Olfactometry is the science of measuring smell, bridging the worlds of chemistry and psychology. An odor is a collection of volatile organic compounds (VOCs) that can be chemically quantified. However, the experience of smell is a subjective, biological, and psychological perception that occurs when these compounds interact with human receptors. Measuring smell is necessary for applications ranging from diagnosing medical conditions to controlling environmental pollution and ensuring product quality in the food and fragrance industries. The challenge is translating the physical properties of a chemical mixture into a reliable measure of human sensory experience.
Subjective Measurement: Assessing Human Olfactory Perception
The most traditional and direct method for assessing odor involves human sensory panels. These panels are necessary because instruments cannot fully replicate the human brain’s interpretation of an odor’s quality or pleasantness. Trained panels are used extensively in industrial and environmental testing to measure perceived intensity and the hedonic quality, or pleasantness, of an odor. Their role is to act as a standardized sensor, providing data on how an odor is experienced by people.
These human evaluations are often performed using an olfactometer, a specialized device that precisely dilutes an odorous air sample with odor-free air. The olfactometer presents the diluted sample at specific, controlled concentrations to the human assessors through a sniffing port. The panel members then determine the point at which the odor is just detectable, which is used to calculate the odor concentration in the original sample.
In healthcare settings, standardized tools are used for the clinical assessment of olfactory function, such as diagnosing anosmia or partial loss of smell. These tools involve threshold tests to determine the lowest concentration an individual can detect, and identification tests. The University of Pennsylvania Smell Identification Test (UPSIT) and the “Sniffin’ Sticks” are common examples that require the patient to correctly identify a variety of odors from a multiple-choice list.
Objective Measurement: Instrumental Analysis of Volatile Compounds
Objective measurement methods quantify the chemical components of an odor without relying on human perception, focusing on the volatile organic compounds (VOCs) present. The gold standard for chemical identification is Gas Chromatography–Mass Spectrometry (GC-MS), which separates the complex mixture of VOCs into individual components. Gas Chromatography uses a column to separate the compounds based on travel speed, and the Mass Spectrometer identifies each separated compound by its unique chemical fingerprint.
GC-MS is highly effective at identifying what chemicals are present and in what concentration, but it does not inherently measure how the sample smells to a person. To bridge this gap, a technique called Gas Chromatography-Olfactometry (GC-O) is used, where the separated compounds are split: one stream goes to the Mass Spectrometer for identification, and the other goes to a human sniffer. This allows researchers to correlate a specific chemical compound with its perceived odor quality and intensity.
Another objective technology is the Electronic Nose (e-nose), which is engineered to mimic the non-separative nature of human olfaction. An e-nose employs an array of chemical sensors, each with a broad and overlapping sensitivity to different VOCs. When an odorous sample passes over the sensor array, it creates a unique electrical “fingerprint” or pattern based on the collective sensor responses.
The e-nose does not identify individual chemicals like GC-MS, but rather recognizes the overall pattern of a complex odor mixture. This makes it valuable for quality control and rapid comparison. While the e-nose offers the advantages of consistency and objectivity, it cannot replicate the perception of odor quality, hedonic tone, or the emotional response a human experiences. These subjective attributes remain the domain of human sensory panels.
Converting Sensory Input into Quantifiable Data
Translating the subjective experience of smell into objective, quantifiable data requires standardized metrics and scales. In environmental science, the Odor Unit (OU) or European Odor Unit (OU\(_{E}\)/m³) is the primary metric for odor concentration, based on data collected from human panels using olfactometers. One European Odor Unit (1 OU\(_{E}\)/m³) is defined as the concentration of odorant that 50% of a trained panel can just detect at the detection threshold. If an air sample must be diluted 200 times with neutral air to reach this threshold, the original concentration is 200 OU\(_{E}\)/m³. This metric is widely used in air quality regulation to set acceptable limits for odor emissions.
In human testing, psychophysical scales convert subjective sensory experiences into numerical data. The detection threshold is the lowest concentration of an odorant that can be sensed, while the recognition threshold is the slightly higher concentration at which the odor can be correctly identified. Intensity scales, such as the Labeled Magnitude Scale (LMS), are used to rate the perceived strength of the odor. Hedonic scales ask the panelist to rate the pleasantness or unpleasantness of an odor, often on a scale ranging from “extremely unpleasant” to “extremely pleasant.” These methods are necessary to quantify the subjective attributes of an odor, such as its relative strength and affective quality.