Odour unit

Odour unit

The odour unit is a unit of measure of odour concentration.  It is defined as follows:

American and Australian Odour Unit (OU)

One odour unit is a dimensionless number where a panel (representing the 50% of the population) is presented odours in decreasing dilution (increasing concentration) until it is detected.  This is termed the detection threshold (DT) and is 1OU.  For example, if a sample had a DT which was diluted 500 times, then the odour concentration in the sample is 500dilutions/1OU of sample = 500OU.  This is expressed as ‘dilutions to threshold’.  Note that the panel can only detect at 1OU (the detection threshold) and not 2OU for example.

European Odour Unit (OUE/m3)

One European Odour Unit, [OUE/m3], is the amount of odourant(s) that, when evaporated into one cubic metre of neutral gas at standard conditions, elicits a physiological response from a panel (detection threshold) equivalent to that elicited by one European Reference Odour Mass (EROM), evaporated in 1 m3 of neutral gas at standard conditions. One EROM is equivalent to 123microg n-butanol.  Subjects are standardized to n-butanol which is the reference material.  When odours are detected at the threshold, it is expressed as a multiple of the reference material.

1 EROM ≡ 123 µg of n-butanol ≡ 1 OUE for mixture of odorants

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