Odor intensity relates to the degree of strength and magnitude of the observed ambient air. ASTM E544-991, Standard Practice for Referencing Suprathreshold Odor Intensity, presents two methods for referencing the intensity of ambient odor or a sample of odorous air: Procedure A - Dynamic Scale method and Procedure B - Static Scale Method. The Dynamic Scale Method utilizes a laboratory olfactometer device with a continuous flow of a standard odorant for presentation to a panelist (assessor). The panelist compares the intensity of an odor sample (gas sample bag) to a specific concentration level of the standard odorant from the laboratory olfactometer device. The Static Scale method utilizes a set of bottles with fixed dilution's of the standard odorant in a water solution. Enforcement field inspectors and facility or citizen odor monitors commonly use the Static Scale method. The Static Scale Method has also been incorporated as a standard of practice by a number of odor laboratories, because of its low cost of set-up compared to an olfactometer device.
The dilution levels for each procedure are determined by the specific olfactometer device or from interpretation of the ASTM Procedure 'B' scale choices. Several scales are in practice, differing by the starting point of the scale, the geometric progression of the scale, and the standard odorant used (i.e. n-butanol or sec-butanol).
For the enforcement field inspector an odor intensity scale is a valuable tool for verifying compliance or determining whether a specific odor episode has exceeded a pre-set limit in the community. Facility or community monitors equally find odor intensity scales essential in their work to document facility performance or track facility compliance.
Laboratory testing of odor samples utilizes odor intensity for determining the dose-response function (persistency) of a sample, which is useful in ranking sources and conducting odor modeling.
This paper presents the relationship between the intensity scales commonly used and further describes their use and applicability to enforcement of odor regulations and verification of odor compliance plans.