An odour is defined as a sensation resulting from the reception of a stimulus by the olfactory sensory system. The way the human response to an odour is evaluated depends on the particular sensory property that is being measured, including the intensity, detectability, character, and hedonic tone of the odour. The combined effect of these properties is related to the annoyance that may be caused by the odour.
Odour intensity is the strength of the perceived odour sensation. It is related to the odorant concentration, which is an entirely different type of measurement. The following equation defines the relationship between the odour intensity (I) and concentration (C), where k is a constant and n is the exponent.
I (perceived) = k(C)n
Log I = log K + nlog (C)
This is known as Stevens's 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. This is an important concept that is related to the basic problem of reducing the odour intensity of a substance by air dilution or other means.
Odour detectability or threshold is a sensory property referring to the minimum concentration that produces an olfactory response or sensation. This threshold is usually determined by an odour panel consisting of a specified number of people. The numerical result is typically expressed as occurring when 50 per cent of the panel correctly detect the odour. With odour intensity levels at or just above 'threshold', odours become difficult to perceive. As a result, the actual values depend on the type of sensory test, the panellist selection, the detectability criterion, and other factors.
Odour characterOdour character or quality is that property that identifies an odour and differentiates it from another odour of equal intensity. 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.
Hedonic tone is a property of an odour relating to its pleasantness or unpleasantness. A distinction should 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.