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El nivel de olor: el parámetro clave para distinguir el abatimiento de odorantes del abatimiento del olor

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Intensity vs. Odour Concentration

   La distinción terminológica entre la cantidad de un estímulo y su efecto en los sentidos es muy importante, y deja poco margen a la especulación a la hora de interpretar los resultados olfatométricos. La introducción del "nivel de olor" como un enlace entre el estímulo olfatorio y sus efectos ayudará a alcanzar este objetivo.

 Autor: Monika Paduch

 Secretaria de los grupos de trabajo sobre olfatometría de la Asociación de Ingenieros Alemana, Düsseldorf, Alemania desde 1980 hasta el año 2007.

   Leer artículo completo en inglés:

    Conflictos de interés: El autor declara que no existe conflicto de intereses.

   Editor académico: Carlos N. Díaz

   Calidad del contenido: Este artículo científico ha sido revisado por al menos dos expertos del olor.

   Cita: Paduch, M. (2017): El nivel de olor: el parámetro clave para distinguir el abatimiento de odorantes del abatimiento del olor. Olores.org

   Copyright: 2017 olores.org. Licencia de Acceso Abierto de Creative Commons. Los autores retienen la propiedad del copyright de sus artículos, pero los autores permiten a cualquier persona descargar, reusar, reimprimir, modificar, distribuir y/o copiar artículos del sitio web de olores.org, siempre que se citen los autores originales y las fuentes. No es necesario permiso específico de los autores o de los editores de esta web.

   Palabras clave: Olfactometria, control del olor, nivel de olor, concentración odorante, terminología

Resumen

   Después de décadas de olfatometría aplicada es el momento de aclarar un malentendido fundamental: Debe hacerse una distinción clara entre el control del olor y el control de odorantes. La diferencia terminológica no es una cuestión lingüística menor. Se enfatiza la relación entre un estímulo (odorante) y su efecto (sensación de olor) con el nivel de olor como un enlace matemático para evitar la malinterpretación de los resultados olfatométricos y los valores límite. Por ello, el nivel de olor ayudará a los (nuevos) olfatometristas y legos en la materia a entender por qué es necesario usar logaritmos y decibelios. Al mismo tiempo, se evaluará la eficiencia de reducción de una corriente de gas oloroso requerida para observar un conjunto de respuestas de la intensidad de olor. 

Introduction

   Decades elapsed since scientists and engineers in environmental protection started to try and quantify the properties of odour in order to avoid odour nuisance near emission sources. When developing measurement methods with the help of (trained) panellists, the sample Dilution to Threshold became the acknowledged basis of “odour” quantification. While in some countries, the D/T number and Stevens’ law of the relationship between stimulus and effect was adopted, European experts preferred Weber-Fechner’s law based on a just noticeable difference (JND) and the artificial quantity ouE/m³ (European odour unit per cubic meter), called “odour” concentration as a unit comparable to emissions such as mass concentrations.

   In Europe, the threshold concentration (at the olfactometer outlet) is 1 ouE/m³ by convention. It is just enough to trigger the physiological sense of smell and provoke an odour perception. The pertinent dilution number set at the olfactometer when the threshold reaction is reached, is the absolute value of the concentration of odorant substances (odorants) in 1 m³ present in the original sample. So lacking a chemical sensor device that could substitute panelists, you find the strength of the undiluted stimulus at the source of emission in the sample bag with the help of the panelists’ physiological sense of smell.

   Although this paper is based on the European methodology to assess an odour situation (see EN standards and VDI guidelines), it is easy to transfer the terminological recommendations to other olfactometric methods.

 

1. Odour is a sensation

   In terms of olfactometry as applied in Europe (Weber-Fechner), I is the odour intensity (strength of odour sensation) and c and c0 are the odorants concentration or the threshold concentration of odorants, respectively.

I = kW ∙ lg (c/c0)                                                                        (1)

   The coefficient kW is a measure of the slope of the curve (function) depicting the typical and individual relationship between odour intensity and odorants concentration of a defined source or production cycle (e.g. olfactometric fingerprint of an emission). In contrast to S.S. Stevens who took the symbol I for the stimulus intensity in decibels (!), the term Intensity in European olfactometry was taken from psychophysics and denominates the strength of a sensation.

