Section I of this Thesis presents the stimulus-response (i.e., psychophysical) functions for the total nasal perceived intensity of two pungent odorants, formaldehyde and ammonia, presented either alone or mixed with varying concentrations of the other irritant. Stimuli comprised formaldehyde at 1.0, 3.5, 6.9, and 16.7 ppm; ammonia at 210, 776, 1,172, and 1,716 ppm; and their 16 binary mixtures. At low, medium, and high concentrations within the ranges selected, the total perceived intensity of the mixtures was, respectively, significantly lower than (hypoadditivity), equal to (simple additivity), and greater than (hyperadditivity) the sum of the intensities of its individual components. In Section II, subjects were again asked to rate the total nasal perceived intensity of the previous stimuli, but now they also rated the olfactory (i.e., odor) and the trigeminal (i.e., pungency or irritation) attributes of the evoked sensations. Psychophysical functions for pungency were steeper than those for odor. Furthermore, odor was always hypoadditive in mixtures, whereas pungency was, mainly, additive, and even reached hyperadditivity. Total nasal perceived intensity of the stimuli, singly and in mixtures, followed the stimulus-response patterns for pungency, which, therefore, emerged as the dominating attribute used by the subjects to rate the explored range of concentrations. In turn, the relationship between total nasal perceived intensity of the mixtures and the sum of the intensities of their components reflected hypoadditivity, resembling the outcome for the odor attribute.

This report addresses the topic of sensory evaluation of indoor air through the use of human subjects. It begins by discussing the chemical senses involved in such evaluation, specifically the senses of smell (olfaction) and chemical sensory irritation (common chemical sense, CCS, now called chemesthesis). An analysis of similarities and differences between these two sensory modalities regarding key measurements and issues follows. Later, the report discusses the quantification of sensory reactions to indoor pollutants, including aspects related to subjects, exposure techniques, sensory methodology, quality assurance, and data evaluation. An Appendix presents the results of a survey of opinion regarding a tentative protocol for sensory evaluation of emissions from indoor activities and materials, based on the responses of 16 out of the 25 indoor air researchers initially contacted to fill out the questionnaire.

We have applied a quantitative structure-activity (QSAR) approach to analyse the chemical parameters that determine the relative sensitivity of olfaction and nasal chemesthesis to a common set of volatile organic compounds (VOCs). We used previously reported data on odor detection thresholds (ODTs) and nasal pungency thresholds (NPTs) from 64 VOCs belonging to seven chemical series (acetate esters, carboxylic acids, alcohols, aliphatic aldehydes, alkylbenzenes, ketones, and terpenes). The analysis tested whether NPTs could be used to separate out “selective” chemosensory effects (i.e., those resting on the transfer of VOCs from the gas phase to the receptor phase) from “specific” chemosensory effects in ODTs. Previous work showed that selective effects overwhelmingly dominate chemesthetic potency whereas both selective and specific effects control olfactory potency. We conclude that it is indeed possible to use NPTs to separate out selective from specific effects in ODTs. Among the series studied, aldehydes and acids, except for formic acid, show clear specific effects in their olfactory potency. Furthermore, for VOCs whose odor potency rests mainly on selective effects, we have developed a QSAR equation that can predict their ODTs based on their NPTs.

Odor detection thresholds, that we have previously obtained, have been analysed by a general equation for selective transport. It is shown that such selective transport can account for some 77% of the total effect. The remainder is due to a specific size effect, that might involve odor-binding proteins, and a specific effect for aldehydes and carboxylic acids. Our analysis raises the question of whether selective transport is physically separable from the specific effects of receptor activation. The model predicts a chemical cut-off in odor detection along any homologous series.

One hundred and ninety three odor detection thresholds, ODTs, obtained by Nagata using the Japanese triangular bag method can be correlated as log (1/ODT) by a linear equation with R² = 0.748 and a standard deviation, SD, of 0.830 log units; the latter may be compared to our estimate of 0.66 log units for the self-consistency of Nagata’s data. Aldehydes, acids, unsaturated esters and mercaptans were included in the equation through indicator variables that took into account the higher potency of these compounds. The ODTs obtained by Cometto-Muñiz and Cain, by Cometto-Muñiz and Abraham, and by Hellman and Small could be put on the same scale as those of Nagata to yield a linear equation for 353 ODTs with R² = 0.759 and SD = 0.819 log units. The compound descriptors are available for several thousand compounds, and can be calculated from structure, so that further ODT values on the Nagata scale can be predicted for a host of volatile or semivolatile compounds.

A general equation set up for the study of transport properties has been applied to VOC nasal pungency and eye irritation thresholds, as log (1/NPT) and log (1/EIT). The equation accounts for 93-95 % of the total effect, thus suggesting that the main step is transport of the VOC from the gas phase to a receptor phase. In the case of odor thresholds, ODT, the general equation accounts for only 77% of the total effect. A modified equation incorporates a factor for aliphatic aldehydes and carboxylic acids, and a general size effect that depends on the VOC maximum length. The size effect is important in that it leads to a cut-off effect that greatly reduces the potency of higher homologues. The various equations can all be used to predict chemosensory thresholds for thousands of VOCs.