What factors affect individual odour perception?
A number of factors influence the assessment of odour by a panelist:
Adaptation: Fatigue from continued exposure to an odour may affect a panelist's sense of smell. This phenomenon is called adaptation. Adaptation may reduce both perceived odour intensity and perceived odour quality. The degree of adaptation resulting from exposure to an odorous air will depend on the odour concentration experienced. The weaker the odour concentration of an air sample, the more does adaptation affect perceived strength. This is because at a lower concentration it may be necessary to sniff harder and to take more than one sniff in order for the odour to be registered. Continuous exposure to an odour should be avoid during testing. Although adaptation takes some time to develop, recovery takes place more quickly. Recovery times may range from seconds to minutes depending upon type of odour, odour concentration and duration of testing. It has been pointed out that while sensitivity to an odour may decrease after sniffing a sample, 80 to 90% recovery generally occurs within a minute with complete recovery in several minutes.
Adaptation begins to reduce perceived odour intensity and quality during the first inhalation. In olfactometry testing, the order of dilution steps is ascending, from a weaker strength to a high strength. This procedure is designed to avoid adaptation. It is essential that panelists commence at the optimal dilution step, such as two steps lower than the threshold. If the operator starts with the dilution too low (high concentration of odorous compounds), the ability of panelists to make the right choices during the test may be affected.
The phenomenon of adaptation frequently reveals itself in industrial situations, with workers reporting that an initially repulsive odour eventually seems less repulsive.
Anosmia: Anosmia is lack of sensitivity to some groups of odours. Unlike the senses of sight and hearing, for which optometric and audiometric instruments can be used to assess sight and hearing, there is no direct technique to examine loss of the sense of smell and sensitivity to odours. Deficiencies in odour sensitivity and odour perception are often hereditary.
It should be noted that the sensitivity of a panellist to butanol is not necessarily reflected in sensitivity to environmental odours in olfactometry testing. In our experience, some panellists perform poorly on butanol screenings but demonstrate high sensitivity in response to environmental odours. Consequently, normalisation of environmental odour testing results on the basis of butanol can be misleading.
Memory: It is also a common experience that one will smell an odour, recognise that it is familiar and belongs to a general class or category, but be unable to come up with a specific label for it. One reason may be lack of a word to describe the odour. It is suggested that it is difficult to form associations between odours and words. Another reason may be lack of training. Fortunately, the ability to recognize odours is superior to the ability to label them. It was reported that an untrained person can identify by label at least 2000 odours and experts can identify as many as 10000.
Odour mixture: In real life, one rarely smells a pure chemical or odorous compound. The odour as perceived in the brain may be a compound response based on a range of different olfactory receptor stimuli experienced as sensations in the individual's olfactory system. Olfactory processing of mixtures of odours involves the perception of both intensity and quality.
Studies have been undertaken on the perceived intensity of odour mixtures obtained by mixing two odorants, both above detection threshold. Typically, it has been found that the perceived intensity of a mixture is less than the arithmetic sum of the individual intensities but greater than their average. In addition, it may be a characteristic of odour perception that there is poor discrimination between changes in quality and changes in intensity, partly because the two are correlated. A change in concentration of an odorant may produce a change in its quality, and quality may dominate in perception at the different concentration.
On the matter of perceived quality of odour mixtures, a special area to be looked into is masking. When the problem is odour unpleasant, strong odours are usually considered "pungent", not just strong. Deodorisers may have a qualitative effect just because they mix with the malodours. The mixtures of the smells may be less intense and thus less unpleasant than the malodours, but presumably also less pleasant than the smell of typical deodorisers. The intensity and unpleasantness of malodours may be reduced. However, the effect depends upon both the concentrations of the odorants and the deodoriser and the overall effect may be an increase in odour intensity. Mostly, deodorizers may only be effective with relatively weak odours. Of course, harmful odorants should be removed rather than masked.
Age and gender: Olfactory responses of individuals vary with age. Increasing age is correlated with decreasing acuity in odour perception. In a study by Amoore, 18 year old persons were found to have more sensitivity (a factor of 2) than 40 year olds 62 year olds were found in turn to have less sensitivity (a factor of 2) than 40 year olds. It is generally accepted that only persons between sixteen and sixty years of age with a normal sense of smell should be included on an odour panel. Female panellists normally have a greater sensitivity than male panellists from the same age group.
Other factors: It has been reported that smokers have less sensitivity than non-smokers. This is particularly true if the sensory test is carried out within half an hour of smoking. Factors such as health (such as cold, nasal allergy), personality, education background and training may contribute in some degree to the ability to assess an odour.