The Five Senses and Beyond: The Encyclopedia of Perception - Jennifer L. Hellier 2017

Odor Intensity Scale

Neuroscientists have identified four dimensions of an odor including odor concentration (an odor’s pervasiveness), hedonic tone (the odor is ranked from extremely unpleasant to extremely pleasant), character (description of the odor to distinguish it from another), and odor intensity. How strongly an odor is perceived through the nose and interpreted by the brain is the odor’s intensity. An odor’s intensity is a property of the olfactory system that helps animals locate an odor’s source. It is also essential for determining if an odor is a nuisance and to warn the animal of potential danger (e.g., spoiled food or a gas leak). Odor intensity is analogous to the perception of temperature sensation in humans, such as identifying whether an object is warm or cold. However, individuals can perceive an odor differently. Just as in the children’s story “Goldilocks and the Three Bears,” an odor may be too strong for one person, barely detectable by another individual, and just right for a third person. Thus, the field of psychophysics was developed to quantify the relationship between a physical stimulus (like an odor) and the perception and sensation it affects in humans.

Psychophysics and Odor Intensity

German physician Ernst Heinrich Weber (1795—1878) was the first person to quantify a human response to a physical stimulus. Using 5-pound dumbbells and slightly changing the weight by half a pound (0.5 pound), he asked individuals if they could detect the difference (if it was heavier). From these experiments, he developed Weber’s law, which states that the just-noticeable difference between two stimuli is proportional to the magnitude of the stimuli and the person’s sensitivity. This means if a person can detect the difference of 0.5 pound on a 5-pound dumbbell, the person should also be able to detect the difference of 1 pound on a 10-pound dumbbell. Weber’s student Gustav Theodor Fechner (1801—1887), who was a German physicist and experimental psychologist, added to Weber’s law to develop a psychophysical scale. This scale described the relationship between the physical magnitude of a stimulus (such as odor, sound, or light intensity) and the perceived intensity by the subject. Fechner’s scale states that subjective sensation is proportional to the logarithm of the stimulus intensity. This scale is now often called the Weber-Fechner law.

In psychophysics, an odor’s intensity is related to its physical concentration and strength and can be measured by the Weber-Fechner law: I = a × log(c) + b, where I is the perceived psychological intensity at the dilution step on the butanol scale, a is the Weber-Fechner coefficient, C is the chemical concentration, and b is the intercept constant (0.5 by definition; Jiang et al., 2006).

Not everyone can quickly calculate the Weber-Fechner law to determine an odor’s intensity, thus neuroscientists have produced a verbal odor intensity scale to describe the odor’s sensation. This numbered scale ranges from 0 to 6 with specific words to describe an odor’s intensity. A score of 0 represents no odor. A score of 1 is a very weak odor and when an odor is first detected, an odor’s threshold. A score of 2 represents a weak odor. A score of 3 is a distinct odor, generally when a person can quickly identify the name or character of the smell. A score of 4 represents a strong odor. A score of 5 means a very strong odor, which for some people can be distracting. Finally, a score of 6 denotes the odor is intolerable, meaning for most individuals it is a very annoying or bothersome smell. Today, this descriptive odor intensity scale is used in the laboratory by highly and suitably trained observers, who can quickly and easily determine the nuances of the scale to identify an odor’s intensity.

Patricia A. Bloomquist and Jennifer L. Hellier

See also: Odor Threshold; Olfactory Mucosa; Olfactory Reference Syndrome; Olfactory Sensory Neurons; Olfactory System; Primary Odors

Further Reading

Alobid, Isam, Santiago Nogue, Adriana Izquierdo-Dominguez, Silvia Centellas, Manuel Bernal-Sprekelsen, & Joaquim Mullol. (2014). Multiple chemical sensitivity worsens quality of life and cognitive and sensorial features of sense of smell. European Archives of Oto-rhino-laryngology, 271(12), 3203—3208. http://dx.doi.org/10.1007/s00405-014-3015-5

Jiang, John, Patrick Coffey, & Brendan Toohey. (2006). Improvement of odor intensity measurement using dynamic olfactometry. Journal of Air and Waste Management, 56, 675—683. http://dx.doi.org/10.1038/17383

Wojcik, Pawel T., & Yevgeniy B. Sirotin. (2014). Single scale for odor intensity in rat olfaction. Current Biology, 24(5), 568—573. http://dx.doi.org/10.1016/j.cub.2014.01.059