Odour is multidimensional. For example, ammonia gas has a pungent and irritating 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.