Odor intensity refers to the perceived strength of the odor sensation. Intensity increases as a function of concentration. The relationship of the perceived strength (intensity) and concentration is expressed by Stevens (1961) as a psy-chophysical power function as follows:


S = perceived intensity of the odor sensation I = physical intensity of stimulus (odorant concentration) n = constant K = constant

This equation can be expressed in logarithm as follows: log S = log K + n log I 5.26(4)


Figure 5.26.2 shows an intensity function on logarithm coordinates of the standard odorant 1-butanol. The slope of the function, also called the dose-response function, varies with the type of odorant. Odor pollution control is concerned with the dose-response function, or the degree of dilution necessary to decrease the intensity. This function can be described by the slope. A low slope value indicates that the odor requires greater dilution for it to dissipate; a high slope value indicates that the intensity can be reduced by dilution more quickly. Compounds with low slope values are hydrogen sulfide, butyl acetate, and amines. Compounds with high slope values are ammonia and aldehydes. This function explains why hydrogen sulfide, butyl acetate, and amines can be detected far away from the odor origin. On the other hand, ammonia and aldehydes cause odor problems at locations near the origin.

Category Scales

Category scales were the first technique developed to measure odor intensity. One widely used scale was developed for a 1930 study of odor used as gas alarms. The scale has the following six simple categories:

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