Odor perception is subjective and is affected by the individual’s experience and physical and emotional sense of odor. To quantify odor levels, two kinds of measurement have been used: (1) measurement of concentration of single or group gases, and (2) human reaction to the comprehensive contribution of odorous gases. Because of the complexity of the odor composition, using concentration of single or group of gases to represent odor level has not been proved to fully estimate the presence or level of odors. Currently, olfactometry is considered as the most accurate method to assess odor level.
Odor can be qualitatively described by its characteristics, such as mint smell, rotting smell, etc. The olfactometry method uses a panel of humans to quantify odor as to its concentration or intensity, persistence, and hedonic tone. Odor concentration is defined as Odor Unit per cubic meter air volume (OU/m3). Odor Unit is ratio of the volume of clean air to the volume of odorous air sampled. The clean air is used to dilute the odorous air to either detection or recognition threshold levels at which 50% of the panelists can detect or recognize the odor.
Odor intensity describes the strength of an odor sample and is measured at concentrations above the detection threshold. Persistence is a calculated value to measure how easily the full-strength odorous air can be diluted to the detection threshold level. Hedonic tone describes the unpleasantness or pleasantness of an odor. It is typically rated using a scale that ranges from -10 to 10, representing from most unpleasant to most pleasant.
Besides the olfactometry method, the scentometer is a simple, hand-held device to measure ambient odor level in the field. A trained individual can use it to measure odor concentration or intensity in the field. The scentometer is simple to use, but is not high in accuracy.
A field sniffer is another way to measure ambient odor intensity. Trained sniffers calibrate their noses in a lab to establish consistent intensity detection levels. The sniffers sniff the air to determine the odor intensity. They wear charcoal filter masks to breath non-odorous air between measurements.