UC San Diego

Nasal chemosensory irritation (i.e., chemesthesis) in humans results form stimulation of the trigeminal nerve. Almost all chemical vapors that produce odor can evoke nasal chemesthesis at higher concentrations, although there is a cut-off point along homologous series beyond which larger homologs fail to be detected by chemesthesis. The failure seems to rest on some aspect of molecular structure or dimensions rather than on a low vapor concentration. In turn, almost all irritants can also elicit an odor with the arguable exception of carbon dioxide (CO₂). To separate the trigeminal from the olfactory response of the nose, investigators have tested subjects lacking olfaction (i.e., anosmics) and have measured nasal lateralization thresholds, i.e., the ability to localize whether a vapor entered the right or the left nostril when air enters the contralateral nostril. Such ability rests on trigeminal, not olfactory input. Detection of nasal chemesthesis from chemical mixtures reveals additive effects among constituents, particularly at low levels of detectability (but still above chance detection). As a rule, increases in time of exposure decrease chemesthetic thresholds (i.e., enhances sensitivity) and produce higher ratings of irritation intensity. Nasal chemesthesis can produce alterations in respiration, including a reflex, transitory apnea, and reductions in the duration and volume of nasal inhalations. Relative consistency has been found for irritation thresholds among two of three anatomical structures subsumed within “sensory irritation” – i.e., the nose and eye. Thus for predictive toxicology and risk assessment purposes, an argument can be made that measurements using one system can often be extrapolated to the other.