On human olfactory sensitivity across odorants and across subjects

We present concentration-detection (i.e., psychometric) odor functions for 28 odorants tested on subgroups from a pool of 132 normosmic, nonsmoker participants (64 female), most of them between 18 and 40 years old (n=123), a few between 41 and 59 years old (n=9, 2 female). Odorants included n-alcohols (n=4), acetate esters (n=4), 2-ketones (n=4), alkylbenzenes (n=5), aldehydes (n=6), and carboxylic acids (n=5). Vapors were presented by dynamic olfactometry. We used a three-alternative forced-choice procedure against carbon-filtered air blanks, and an ascending concentration approach. During subject testing, gas chromatography (flame ionization detector) served to quantify delivered concentrations. The results from 12 subjects (8 female) tested on 10 or more odorants showed strong across-subject agreement regarding the least potent (highest odor detection thresholds) and the most potent (lowest thresholds) odorants. Odor functions from 6 subjects (3 female) tested on a common set of 6 odorants, agreed fully on which were the least and the most potent odorants, and agreed partially on the odorants of intermediate potency. Across these 6 subjects, the odor threshold range between the least and the most potent odorant covered 2 to 3 orders of magnitude. For each of the 6 odorants in common, the ratio between the least and the most sensitive of the 6 subjects was at or below 1 order of magnitude. All odorants had thresholds below 1 part per million by volume (ppm). Notably, carboxylic acids with 4 or more carbons in the chain length, and the aldehydes were the most potent olfactory stimuli with thresholds often below the part per billion (ppb) level