69 76 87 93 65 76 84
86 91 95 95 88 91 92 92 79 84 92 93 77 86 88
...
...
who correctly associated the odor of EtSH with gas was higher than for THT. A sample of the data, divided according to sex and age, where junior is 15-30, middle is 31-55, and senior is 56 and above, is shown in Figure 2. Females gave positive perception responses more often than did men and also correctly associated the odor with gas more often. Generally, the younger age groups were more accurate and perceptive than the older age groups. This fits the general concept of deterioration of the senses with age ( 1 1 ) . The effect of “experience” can be seen even in the small amount gained by participants during the short semidirected experiments. Table I11 presents data showing the percentage of the respondents whose answers for two consecutive runs were in the proper direction at various stages of “experience”.
Summary From these results, it can be concluded that classical methods of evaluating odorant levels required to provide a warning to LP-gas consumers probably approximate the levels that can be detected more nearly than the levels that will be detected. Classical testing methods, therefore, may not define the level of odorants required to warn persons of the presence of gas in distracting or unfamiliar surroundings. Actually, since the classical methods depend on scaled rating systems, tests are relative; results can be arbitrarily obtained depending Volume 12,Number 12,November 1978
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Table 111. Effect of Experience on Odor Detection and Perception percentages ‘right a
tests 1 and 2 (no previous experience) tests 4 and 5 (after 3 observations of same odorant) tests 1 and 2 (after 6 observations of the other odorant ) tests 4 and 5 (after 6 observations of other odorant and 3 of same odorant
eihanethloi same
wrong
70
10
20
100
... ...
...
90 70
10
10 20
ietrahydroihiophene ’right same wrong
40 80 70 80
... 10 20 10
60 10 10 10
The change in rating was in the proper direction for the change in concentration of odorant. “Same” means the rating did not change, even though the levels of odorant did.
-1
I
Combined
rest
modis
KEY
walk-in test facility is superior to an odorimeter system, but the size and interior treatment of this facility should be standardized, as well as the techniques for preparing blends in the room, and the training (or misdirecting) of the evaluators. The number of participants used is important, since one is attempting to make some sampling of the total population, but the number of times each person is used is also important, since training or experience is a significant factor. Until some standardization is achieved, there will probably continue to be disagreement among investigators regarding the significance of test results, not to mention variations in estimated threshold concentrations and divergence of odorant levels recommended to provide reasonable protection to consumers. Acknowledgment
We gratefully acknowledge the help of Fred W. Gragg and Gregory Steele in collecting field data; William Mendenhall and Ramon Littell, University of Florida, in designing sensory perception procedures; and all the participants who often gave of their time without full knowledge of the experimental objectives. L i t e r a t u r e Cited Jr
Mid
Flgure 2. Percentage of positive responses to ethanethiol correctly relating odor to gas
on the reference point(s) given and the procedures followed. Persons in unfamiliar surroundings, or mentally distracted, fail to detect odors they might perceive in familiar, neutral environments. Persons told to detect an odor most often do so, whereas persons told to concentrate on other sensory responses will do so, frequently a t the expense of olfactory sensitivity. It is thus felt that no one level can be set down at this time as being ideal, or even sufficient, with confidence. The results obtained in this extensive study point out a need for redefining the methods for estimating appropriate dosing levels for odorants. Most of the analytical chemistry problems in conjunction with measurements and monitoring have been resolved satisfactorily in this project. However, further refinements of the various modes of testing human subjects are necessary in an attempt to converge on one acceptable mode, and continued careful parameter control will be necessary for final resolution of this investigation. Totally subjective rating systems need to be replaced with positive-negative perceptions. This would necessitate a new standard, based upon a percent positive perception in a defined environment. A 1288
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(1) Reitmayer, K., J . Gasbeleucht. Wasseruersorg., 50,318 (1907). (2) Katz, S. H., Talbert, E. J., “Intensities of Odor and Irritating Effects of Warning Agents for Inflammable and Poisonous Gases’’. Bureau of Mines ;i’P%O, 1930. (3) Fieldner, A. C., Sayers, R. R., Yant, W. P., Katz, S. H., Shohan, J. B.. Leitch. R. D.. “Warning Aeents for Fuel Gases”. Bureau of Mines Monograph 4,1931. (4) National Fire Protection Assoc., “Storage and Handling Liquefied Petroleum Gases, 1974”, NFPA No. 58, 1974. ( 5 ) Whisman, M. L., Goetzinger, J. W., Cotton, F. O., Brinkman, D. W., Thompson, C. J.,“A New Look a t Odorization Levels for Propane Gas”, 91 pp, BERC/RI-77/1, DOE, 1977. (6) Hurn, R. W., Marshall, W. F., Soc. of Auto. Eng. Meeting, Paper No. 680418, Detroit, Mich., May 20-24, 1968. (7) Deininger, N., McKinley, R. W., “The Design, Construction and Use of an Odor Test Room”, p 23, American Soc. of Testing Materials, Symp. on Odor., Spec. Tech. Publ. No. 164, 1954. (.8_ ) Leonardos. G.. Kendall.. D.., Barnard.. N.., J . Air Pollut. Control ASSOC.,19 cz), si (1969). (9) . . Goetzinger. J. W.. Brinkman, D. W., Poling, B. E., Whisman, M. L.,J. Chem. kng. Data, 22 (4), 396 (1977). (10) Turk, A,, “Selection and Training of Judges for Sensory Evaluation of the Intensity and Character of Diesel Exhaust Odors”, P H S Publ. No. 99-AP-32, Washington, D.C., 1967. (11) Amoore, J. E., “Properties of Olfactory System”, Odorization Symp., Inst. of Gas Tech., IIT Center, Chicago, Ill., July 12-15, 1976. (12) Wilby, F. V., J. Air Pollut. Control. Assoc., 19 (2), 96 (1969). (13) Finney, D. J., “Probit Analysis”, 3rd ed., Chap. 7 , Cambridge Univ. Press, 1971.
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Receiued for review February 14,1978. Accepted June 5,1978. Work partially funded by the National LP-Gas Producers Association. Mention of trade names is for identification only and does not imply DOE recommendation.