A simple verbal scale for odour intensity was chosen in Germany [VDI 3882-1] in order to include also panellists who are not acquainted with logarithmic numerical scales to express a sensory order of magnitude. The distances between the steps are assumed to be equal.

TABLE 1: Odour intensity response as per VDI 3882 Part 1

6

extremely strong

5

very strong

4

strong

3

distinct

2

faint

1

very faint

0

no odour

 

   Although the intensity measurement method is sometimes deemed to be subjective, it is an objective method in the framework of and according to the definition of EN ISO 5492:2009 (Sensory analysis; multilingual version). The intensity measurement to describe the strength of an odour sensation only makes use of the physiological properties of the (selected and trained) panelists, just as is the case with the threshold determination. The required spontaneity of the intensity response does not involve any personal reflection, but just a physiological reaction. Of course, this response is physiologically individual, but not necessarily the psychologically subjective interpretation of a sensation. Statistics are generally needed to handle natural physiological differences in order to find a reference value (limit value) for the population.

   Unfortunately, the fact that panelists and their sense of smell are used to determine the dilutions to threshold led to the misinterpretation that the so-called “odour” concentration is already the sensory effect required to make conclusions on a potential odour annoyance in the neighborhood of an emission source.Plenty of efforts have been made particularly in Germany as a densely populated country to avoid any subjectivity in the assessment of odors [various VDI Guidelines and input to EN Standards].

   However, the “odour” concentration is the quantification of the stimulus, the odorants concentration. The damping effect of the human sense of smell towards that stimulus has been and still is ignored in many cases.

   The knowledge of this background requires not only revision of the interpretation of odorants abatement (commonly called odour abatement), but also of the necessary degree of waste gas cleaning efficiency to meet any emission limits and their admissible spread of results.The following sections serve to become aware of the difference between stimuli and effects and the importance to distinguish them in terminology.

TABLE 2. Terminological distinction between quantities/properties of stimuli and effects (sensations)

Stimuli = Odorants

Effects (Sensations = Odours)

 
Source:
Odorants emission
Odorants concentration cod
Volume flow of odorants od
Odorants abatement (technical degree of reduction in odorants) η od
 
 
Atmosphere:
Dispersion of odorants
(Plume of odorants)
 
 
Ambient air :
Odorants in ambient air
Plume of odorants
 
Sensitivity threshold :
Odour perception
Frequency of odour perception
 
 
Identification threshold :
Odour identification (Identification of the odour quality and of the pertinent source of odorants)
 

Periods of odour identification
Odour hour (German definition),
Frequency of odour hours (German def.)
 

Strength of odour sensation :
Odour intensity response (verbal scale)
Odour intensity Iod
Odour level Lod (1)
Distance between odour levels ∆Lod
Odour reduction index Dod (2) (sensory efficiency or odour damping characteristic)
 
 
Other odour properties :
Hedonic odour tone (pleasant – unpleasant scale),
odour quality (type of odour)
 

  (1) Strictly speaking, the odour level is a mathematical transformation of the odorant concentration, i.e. a stimulus. However,  the term odour level was deliberately chosen in order to create a link between stimulus and sensation, and to facilitate the change from concentration to level in practice.

  (2) This term was chosen in analogy to the acoustic term „sound reduction index” and its definition.

   An intensity measurement following the threshold determination at the olfactometer is only a minor expenditure of time and money when taking into consideration the gain in information involved in this suprathreshold odour assessment with regard to the characterization and control of an industrial emission. Even if one day the odour intensity Iod should be calculated in accordance with Weber-Fechner, the mathematical results will not tell us anything unless they are matched with the trained panellists’ odour intensity response (see Table 1).

 

2. The odour level Lod – the link between stimulus and effect (sensation)

   In natural sciences, a level L is defined to be the dimensionless logarithmic relationship between two quantities of the same dimension (here: lg (c/c0) the denominator of which is a defined basic quantity (here: 1 ouE/m³). In order to mark the values as logarithms and transfer them into a suitable order of magnitude for calculations, they are multiplied by 10 and followed by the denomination decibel (dB), which is not a dimension (cf. acoustics and pH-values). Therefore, weighting of the result (cf. dB(A) in acoustics; Maue, J.H.) is not a prerequisite to apply decibels in olfactometry.

