First, it must be understood that an odour is a perception. For the same mixture of chemical compounds, humans will have very different perceptions. For the purpose of this Blog, we will focus on odour concentration and how humans perceive its intensity.
Odour concentration and intensity are two different dimensions of an odour. More generally, there are four (4) different dimensions to describe each odour:
Odour Concentration: It is based on the detection threshold of an odour. It corresponds to the lowest concentration at which 50% of the human population would detect an odour. This concentration is defined as 1 odour-unit per cubic meter (o.u./m3) for that specific odour. Olfactometry is used to determine odour concentrations.
Odour Intensity: It is the perceived strength of an odour above its threshold. It is determined by an odour panel and is described in categories which progress from “Not perceptible”; “Very Weak”; “Weak”; “Distinct”; “Strong”; “Very Strong” to “Extremely Strong”. These categories are benchmarked using n-butanol as the reference odorant (ASTM E544).
Hedonic Tone: It is the degree to which an odour is perceived as pleasant or unpleasant. Such perceptions differ widely from person to person and are strongly influenced by previous experiences and emotions at the time the hedonic tone is evaluated.
Odour Character: It is basically what the odour smells like. It allows one to distinguish between different odours. For example, ammonia gas has a pungent and irritating smell. The character of an odour may change with dilution.
Hedonic Tone and the Odour Character are highly subjective and dependent on the people evaluating an odour. Odour character can be made less subjective if a standard list of descriptors is used. On the other hand, odour concentration and odour intensity are usually considered easier to quantify by regulators so used as part of odour methodology guidelines to evaluate nuisances. The frequency and duration of exposure to an odour and the location where it was perceived are also frequently part of nuisance evaluation.
In summary, five (5) parameters may be used by regulators to define the impact of an odour or its level of nuisance:
• Odour concentration
• Odour intensity
• Frequency of exposure
• Duration of exposure
• Location where the odour was perceived
Some odours are perceived as being stronger than others so their intensity is different. However, they all will be just detectable at a concentration of 1 o.u./m3. In other words, at 2 o.u./m3, some odours may be perceived as “Very Weak” while others may be perceived as “Distinct”. At 20 o.u./m3, one odour may be perceived as “Distinct” while another may be “Very Strong”.
Using dynamic olfactometry to determine the concentration of an odour and its intensity, a relationship between concentration and intensity can be determined. The Stevens Law and the Weber-Fechner Law are examples of formulae accepted to define the relationship between intensity and concentration for a particular odorant. However, the Weber-Fechner law is more widely used because it is simpler to derive from experimental data.
The Weber-Fechner law is as follows: