FIG. 5.26.1 Odor threshold ratio.
5.26.1 where a 50% panel response occurs at 235 D/T for one sample and 344 D/T for the other sample.
Over the years, many terms have been used to express the concentration of odor including the following:
ODOR UNIT—one volume of odorous air at the odor threshold; often the volume is defined in terms of cubic feet as follows:
odor units volume of sample diluted to threshold (cu ft)
cu ft original volume of sample (cu ft)
ODORANT QUOTIENT—Expressed by the following equation. The Z is for Zwaardermaker who was the earliest investigator to use dilution ratios for odor measurement (ASTM 1993).
Co odorant concentration of a sample
Both odor units/cu ft and Zt are dimensionless and are synonyms of the D/T; the odor unit is a unit of volume and is not a synonym of D/T (ASTM 1993; Turk 1973).
Another term, the odor emission rate, is often used to describe the severeness of the downwind impact, i.e., the problem a typical odor source creates at downwind locations. The odor emission rate, expressed in unit volume per minute, is the product of the D/T value of the odorous air and the air flowrate (cfm or its equivalent).
Listings of threshold values for pure chemicals are common in many environmental resource references. However, the reported threshold values are limited and vary as much as several orders of magnitude, suggesting that thresholds are limited in usefulness. However, a review of threshold value methodology finds that when threshold values are subjected to basic methodological scrutiny, the range of experimentally acceptable values is considerably reduced. Two recent documents of odor threshold reference guides are available. One published by the American Industrial Hygiene Association (1989) is based on a review for 183 chemicals with occupational health standards. The other document is published by the U.S. EPA (1992). It compiles odor thresholds for the 189 chemicals listed in the CAAAs.
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