PERCEPTUAL CHARACTERISTICS OF BINARY, TRINARY, AND QUATERNARY ODOR MIXTURES CONSISTING OF UNPLEASANT CONSTITUENTS

Among the most obnoxious stimuli that the population at large is expos ed to during everyday life are odorous emissions from sewage treatment plants. Such emissions are complex and contain many different types o f odorants that vary in quantity depending upon the contents and effic iency of treatment processes. Because little is known about how indivi dual odorants in complex mixtures affect the perception of each other, it is difficult to develop mathematical models that can predict the p leasantness, strength, and quality characteristics of an emission at d ifferent distances from a source. In the present study, the interactio ns of the four major types of odorants emitted by treatment plants wor ldwide, namely, hydrogen sulphide, isovaleric acid, butanethiol, and s katole, were investigated by measuring the perceived intensity of indi vidual odorants alone and in mixtures, and the overall perceived inten sity, unpleasantness, and qualities of mixtures. In addition, models f or predicting odor strength were investigated. The results indicated t hat (i) the perceived odor intensity (odor strength) of mixtures of th e odorants was equal or greater than that of any of the individual con stituents, but less than the sum of their intensities. However, as the number of components in a mixture increased, the intensity of the mos t dominant component provided a good approximation of the intensity of the mixture. (ii) The vector model of intensity summation also satisf actorily predicted the odor intensity of mixtures containing hue, thre e, or four of the odorants investigated. (iii) In no instance was the intensity of one odorant enhanced by another, i.e., no synergistic int eractions occurred; the greater the number of odorants in a mixture, t he greater was the degree of suppression of the individual constituent s. (iv) The greater the number of constituents in a mixture the more d ifficult it became to identify individual constituents. (v) Hydrogen s ulphide was the least frequently suppressed constituent, and isovaleri c acid and skatole were the most frequently suppressed constituents in mixtures. (vi) The unpleasantness of mixtures was usually greater tha n that of the individual constituents, indicating that models used for predicting complaint levels in communities affected by sewage odor an d based on assumptions related to a single odorant, e.g., hydrogen sul phide, will underestimate the number of complaints. Even mixtures with low but above threshold concentrations of these odorants are likely t o generate complaints.