   At present, the statistics applied in Europe to the odorants concentration (“odour” concentration) is on the log10-scale. It requires several steps from antilog to log and back. The logic of this procedure leaves to be desired at first glance because the evaluation of other pollution concentrations does not need these mathematical deviations. The statistics on the log-scale are tailored to the statistics needed for the evaluation of the odour level Lod. What is required is at least the simple parallel quotation of the odorants concentration cod and the pertinent log-value Lod = 10 lg cod [dB] as the measurement result of an odorant concentration (“odour” concentration; see Table 3). While cod represents the quantity of the odour stimulus, L od reflects the damping of the transmission from stimulus to effect and represents the link to the pertinent sensory strength of that stimulus (see Equation (1)).

   This level has so far been related only to the odour threshold. If the level should be related to the odour intensity I, the coefficient kW may play a decisive role (fingerprint of a source) depending on the facility and its type of odour as well as the resulting odour intensity response (see also Sections 3 and 4).

TABLE 3. Transformation of the odorants concentration cod into the odour level Lod (examples)

Odorants concentration cod,
in ouE/m³

Odorants concentration cod, as decimal power,
in ouE/m³

Odour level Lod
in decibel (dBod)

1

100

0

10

101

10

100

102

20

360

102,56

25,6

500

102,7

27

800

102,9

29

1.724

103,236

32,4

13.476

104,129

41,3

50.000

104,7

47

673.592

105,828

58,3

1.000.000

106

60

 

3. Reveal wrong promises and expectations regarding odour abatement

   Due to the quotation of the results in ouE/m³, there is an irresistible temptation to regard the function between stimulus and effect as a proportional one, just like other emission concentrations (e.g. mass concentration per cubic meter) dealt with in air pollution prevention. Consequently, the interpretation of the statistics applied to olfactometry is liable to be wrong.

Example 1

   The authorities of country X set the emission limit for odorous substances to be 100 ouE/m³ and allow a deviation of 3 % from the calculated mean value (antilog of the geometric mean of logarithms – you know it), so that an emission concentration of 104 ouE/m³ would be a violation of the set emission limit. Several years later, they decided to set the limit at 300 ouE/m³, and 310 ouE/m³ were regarded as an unacceptable emission concentration.

   Error: The underlying logarithm of the stimulus-effect function is ignored, and the calculated spread of results near the limit value is completely inadequate in practice. This type of error is in particular affecting the adequate efficiency assessment of biological odour abatement equipment.

   Prove of error: Present the undiluted suprathreshold concentrations of 100 ouE/m³ or 300 ouE/m³ and that of 104 ouE/m³ or 310 ouE/m³, respectively, to a group of panellists. Will they detect a difference in the strength of sensation between 100 and 104 ouE/m³ or else 300 and 310 ouE/m³, respectively? No. Is the spread (here: upper confidence limit) suitable to decide on a violation of the set limit value? No. It is mere theory and application of the wrong statistics because the focus is still on the stimulus and its statistics rather than on the pertinent effect, i.e. "any perceivable change in odour intensity from mean value to upper confidence limit.”

   If companies have to invest 50.000 € or more or even close down because of wrong mathematics for the admissible spread of the limit value in ouE/m³ (no matter what the limit value is), it is necessary to protect them from that economic disaster by being precise and unambiguous in terminology and in the formulation of standards on olfactometry.

Example 2

   Imagine a seller of odour abatement equipment offering his customer a system which would yield an “odour” reduction of 99 %. What will the customer expect? Equipment that will reduce the “odour” emanating from his plant down to a value next to nothing. When comparing the crude and clean gas concentrations of a source in ouE/m³ (e.g. 500.000 ouE/m³ versus 5.000 ouE/m³), which mathematically is the correct odorants abatement, the remaining clean gas concentration may not at all meet the requirements of odour abatement necessary for the environment.

