UC San Diego

We gathered from the literature 47 odor and 37 trigeminal (nasal and ocular) chemesthetic psychometric (i.e., detectability or dose-response) functions from a group of 41 chemicals. Vapors delivered were quantified by analytical methods. All functions were very well fitted by the sigmoid (logistic) equation: y = 1 / (1 + e^{-(x-C)/D}), where parameter C quantifies the detection threshold concentration and parameter D the steepness of the function. Odor and chemesthetic functions showed no concentration overlap: olfactory functions grew along the parts per billion (ppb by volume) range or lower, whereas trigeminal functions grew along the part per million (ppm by volume) range. While, on average, odor detectability rose form chance detection to perfect detection within two orders of magnitude in concentration, chemesthetic detectability did it within one. For 16 compounds having at least one odor and one chemesthetic function, the average gap between the two functions was 4.6 orders of magnitude in concentration. A quantitative structure-activity relationship (QSAR) using five chemical descriptors that had previously described stand-alone odor and chemesthetic threshold values, also holds promise to describe, and eventually predict, olfactory and chemesthetic detectability functions, albeit functions from additional compounds are needed to strengthen the QSAR.