Odour is multidimensional. To completely describe an odour, four different dimensions are considered:

• Odour character allows one to distinguish between different odours. For example, ammonia gas has a pungent and irritating smell. It may be evaluated by a comparison with some known odours (direct-comparison method) or through the use of descriptive words (describing-profile method). The character of an odour may change with concentration level, for example, hydrogen sulfide at levels of 20 ppm or above ceases to be perceived as a "rotten egg" smell.

• Hedonic tone is the degree to which an odour is perceived as pleasant or unpleasant. Such perceptions vary widely from person to person and are strongly influenced by previous experience and the emotional context in which the odour is perceived.

• Odour concentration is the odour strength expressed in terms of the number of times the sample has to be diluted to reach its odour detection threshold. Using CEN terminology, odour concentration may be expressed as the number of European Odour Units in a cubic metre of gas at standard conditions.

• Odour intensity is the relative perceived psychological strength of an odour above its odour detection threshold. Odour intensity represents the increase in sensation intensity experienced by an individual as the chemical concentration increases (ie as the number of dilutions of the environmental sample decrease). Extensive experiments on individual gaseous compounds have shown that odour intensity increases as a power function of chemical concentration within a certain range of chemical concentrations immediately above the detection threshold (Steven's Law). However with increasing concentration, a limit at which the sensation intensity may be considered as "extremely strong" is reached. With further concentration above the "extremely strong" concentration, an individual will experience no further increase in sensation intensity. Similarly and in summary environmental odour samples at dilutions below the detection threshold will be perceived as "not perceptible" (ie as having "no odour"). As the odour concentration rises above the threshold (ie the number of dilutions decrease), the odour intensity ranges through "very week" just above the threshold up to a plateau at the "extremely strong" level.