   Here again, the underlying error is obvious. One should be very skeptical about statements with percentages in olfactometry. Although speaking about 99 % odour abatement, the seller probably means “reduction in odorant concentration” (strength of stimulus) because he cannot give any other warranty. The customer understands “reduction in the perceivable odour intensity” (strength of effect) from his source. Misunderstandings like this one underline the importance of being precise in terminology.

 

4. Setting sensible limit values

   In view of the fact that the reduction in odour sensation (odour intensity) is the true aim of emission control and taking into consideration that the relationship between stimulus and effect is a logarithmic one, the close look at the sensory effect of the reduction in stimulus is vital for sensible measures of emission control.

   In particular, the interpretation of the “fingerprint” of a defined odorants source by means of the intensity function allows of comparing the reduction in odorants concentration and the odour levels to the resulting odour intensity category. It reveals the slope kW (Weber-Fechner coefficient) of the intensity function and also the range of concentrations/levels that lead to the same intensity response. As a consequence, any waste gas cleaning equipment will most probably be a waste of money if the clean gas concentration achieves only the same category of odour intensity response as the crude gas concentration.

MPaduch

 Figure 1. Draft of the odour intensity Iod as a function of odorants concentration cod and odour level L od

   These new aspects of assessing an odorants source need reflection upon the criteria that were applied so far and upon the transferability of criteria from other fields of science. To make sure that all readers have the same concepts of defined parameters, here are the ones which are based on the following interpretations:

Performance of a waste gas cleaning equipment

   Difference of the crude and clean gas concentrations c cr and ccl

c = ccrccl                                                                                               (2)

Technical efficiency of the waste gas cleaning equipment ɳ od

   The technical efficiency is the ratio of the benefit of the equipment and the original emission concentration.

ɳ od = (∆ c / ccr) = (ccrccl) / ccr                                                 (3)

   In contrast, the residual emission index is the relationship between clean and crude gas concentrations (ccl / cc). This index must not be confused with the efficiency of a process.

Odour damping index D od ( Relationship of the level distance and the crude gas level)

   This quantity shall be the sensory equivalent to the technical efficiency ɳod and in analogy with the damping index of transmissions systems. In fact, the physiological transmission system is an electrical one.

D od = (∆L / Lcr) = (LcrL cl) / Lcr                                                 (4)

   Table 4 contains some mathematical experiments to show what happens if stimulus quantities or the pertinent odour levels and their ratios are taken as limit values.

TABLE 4. Fictitious examples of emission control criteria

Assumptions :
Limit 1 : the max. emission concentration of odorants (so-called “odour concentration”) be c od, max = 500 ouE/m³, the pertinent max. odour emission level is Lod,max = 27 dBod,
Limit 2 : min. technical efficiency of the waste gas cleaning equipment be ηod,min = 95 %,
Limit 3 : min. sensory efficiency achieved by the waste gas cleaning equipment be Dod = 0,4

c cr

in ouE/m³

c cln

in ouE/m³

ηod

in %

L od,cr

in dBod

L od,cln

in dBod

D od

Limit 1:

cod,max met?

Limit 2:

ηod,min met?

Limit 3:

Dod met?

500.000

500.000

5000

1000

99

99,8

57

57

37

30

0,35

0,47

no

no

yes

yes

no

yes

100.000

100.000

5000

500

95

99,5

50

50

37

27

0,26

0,46

no

yes

yes

yes

no

yes

10.000

10.000

1000

500

90

95

40

40

30

27

0,25

0,33

no

yes

no

yes

yes

yes

6000

6000

600

450

90

92,5

37,8

37,8

27,8

26,5

0,26

0,30

no

yes

no

no

yes

yes

1200

1200

800

380

33

68

30,8

30,8

29

25,8

0,06

0,20

no

yes

no

no

yes

yes

 

   The limit values yield various extents of emission control. A decision has to be taken regarding the objective of emission control. What is closer to odour nuisance – odour stimulus or odour effect? What should be the future (combination of) criteria to limit odorants emissions as the first measure against odour nuisance abatement?

