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.