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What is an odor unit? What is the perception threshold?


When measuring odor with an olfactometer, panelists are exposed to the odor sample via the dilution unit of the olfactometer. Initially, the odor is highly diluted, all panelists indicate that they cannot smell it. The operator then increases the concentration by diluting the sample a little less with pure air in a given, accurate ratio, and the panelists respond. The operator keeps reducing the odor sample dilution until half of the panelists indicate that they can smell the odor, but the other half still cannot. By definition, the point at which 50% of the panelists cannot smell the odor but 50% can, is called the PERCEPTION THRESHOLD and is equal to 1 odor unit per cubic meter.

olfactometer sniffing post resized 600

To find out how many odor units the sample had in the first place, the operator – once the perception threshold has been reached – adds up all the dilutions that were required to reach that threshold; if the sample was diluted, say, 536 times, in order to reach 1 odor unit, then the sample odor concentration was 536 odor units (o.u./m3), initially.

Olfactometry panel

This method is sometimes called dilution-to-threshold, or D/T.

olfactometer EN13725

Olfactometry provides solutions for verifying compliance with standards and regulations, designing waste collection sites, designing equipment for odor mitigation, and for monitoring the respective performance.

  • Environmental odour monitoring
  • Standard product odor compliance
  • Production line quality control: foods, cosmetics, materials
  • Atmospheric odors: automobiles, offices, factories
  • Disaster assessment: fire, flooding
  • Optimization or odor control technologies
  • Medical research


Let me understand. If the odor sample need to be diluted by 50 percent to acheive the perception threshold, does that mean that the odor concentration is 2 o. u./m3 initially?
Posted @ Wednesday, November 03, 2010 3:27 PM by John Bouey
Mr Bouey,  
Actually, the concentrations of the collected samples are typically at much higher values, for example of the order of 500 o.u./m3 on top of a biofilter, 10 000 o.u./m3 in a reception hall, etc. The measurement of the actual concentration (by olfactometry) is based on dilutions of the sample before jury exposure (starting with an extremelly diluted sample), at some point (with reduced dilution factors) half of the jurors will detect the odor, the level of dilution at that point will be taken as the odor concentration. You may be interested in our blog on Olfactometry versus chemical analysis  
Also, be aware that there are established analysis methods (i.e. EN 13725 and ASTM E679-04) that guide the approach.
Posted @ Thursday, November 04, 2010 4:16 PM by Denis Dionne
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