 

5. Conclusions

   Always remember that no odour measurement, calculation or assessment method needs to have a better resolution than the human sense of smell. It would only lead to make-believe precision.

  • Review today’s methods for discrepancies in logics and lack of precision in terminology.
  • Remember that the odorants concentration and the pertinent odour levels are values related to the source, not to the quantity after dispersion/dilution and arrival in the ambient air of the neighbourhood.
  • Consider the definition of a negligence criterion for any additional emission reduction to avoid relocation or shutdown of small facilities.
  • Improve the authorities’ power of discretion and their observance of a reasonableness of means. To this end, found a central institute where individual real cases of odour abatement are collected and characterized. The cost of the achieved improvement should also be disclosed (reasonableness of means).
  • Check what benefits the odour level in decibels may have to dispersion calculation of odorants and to the characterization of multisource areas.
  • Remember that odorants abatement as well as odour abatement do not aim at preventing severe health risks but “only” at preventing annoyance, particularly significant annoyance (nuisance).

 

References

   EN ISO 5492: Sensory analysis - Vocabulary (ISO 5492:2008), plus Amendment 1 (2017)

   EN 13725 (2003): Air quality - Determination of odour concentration by dynamic olfactometry (plus corrigenda (2006)). European Committee for Standardization, Brussels

   Maue, J.H.: 0 Dezibel + 0 Dezibel = 3 Dezibel. Berlin, Erich Schmidt Verlag, 2009

   Stevens, S.S.: Cross-modality validation of subjective scales for loudness, vibration and electric shock. Journal of Experimental Psychology, Vol. 57 No. 4, April 1959, pp. 201-209

   VDI 3882-1 (1992): Olfactometry - Determination of odour intensity (plus corrigendum 1992); (approved for further application 2015-04); bilingual German/English version. Berlin, Beuth Verlag

   VDI 3882-2 (1994): Olfactometry – Determination of hedonic odour tone (approved for further application 2015-04); bilingual German/English version. Berlin, Beuth Verlag

   VDI 3883-1 (2015): Effects and assessment of odours - Psychometric assessment of odour annoyance – Questionnaires; bilingual German/English version. Berlin, Beuth Verlag

   VDI 3883-2 (1993): Effects and assessment of odours – Determination of annoyance parameters by questioning - Repeated brief questioning of neighbour panelists (approved for further application 2015-04); bilingual German/English version. Berlin, Beuth Verlag

   VDI 3883-3 (2014): Conflict management in air pollution abatement - Fundamentals and application to ambient odour; bilingual German/English version. Berlin, Beuth Verlag

   VDI 3940-1 (2006): Measurement of odour impact by field inspection - Measurement of the impact frequency of recognizable odours - Grid measurement (plus corrigendum 1); bilingual German/English version. Berlin, Beuth Verlag; cf. prEN 16841-1 (2015), CEN, Brussels

   VDI 3940-2 (2006): Measurement of odour impact by field inspection – Measurement of the impact frequency of recognizable odours - Plume measurement (approved for further application 2012-03), bilingual German/English version. Berlin, Beuth Verlag; cf. prEN 16841-2 (2015), CEN, Brussels

   VDI 3940-4 (2010): Determination of the hedonic odour tone - Polarity profiles (approved for further application 2015-04); bilingual German/English version. Berlin, Beuth Verlag

 


 

Monika Paduch   Monika Paduch was born in Germany on 3rd Oct., 1948. After finishing school in 1967 (A-level; SAT-exam), she had professional educations to work for several years as a multilingual secretary (English, French, office administration) in various industrial areas. In 1975, she decided to start studying design engineering at the University for Applied Sciences in Cologne, and finished studies with a diploma in 1980. After that, she was an academic staff member of the Commission on Air Pollution Prevention of VDI (Association of German Engineers) and DIN (German Standardization Institute). Her main task was that of a secretary to working groups specialized in odour emission, odour measurement and assessment as well as odour abatement by biological waste gas cleaning methods. She is no longer involved with odour issues as she retired in 2008. However, she attended the last Conference of the German Association of Engineering in Karlsruhe. When asked about her presence in that conference being already retired she replied "you know what?, I am just addicted to odours ;-)".

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