Air Emissions and Animal Agriculture


Odor measurement is a complicated task. While a number of methods are available, none are without drawbacks. Current odor measurement methods use human panels for assessment. Work has shown that the same panelist’s response from one day to the next can vary by as much as three-fold, possibly due to health or mood of the individual. Variability in the sensitivity of the individual conducting the evaluation and odor fatigue are further concerns that are commonly addressed in procedural protocol. Odor fatigue is a temporary condition where a person becomes acclimated to an odorant or odor to the point that they are no longer aware that the odor is present. An example would be when you walk into a barbeque restaurant and by the time you leave, you are unaware of the aroma that attracted you in the door. On-site methods are complicated by the influence that visual perception might have in an evaluation (smelling with your eyes, so to speak). Each of us has a unique odor acuity. While methods try to minimize panelist variation, the difference in sense of smell from one person is another consideration in human assessment methods.

Dilution-to-threshold methods are the most widely accepted methods at the current time. The measurement of odor concentration by dilution is more direct and objective than that of odor quality or intensity. Dilution-to-threshold techniques dilute an odor sample with odorless air at a number of levels and the dilution series is presented in ascending order of odor concentration.  From one level to the next, the dilution decreases and the amount of odorous air increases.  The first few levels include the sample diluted with a large amount of odorless air so evaluation can begin below the threshold of detection. This method, too, requires the use of the human nose as a detector, so not one is completely objective.  The AS’CENT International Olfactometer® (St. Croix Sensory, St. Elmo, MN) allows samples to be presented at 14 dilutions that represent a range in dilution-to-threshold of 8 to 66,667. These units are often used in a laboratory setting and a number of panelists (7 to 10) evaluate each sample rather than the small number of evaluators that are used in the field measurements. Researchers have identified the imprecision that results from the large difference between the dilution levels that might exist in dilution-to-threshold methods as a concern as well. Use of a forced-choice method, such as that used with dynamic olfactometers, is generally a better method than ranking (an arbitrary scale is used to describe either the intensity or offensiveness of an odor), as the human nose cannot distinguish small differences between levels of intensity. When a forced-choice method is used, a panelist, typically trained to conduct these evaluations, must simply identify the presence or absence of an odor, even if it is a guess.

The AS’CENT International Olfactometer
Photo courtesy of St. Croix Sensory

Efforts are underway across the United States to develop evaluation methods that can be used on-site and without the influence of human subjectivity with the goal of providing an objective and affordable means of quantifying odors. In the interim, human assessment methods remain standard procedure.


Last Updated 05/30/2011