Airborne substances can stimulate both the olfactory and the trigemina
l nerve in the nose, giving rise to odor and pungent (irritant) sensat
ions, respectively. Nose, eye, and throat irritation constitute common
adverse effects in indoor environments. We measured odor and nasal pu
ngency thresholds for homologous aliphatic aldehydes (butanal through
octanal) and carboxylic acids (formic, acetic, butanoic, hexanoic, and
octanoic). Nasal pungency was measured in subjects lacking olfaction
(i.e., anosmics) to avoid odor biases. Similar to other homologous ser
ies, odor and pungency thresholds declined (i.e., sensory potency incr
eased) with increasing carbon chain length. A previously derived quant
itative structure-activity relationship (QSAR) based on solvation ener
gies predicted all nasal pungency thresholds, except for acetic acid,
implying that a key step in the mechanism for threshold pungency invol
ves transfer of the inhaled substance from the vapor phase to the rece
ptive biological phase. In contrast, acetic acid - with a pungency thr
eshold lower than predicted - is likely to produce threshold pungency
through direct chemical reaction with the mucosa. Both in the series s
tudied here and in those studied previously, we reach a member at long
er chain-lengths beyond which pungency fades. The evidence suggests a
biological cutoff, presumably based upon molecular size, across the va
rious series.