Appendix 6. Odour Buffer Assessment [As Published] · 132007.2399

                                                                                            [4]
               Odour Buffer Assessment
               StarCo Developments


3.1 Factors Affecting Odour Response

Analysis of community odour exposure has identified five factors that are important in determining
the potential for annoyance and complaint:

              Frequency;

              Intensity;

              Duration;

              Offensiveness; and

              Location.

These are together called the FIDOL factors (NZ Ministry for Environment, 1995). Generally, the
greater the frequency, intensity, duration and offensiveness of an odour the more likely it is to
cause annoyance and lead to complaints.



3.1.1         Frequency

The frequency of odour exposure simply refers to how often odour events occur. It is a function of
the variations of odour emissions over time, and of the meteorological conditions in the area
around an odour source. The frequency of odour events is generally greatest in areas that are
most often downwind of the source.



3.1.2         Intensity

The intensity is the perceived strength of an odour and it is related to the odour concentration, or
the mass concentration of the compounds involved. However, the relationship is not direct.

For simple odours associated with single compounds, such as hydrogen sulphide, the relationship
between the intensity and the mass concentration of the compound (µg/m3) or its odour
concentration (ou), has the general form of a power law or a logarithmic relationship. Thus, as the
odour concentration increases, the perceived strength or intensity increases by a much smaller
amount. The relationship between odour intensity and odorant concentration may be described by
the Weber-Fechner law (Frechen, 1997), which is written as:

        I = kw log(COD)

where

        I        = intensity level

        kw       = Weber-Fechner coefficient

        cOD      = odorant concentration

What this means, for example, is that a doubling of the odour concentration leads to a
proportionally much smaller increase in odour intensity, the parameter detected by human noses.
An important implication of this relationship is that in order to reduce the impact of an odour that




1898a report final.doc                                                   PAE Job Number 1898a