How are odour threshold and odour concentration related?
The odour concentration of an odour sample (single compound or mixture) is defined as the dilution ratio at which the odour threshold level is reached during olfactometry testing. At the odour threshold, 50% of panelists in olfactometry analysis respond to the odour and 50% do not. In practice, odour concentration is calculated from a series of panel responses to samples presented by olfactometer over a range of dilution ratios.
In the draft European and Australian standards the unit adopted for odour concentration measurement is expressed in terms of odour units per cubic metre.
Three different threshold levels are commonly used in environmental odour measurement on the basis of panel responses in olfactometry testing. These are the "guessing threshold", the "detection threshold" (or certainty threshold) and the "recognition threshold". The value of the detection threshold is generally 3 - 5 times higher than the guessing threshold and the recognition threshold generally 3 -10 times higher than the detection threshold. Although the guessing threshold is more sensitive than the detection threshold, the detection threshold can be measured with better repeatability (within a laboratory) and better reproducibility (between laboratories) and so is preferable for environmental assessment purposes.
Furthermore the odour (detection) threshold of a single compound can be calculated by dividing the chemical concentration of the compound by the odour concentration determined by olfactometer. The odour (detection) threshold of a single chemical compound may be expressed in chemical concentration units. For example, the odour (detection) threshold for n-butanol is 40 ppb.
In summary, the odour concentration of a sample indicates the number of dilutions required to reach its odour detection threshold level. Odour threshold should only be used for a single compound.