Olfactometry is a psychophysical method based upon the olfactory
responses of individuals sniffing diluted odours presented by
an olfactometer to determine odour strength or odour concentration.
Recent developments in the methodology of olfactometry have
dramatically improved the repeatability and reproducibility
of olfactometry measurements. Developments of particular importance
include refined dilution instrument calibration and panelist
management techniques. Olfactometry can provide an effective
approach to the measurement of the odour concentration of complex
odours.
Olfactometry employs a panel of human noses as sensors. A human
nose can detect odour at concentrations well below the sensitivity
levels of chemical analytical methods. In the olfactometry testing
procedure, a diluted odorous mixture and an odour-free gas (as
a reference) are presented separately from two sniffing ports
at 20 l/min to a group of eight panelists in succession. In
comparing the gases emitted from each port, the panelists are
asked to report the presence of odour together with a confidence
level such as guessing, inkling, or certainty. The gas diluting
ratio is then decreased by a factor of two (ie chemical concentration
is increased by a factor of two). The panelists are asked to
repeat their judgment. This continues for five - six different
dilution levels, resulting in a total of 8x6x2 = 96 judgments
(sniffings) from eight panelists. Using panelist responses over
a range of dilution settings, odour concentration expressed
as odour unit per cubic meter can be calculated from individual
threshold estimates.
Currently, the preferred and internationally standardized methods
of measuring odour are the Dutch NVN2820 and the more recent
draft CEN standards. A joint Australia New Zealand standard
based on the draft CEN standard is in the course of preparation.
There are some limitations in odour measurement:
-
Odour concentration is only one of the four dimensions
that are used to express odour sensation as experience by
humans. The odour concentration is determined in an odour-free
environment and does not reflect the actual perception of
the odour. The odour concentration may only be linked to
the detectability of an odour. In a real environmental situation,
a person exposed to ambient air may describe his or her
responses ranging through terms such as " I smell something
", "there is a strong smell", " this is a bad smell" and
"the smell is like …". Measurement of odour intensity can
provide information on the likely degree of impact of an
odour.
-
Common standardized instrument calibration and panel selection
procedures are a prerequisite to comparison of odour concentration
data reported in the literature. In the absence of standardized
procedures, odour concentration levels reported might simply
reflect the experience of the operator, the design of the
olfactometer, its operational mode (manual or automatic),
its mixing method, the flow rate presented to panelists
and the number of panelists employed.
Odour measurement data can be used to:
-
Predict odour impact in the vicinity of an operation for
odour impact assessment purposes.
-
Provide information on the strength and intensity of odours.
-
Identify the causes of an odour problem and quantify the
scale of odour emission from a particular source.
-
Measure the performance of a pollution reduction program
implemented by a company.
-
Evaluate the removal efficiency of odour control technology.
