Page 244 - Fundamentals of Air Pollution

III. Analysis and Measurement of Odors 207

flow conditions. With this type of apparatus and standard operating condi-
tions, it is possible to determine the detection threshold and the recognition
threshold. At high dilution, the panel will be able to tell only whether an
odorant is present or absent. Only at concentrations higher, typically by a
factor of 2-10, will the subjects be able to identify the odorant.
The olfactometric procedure contains the following elements:

1. Dynamic dilution.
2. Delivery of diluted odorant for smelling through a mask or port.
3. Schedule of presentation of various dilutions and blanks,
4. Obtaining responses from the panelists.
5. Calculation of a panel threshold from experimental data.
6. Panelist selection criteria.
The first element, dynamic dilution, provides a reproducible sample for
each panelist. The system must minimize the loss of the odorant to the
walls of the delivery apparatus, provide clean dilution air of odor-free
quality, maintain a constant dilution ratio for the duration of a given test,
and have no memory effect when going from high to low concentrations
or switching between odorants of different character. The type of mask or
port and the delivery flow rate have been found to influence the response
of panelists in determining odor threshold and intensity.
The schedule of presentation may influence the results. The sensory
effects are judgment criterion, anticipation, and adaptation. The judgment
criterion determines how the panelist will respond when asked whether
or not an odor is sensed. Individuals differ in their readiness to be positive
or negative. The anticipation effect is a tendency to expect an odor over a
given series of trials. Subjects show some positive response when no odor-
ant is present. The adaptation effect is the temporary desensitization after
smelling an odorant. This is also called olfactory fatigue and often occurs in
occupational settings. Because of olfactory fatigue, investigators evaluating
odor concentration in the field must breathe air deodorized by passage
through an activated carbon canister before and after sniffing the ambient
air being evaluated.
Individuals differ in their sensitivity to odor. Figure 14-7 shows a typical
distribution of sensitivities to ethylsulfide vapor (17). There are currently
no guidelines on inclusion or exclusion of individuals with abnormally
high or low sensitivity. This variability of response complicates the data
treatment procedure. In many instances, the goal is to determine some
mean value for the threshold representative of the panel as a whole. The
small size of panels (generally fewer than 10 people) and the distribution
of individual sensitivities require sophisticated statistical procedures to find
the threshold from the responses.
Thresholds may also be determined by extrapolation of dose-response
plots. In this approach, the perceived odor intensity is measured at several