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 panelist to butanol is not necessarily reflected in sensitivity
to environmental odours in olfactometry testing. In our experience,
some panelists 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 panelists normally have a greater sensitivity than
male panelists 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.
