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What are Odours?

Odours are defined (1) as the resulting sensation of the reception on a stimulus by the olfactory sensory system. The way the human respond to an odour stimulus depends on its sensory properties, in particular, Odour Concentration, Intensity, Quality and Hedonic Tone. The combined effect of these properties is related with the degree of annoyance that an odour can cause.

Odour Concentration

Following the European UNE EN 13725 Norm, Odour Concentration is "the number of European odour units in a cubic metre of gas at standard conditions". Therefore the odour concentration is measured in "European Odour Units" and its symbol is "uoE".

In short, Odour Units works for "odours" as Decibels for "noise". In general, the odour unit is calculated from the number of times that a gas sample has to be diluted in order to be detected by a percentage of 50% of a group of people adequately trained for this purpose (panel).

A simple example will show us how to calculate the odour concentration:

Let us suppose that we have taken a bag with odours from some place. If we dilute this sample 1000000 times with odourless air (or nitrogen), it is probable that when the diluted sample is presented to a group of, let's say six people, none of them will detect the odour. If we dilute step by step the "odorous sample" with a smaller concentration of odourless air, there will be a point when half of the people will agree that they do detect the odour, and the other half will say that they don't. In this way, if the sample has been diluted 30 times, it is concluded that the odour concentration of the original sample was 30 Odour Units (30 uoE).

The measured values depend, of course, on the kind of sensory test, the selection of the panel, the criteria of detectability and other factors.

Odour Intensity

In fact, Odour concentration does not inform about how strong is an odour perceived. In contrast, odour intensity does bring information about the degree of annoyance of an odour, that is, odour intensity express the strength of the odour sensation.

Let us suppose using another example that we could measure a sample of odour collected from the vicinity of a restaurant. If we got a concentration of, let's say 10 uoE. ¿Does this mean that the annoyance perceived by the neighbours is 10 times higher than when there was no restaurant?

The answer is NO.

Unfortunately, the "Concentration" magnitude is not lineal and depends on many factors. An odour can remain undetected above a certain concentration, its composition may have alterations, its hedonic tone could change from an "annoying odour" at low concentration, to a "pleasant odour" at higher concentration (or the opposite), or it could be perceived with the same degree of annoyance with 2 uoE or with 15 uoE.

Odour intensity is the strength of the perceived odour sensation and in many cases It is related with the concentration of an odorous substance. The following equation could define the relation between Odour intensity (I) and concentration (C). “K” is a constant and “n” is the exponent.

Stevens law


Stevens's law formula

This is known as Stevens' law or the power law. For odours, n ranges from about 0.2 to 0.8, depending on the odorant.

For an odorant with n equal to 0.2, a 10-fold reduction in concentration decreases the perceived intensity by a factor of only 1.6; whereas for an odorant with n equal to 0.8, a 10-fold reduction in concentration lowers the perceived intensity by a factor of 6.3.

The dependence can also be described as a theoretically derived logarithmic function according to Weber and Fechner (2):

Weber Fechner relation

where C0 is the threshold concentration; and kw is the Weber-Fechner coefficient or Weber ratio.

Which one of these 3 equations applies depends on the method used. To this date, no theory has been able to derive the psychophysical relationship from knowledge about the absolute odour threshold of various substances.

When immissions are tested on site, odour intensity is determined by means of an odour inspection. Determination by means of olfactometry is performed under the same conditions like those of the odour threshold determination. The method is well described in the VDI Guidelines 3881 sheet 1-4 and 3882 sheet 1. (4)

The following table illustrates the different levels of intensity that can be registered in a field inspection.

OdourIntensity stage
not perceptible 0
very faintly 1
faint 2
distinct 3
strong 4
very strong 5
extremely strong 6

Odour Quality

Odour quality or character is that property that identifies an odour and differentiates it from another odour of equal intensity. Basically this parameter provides information about what the substance smells like.

The odour character is described by a method known as multidimensional scaling or profiling. In this method, the odour is characterised by either the degree of its similarity to a set of reference odours or the degree to which it matches a scale of various descriptor terms. The result is an odour profile.

Odour Hedonic Tone

Hedonic tone is a property of an odour related to its pleasantness.

When the hedonic tone of an odour is evaluated in the neutral context of an olfactometric presentation, the panellist is exposed to a controlled stimulus in terms of intensity and duration. The degree of pleasantness or unpleasantness is determined by each panellist experience and emotional associations (3).

A distinction has to be made between the acceptability and the hedonic tone of an odour. When an odour is evaluated in the laboratory for its hedonic tone in the neutral context of an olfactometric presentation, the panellist is exposed to a controlled stimulus in terms of intensity and duration. The degree of pleasantness or unpleasantness is determined by each panellist's experience and emotional associations.

Other Properties

  Although Intensity, Concentration, Character and Hedonic tone are intrinsic properties of the odours, there are other important parameters related with this sensation that have a certain importance, sometimes a neat one. For example the persistence of an odour is related to the change in the odour intensity when diluting an odour. The frequency of an odour is the percentage in time that is possible to detect this odour in the atmosphere. For example the German norm VDI 3940 defines the odour frequency as the percentage of odour hours in a year, defining the percentage odour time as the ratio between the time fractions that an assessor is able to recognise an odour and the total measurement duration.



  1. (1) http://www.epa.nsw.gov.au/mao/odourcontrol.htm
  2. EN 13725, Air quality — Determination of odour concentration by dynamic Olfactometry
  3. Bidlingmaier et. al (1997): Odour Emissions from Compost Plants - Dimensioning Values forEnclosed and Open Plants (Translation), Rhombos-Verlag, Berlin

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