Human Olfaction Measurement

Besides the stimulus, which is a mixture of volatile compounds at given concentrations, the processing of the odour information by the brain is rather complex and leads to a multidimensional sensation. Three main dimensions can be considered the intensity, or the strength of the odour, the quality, or the nature of the odour and the hedonic tone, or the affective reaction to the odour. 

Intensity refers to the perceived strength of the odour sensation. To generate such perception, the physical stimulus, i.e. the mixture of odorous molecules, must be detectable. That means that its concentration must exceed a given threshold. 

The relationship between the perceived intensity and the concentration of the stimulus, or the odour concentration, is non-linear and depends on the nature of the odorants. 

Two famous psychophysical laws express this relationship the Stevens and the Weber-Fechner laws (Nicolas 2001, Misselbrook et al. 1993, Sperber et al. 2003). 

The intensity can be measured by ranking the odour impression on a predetermined scale or by comparison with a series of samples of known concentration of a reference substance (see for example VDI guideline 3882 -1997- Determination of odour intensity). 

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Human olfaction measurement considers the odour as a global concept and provides the true dimensions of the human perception. Yet, physiological differences in the smelling of various people often lead to subjective results with large uncertainties. Analytical techniques identify the various volatile compounds involved in the odour and give their chemical concentration. They have better scientific standing than sensory methods. However, the chemical composition of the gas mixture doesn t represent the odour perception. 

Because of the relative ease of measurement of many of nitrobenzene s properties and its ready detectability by both chemical analysis and human olfaction (sense of smell), its release, transport and fate, and the consequent exposure of human beings have been studied over a considerable period of time. Thus, the potential for human exposure to nitrobenzene is better understood than that of many other chemicals. 

In this framework, the state-of-the-art generally reports two complementary types of measurement methods human olfaction methods and analytical techniques (Van Harreveld 2003, Hammers et al. 2004, Stuetz et al. 2001). 

It has to be remarked that in spite of the widely accepted term electronic nose, current devices are still far from the structure and functions of natural olfaction sense. The unique common feature between artificial and natural system is that both are largely based on arrays of nonselective sensors. The concept underlying electronic nose systems has been demonstrated to be independent on the particular sensor mechanism indeed during the last two decades almost all the available sensor technologies have been utilized as electronic noses. Clearly, all these sensors are very different from the natural receptors. These dissimilarities make the perception of electronic nose very different from that of natural olfaction, so that the instrumental perception of the composition of air cannot be called odor measurement because odor is the sensation of smell as perceived by human olfaction. Nonetheless, the term odor analysis with electronic noses is now largely adopted, but it is important to keep in mind, especially in medical applications, that the electronic nose measurement may be very distant from the human perception. 

Stearic acid was successfully delivered to the human oral cavity by emulsions at 67-69 °C where this stimulus is in liquid form [4, 22]. Detection thresholds were identified by orthonasal olfaction, retronasal olfaction, gustation, and a multimodal presentation where the lipid emulsion was placed in the oral cavity in the absence of nose clips [4]. Although measured at different temperatures, intensity responses for stearic acid were similar to the 18-carbon cis- unsaturated fatty acids linoleic and oleic acid. Oral detection thresholds for stearic acid in the human oral cavity with emulsions, yielded thresholds near 0.032% (w/v) [4, 22]. In addition, most study participants were able to detect stearic acid in the oral cavity [4,22]. 

Several eore questions emerge When is the potential coimeetion between the human oral and nasal eavities open, and when is it closed Can odorants known to be present in the human oral cavity be detected by physical means in the nasal cavity, and, if so, under what circumstances As a measure of overall function of retronasal olfaction, with what accuracy can human observers identify odorants originating in the oral cavity ... 

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