While odor issues are often addressed on a case-by-case basis when nuisances occur, state laws and local ordinances are increasingly adopting specific odor guidelines and criteria. Some jurisdictions have even established odor testing protocols and air dispersion modeling practices tailored specifically for odor. Hence, odor measurement, modeling, minimization, and management are becoming more critical for industry.
Many jurisdictions in North America define odor criteria using a relative strength scale in terms of “odor units,” or simply, OU. As a reference, one odor unit (or 1-OU) represents the level where 50% of the population can begin to detect an odor. For example, if an established odor criteria was 7-OU, then the odors present in the air would be at a strength that would require 7 dilutions with 'clean' air to meet the threshold where half the population could no longer smell the odor.
Odor is inherently complex and difficult to quantify because it is often caused by a mixture of chemical substances. Odor is also quite subjective by its nature and can affect individuals differently. Therefore, odor assessments typically employ FIDOL observations to further characterize impacts.
Frequency - how often the odor impacts occur
Intensity - the relative odor strength (faint to overwhelming)
Duration - the length of time for a given odor event
Offensiveness - the character or description of the odor
Location - mapping impact location and identifying potential off-property contributing sources
Trinity conducts ambient field studies using olfactometers using pre-screened, certified assessors. (Pre-screened assessors fall into a specific range of odor tolerance to ensure assessors are not overly sensitive or insensitive to odor.) We also use atmospheric dispersion models such as AERMOD for assessing odor impacts and compliance at sensitive points of reception. Odor modeling assessments typically employ a source testing component to quantify odors directly at the source. Samples collected at the source are sent to a certified odor panel for analysis.