Perceptual characteristics of binary, trinary, and quaternary odor mixtures consisting of unpleasant constituents

Among the most obnoxious stimuli that the population at large is exposed to during everyday life are odorous emissions from sewage treatment plants. Such emissions are complex and contain many different types of odorants that vary in quantity depending upon the contents and efficiency of treatment processes. Because little is known about how individual odorants in complex mixtures affect the perception of each other, it is difficult to develop mathematical models that can predict the pleasantness, strength, and quality characteristics of an emission at different distances from a source. In the present study, the interactions of the four major types of odorants emitted by treatment plants worldwide, namely, hydrogen sulphide, isovaleric acid, butanethiol, and skatole, were investigated by measuring the perceived intensity of individual odorants alone and in mixtures, and the overall perceived intensity, unpleasantness, and qualities of mixtures. In addition, models for predicting odor strength were investigated. The results indicated that (i) the perceived odor intensity (odor strength) of mixtures of the odorants was equal or greater than that of any of the individual constituents, but less than the sum of their intensities. However, as the number of components in a mixture increased, the intensity of the most dominant component provided a good approximation of the intensity of the mixture. (ii) The vector model of intensity summation also satisfactorily predicted the odor intensity of mixtures containing two, three, or four of the odorants investigated. (iii) In no instance was the intensity of one odorant enhanced by another, i.e., no synergistic interactions occurred; the greater the number of odorants in a mixture, the greater was the degree of suppression of the individual constituents. (iv) The greater the number of constituents in a mixture the more difficult it became to identify individual constituents. (v) Hydrogen sulphide was the least frequently suppressed constituent, and isovaleric acid and skatole were the most frequently suppressed constituents in mixtures. (vi) The unpleasantness of mixtures was usually greater than that of the individual constituents, indicating that models used for predicting complaint levels in communities affected by sewage odor and based on assumptions related to a single odorant, e.g., hydrogen sulphide, will underestimate the number of complaints. Even mixtures with low but above threshold concentrations of these odorants are likely to generate complaints.