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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 to the degree of annoyance that an odour can cause.
Odour Concentration
The odour concentration is defined within the European Standard EN 13725 as 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, the odour concentration has similar characteristics than decibels in the sense that they are both properties linked to the human senses. The odour concentration is calculated from the number of times that an odorous gas has to be diluted in order to reach the odour threshold of a group of people or panel specially trained and selected, but with a normal sense of smell. In fact, the geometric mean of the individual odour threshold of each person is calculated.
Wait a minute ¿what is the odour threshold?
The odour threshold of any odorant is the minimum concentration at which anybody with a normal sense of smell is able to detect it. It is also called the detection threshold and it has a couple of more technical definitions in the EN 13725, but we will not go there.
The process can be better explained with a simple example. Let's suppose that we have an odorous gas (that is, an odorant) and we dilute it with odourless air 10 million times. Probably, when we present this gas to be smelled by a member of a panel, she (or he) will not detect anything. If we dilute the odorant less and less, there will be a moment in which this person will be able to detect the diluted mix. This way, if the gas has been diluted 30 times with odourless gas in order to reach the detection threshold, it is estimated that the initial odour concentration of the gas was 30 uoE... for this person, of course.
Statistically that is a bit weak, so the same procedure is made for a group of minimum 4 people. In this case, the geometric mean of the individual odour thresholds is calculated and this way we get the odour concentration of an odorant. The important fact to take into account here is that it does not matter the people that participates in the measurement, it does not matter if they are male or female, young or not so young, if they are from China or from Belgium, the result will be repeatable and reproducible within an uncertainty range.
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.
Also
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):
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.
Odour intensity is determined in ambient air by means of odour inspections. There are a few standards on the calculation of odour intensity in ambient air such as the ASTM E544-18 or the much used VDI Guideline 3882 sheet 1.
The following table illustrates the different levels of intensity that can be registered in a field inspection according to the VDI 3882.1.
| Odour | Intensity 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 characterized 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 recognize an odour and the total measurement duration.
Issues with ambient odours could be annoying and exhausting for the people involved in this context. For the plant responsible of the odour emission, for the people that is exposed to environmental odours and for the authorities responsible of mantaining an adequate vital and equilibrated ambient air at work. 
