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Sensory Fatigue
Sensory fatigue refers to the inability or decreased ability to perceive an odour. This increases over time as exposure continues. An odour may initially be strongly detectable but may diminish completely in a few minutes. For example, Hydrogen sulphide is an example of a chemical that emits a rotten egg smell. However, depending on concentration and exposure time, the rotten egg smell will no longer be perceived. Olfactory fatigue is not desired during olfactometry analysis or ambient odour monitoring.
During ambient odour monitoring, it is desired to provide rest periods in a neutral environment compared to the one odour is present if possible. An example may include a vehicle, which could act as a separate environment to the one ambient odour monitoring is taking place. Perfumeries use coffee bean odour to ‘reset’ the olfactory sense. Also, having a neutral odourless air to be smelled for a few minutes may help reset the olfactory sense.
Sensory Fatigue vs. Olfactory Fatigue
In practice, these two terms describe the same receptor-level effect and are used interchangeably in odour work. “Olfactory fatigue” simply makes explicit that the effect is specific to the sense of smell, while “sensory fatigue” is the more general term also used in odour standards and panellist training.
Sensory Fatigue vs. Habituation
Habituation is a related but distinct phenomenon. Sensory fatigue is a short-term, receptor-level desensitization that recovers within minutes once exposure stops. Habituation is a longer-term, central-nervous-system-level decrease in response to a repeated stimulus, and it can persist even after the receptors themselves have recovered. It also applies more broadly across the senses, not just smell.
Why the Distinction Matters
During olfactometry (EN13725) and ambient monitoring, both effects can suppress a panellist’s perceived intensity. Recognizing whether a panellist is fatigued, and needs a short rest, versus habituated, and needs a longer break or rotation out of the environment, helps control data quality and keeps measurements representative.