Odor Threshold

The responses of panelists using the olfactometer are recorded and statistically evaluated to determine the effective dilution (ED) at which 50% of the panelists can detect the presence of the odor. This dilution is called the ED₅₀ or odor threshold.

The odor strength represented by the ED₅₀ is frequently expressed as "Z units". When multiplied by a flow rate, it can be used to rank the sources in order of odor emission rate. The terms "Z unit" and "odor emission rate" have no relation to the quantity of odorous material being emitted. The ED₅₀ also represents an estimate of the dilution or removal factor needed to reduce the odor emissions at the point of sampling to a level detectable by 50% of the population.

Odor Intensity and Pervasiveness

To determine how odor intensity will be affected by dilution or removal (if the removal is proportional to the concentration), it is necessary first to plot the butanol rating determined by panelists using the butanol wheel versus odorant concentration or dilution, and then to measure the slope. Using Steven's psychophysical equation:

I = aCƒ~

Where:

I = odor intensity
a = unit dependent constant
C = concentration or dilution
ƒ~ = slope characteristic of the odor being evaluated, pervasiveness

We can calculate the slope of the butanol rating, as well as the intensity of the undiluted odor in terms of the part per million by volume (ppm V) butanol concentrations. The larger the slope, the more readily will the intensity be reduced by dilution.

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For more information:
2 South 631 Route 59, Suite B
Warrenville, Illinois 60555
630.393.9000
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