Threshold, or detectability, refers to the theoretical minimum concentration of odorant stimulus necessary for perception in a specified percentage of the population, usually the mean. A threshold value is not a fixed physiological fact or a physical constant but is a statistical point representing the best estimate from a group of individual scores. Two types of thresholds can be evaluated: detection and recognition.

The detection threshold is the lowest concentration of odorant that elicits a sensory response in the olfactory receptors of a specified percentage, usually 50%, of the population being tested (see Figure 5.26.1). The detection threshold is the awareness of an odor without necessarily recognizing it.

The recognition threshold is the minimum concentration recognized as having a characteristic odor quality by a specified percentage of the population, usually 50%. It differs from the detection threshold because it is the point at which people can describe a specific odor character to the sensory response.

In the measurement of environmental odors, which are generally complex mixtures of compounds, the threshold is not expressed as a concentration level. Instead, the threshold is expressed as dilution-to-threshold ratios (D/T); it is dimensionless. A D/T ratio of 100 means that a given volume (i.e., 1 cu ft) of odorous air requires 100 volumes (i.e., 100 cu ft) of odor-free air to dilute it to threshold, or a barely detectable odor. This concept is shown in Figure

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