Trigeminal and olfactory chemosensory impact of selected terpenes

In Experiment 1, four normosmics and four anosmics (three congenital, one idiopathic) provided odor and nasal pungency thresholds, respectively, for the following terpenes: delta3-carene, p-cymene, linalool, 1.8-cineole, and geraniol, plus the structurally related compound cumene. Additionally, all subjects provided nasal localization (i.e., right/left) and eye irritation thresholis. Trigeminally mediated thresholds (i.e., nasal pungency, nasal localization, and eye irritation) lay about three orders of magnitude above odor thresholds, which ranged between 0.1 and 1.7 ppm. The results implied uniform chemesthetic sensitivity across tasks and sites of impact. In Experiment 2, normosmics and anosmics provided odor and nasal pungency thresholds, respectively, for three pairs of isomeric terpenes: alpha- and gamma-terpinene, alpha- and beta-pinene, and R(+)- and S(-)-limonene. Odor thresholds ranged between 1.4 and 19 ppm, that is, about an order of magnitude higher than those of the previous terpenes, with no substantial differences between odor thresholds of members of a pair. Regarding chemesthetic impact, only alpha-terpinene evoked nasal pungency. The overall outcome suggests comparable trigeminal chemosensitivity between nose and eyes and between normosmics and anosmics, as shown before for homologous n-alcohols. It also lends support to a previously derived solvation model of the chemesthetic potency of airborne substances, and indicates the likely importance of certain molecular-size restrictions for effective trigeminal impact.