Is an "odour unit" a scientific measure sufficiently
reliable to be a legally enforceable condition in an environmental
approval? The best analysis to date is a decision of the
British Columbia Environmental Appeal Board, West Coast Reduction Ltd. v. Greater
Vancouver (Regional District). According to the Board,
odour units are not scientifically reliable enough for this
West Coast Reduction operates a rendering plant, which provides
an essential public service, but inevitably has some odours.
A community campaign to "stop the stink"
created an "explosion" of complaints, which persuaded the
Greater Vancouver Regional District to impose stringent new
requirements on the company's air permit. Some of those
requirements imposed odour unit limits on various parts of the
plant. The company appealed, and called expert evidence about the
subjectivity and unreliability of odour unit measurements.
The Board decided:
"... the Panel finds that the use of odour units in this
context is not reasonable and appropriate. The notion that
odour units can be used as an indicator of an environmental
"smell" is simply too flawed to be used as a method
of determining compliance, and is therefore not suitable for
determining whether the environment is adequately
332 To begin with, an odour unit is a dilution ratio. The
mathematical definition of "ratio" is dimensionless.
Therefore, to give an odour unit a "unit of
measure", is already predisposing it to a "mass",
which it is not, and is therefore arbitrary.
333 Further, the dilution ratio is equal to the volume of clean
air divided by the volume of diluted air (or the
diluted odour). In order to attribute a "measure" of
odour units to a sample of air, human panelists are used. The Panel
appreciates that this is considered the best, and possibly the
only, means of measuring smell. However, the basis upon which
the panellists are chosen, the use of n-butanol, is not
without its problems, particularly when it comes to the
correlation between sensitivity to n-butanol and to
334 The European Standard is based on an assumption that the
performance characteristics as determined on
reference materials are transferable to other odourants.
Specifically, that there is a linear relationship between a
person's sensitivity to n-butanol and to other odours. If
the person can detect between 20-80 parts per billion, they qualify
to be a panellist. The response obtained to 40 parts per
billion in n-butanol is the standard upon which other odourants are
335 The Panel finds that there is no credible support for this
assumption in the context of the environmental odours at issue
in this case....
that bias and subjectivity are present at many stages during the
capture and analysis process:
In order to assess accuracy of analysis one needs to compare
the result with a standard, a true value. In olfactometry, human
panellists are used in place of analytical instruments. The
difficulty in assessing the accuracy of the odour unit is that
there is no clear measure as to the true odour unit value
because the response to the sensor (i.e., the nose) varies among
humans. Thus, bias and subjectivity is introduced at this
Odour units may be derived from the laboratory and from the
field. Many times the values obtained from these two different
methods are used interchangeably, which is erroneous. Bias is again
introduced at this point.
The human panel process does not account for the possibility of
residual odour in a panellist's nose after completing multiple
rounds with several presentations. Bias can be introduced at this
The Tedlar[TM] bags used for the samples emit odour powder plus
solvent. The solvent is emitted from the bag and could interfere
with the measurement as well. Background odour from a bag is a
problem if measured levels are so low which causes interference
with lab analyses. Thus, another bias is introduced....
345 Given that there are many steps in the process of attempting
to calculate odour units which are problematic, and which contain
so many points of bias and subjectivity, the Panel finds that the
ultimate number or value coming out of an odour unit measurement
cannot be relied upon as meaningful, particularly for the purposes
of evaluating compliance with a mandatory term of a permit.
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