286 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS When it is possible to control the intensity of the stimulus and also mea- sure the resulting magnitude of the response, sufficient information becomes available to determine how the apparatus of the system operates. If this information is combined with that obtained from gross and microscopic dissection of the organ complex, one is usually able to develop a model which can be studied in the laboratory. It is at this stage in the process of evolution of knowledge that the responses normally expected from a given stimulus can be predicted. We can then prescribe corrective measures to return the system to normal, or we can control a system away from normal if that is our desire. Because it has been possible to measure the stimuli and the responses, we have an understanding of the workings of the systems in the body which detect light, sound, pressure, heat, and, to a lesser degree, solid and liquid chemicals. The advancement of our knowledge of the operation of the sys- tem in the body for the detection of chemicals in the form of gases and vapors has been limited. We have not been able to monitor the responses in the olfactory apparatus directly. The complexity of the olfactory system together with its proximity to the brain have limited our ability to envision a model of the system. The number and ranges of concentration of the stimulants is so great that it has been impossible to determine the operation of the system through a knowledge of its limits. We are faced with a rather unique problem. We have too many stimuli to deal with, too much sensitivity in the organ, too wide an operating range, and too little objective output data to make any analysis of the process of oilaction. We have been required to make appraisals of odors in the intact system, i.e., through the judgments of persons trained to distinguish par- ticular odor signals in odor noise backgrounds that are at times in ratios of less than unity. Two alternative approaches to the detection of odors have been of value in the appraisal of their type and intensity. These are: objective methods and dynamic analytical methods. Within the past ten years, objective methods for the qualitative (and to a more limited extent, quantitative) evaluation of chemicals in gases and vapors have developed rapidly. The applicability of these techniques to the detection of type and magnitude of oral odor will be discussed later. The second approach results from the study of the dynamics of the physiology of sensation in the body. When it is not possible to utilize the method of evaluation, of prin- ciples of operation of a body function as outlined earlier, a study of the dynamics of the system in operation can be undertaken. While it is not possible to obtain exact amounts from a subjective study, it is possible to obtain reliable information from the first derivatives of these values, i.e., while we cannot know from a human judgment an exact value of a particular response, we can obtain accurate information about the rate of change in the response as judged by the subject. This is because the body utilizes

EVALUATION OF ORAL ODOR 287 the method of comparison of changes in the amount rather than the quanti- fication of exact amounts, which is the mechanism used by all but a few of our most recently developed instruments. It is this form of combined ratio and rate detection that makes it possible to select small stimulant signals from a background of several stimulants of high intensity striking the organ simultaneously (the signal to noise ratio of less than unity men- tioned above). In the case of oilaction, this is important for the survival of the organism. It is equally essential for a worker to detect the odor of smoke in a laboratory heavily laden with odor of an organic solvent as it is for the deer in a thick pine forest to detect the faint odor of an enemy through the pine scent. This highly developed differential sensing device enables us to sense minute quantities of chemical stimulants. We have no mechanism by which we can shut off the sensation of odors which are not pleasing to us, and this in fact is another protective device of nature to cause us to leave an area of danger. The olfactory system operates on a dif[k:rential system so that, while we are not spared the unpleasantness of detecting an odor which is objectionable to us, we become adapted to it if we must remain in its environment for extended periods. As we shall see, the olfactory system is most sensitive to the first appearance of an odor or to odors presented intermittently for short periods of time. Any odor generator which produces odors in small amounts and in an intermittent manner will be highly efficient from the standpoint of the amount of product (stimulant) used to obtain the greatest response in the olfactory sensors. This fact has its implications, for example, in the economics of perfume sales. If one wishes to disperse the scent of an expensive perfume in a display atomizer, the perfume should be ejected in extremely small amounts with sufficient intervals between the ejections to allow for almost complete theoretical dispersal of the odor throughout the area of sale. The oral cavity is one of the best examples of a highly efficient natural odor generator, one in which amount of stimulant chemical is small with re- spect to the olfactory response. The oral cavity generates odors, some of which we sense as pleasant and others of which we categorize as definitely un- pleasant. It is the efficiency of the oral cavity as an odor generator, to- gether with its frequent proximity to another's olfactory detection apparatus in our present crowded society, which would make each aware of objection- able oral odors unless some form of restraint had been placed upon the generation of odors within the oral cavity. SUBJECTIVE METHODS In order to develop products which reduce the detectability of oral odors, it is necessary to measure them. The human nose is still the most versatile detector available to us in the present state of our odor detecting art. We