Page 191 - Materials Chemistry, Second Edition

10 Life Cycle Impact Assessment 177

corresponding (1) problem observed, (2) principal environmental mechanism,
(3) main causes and (4) most widely used characterisation models.

10.2.3.1 What Is a Characterisation Factor?
A characterisation factor (CF) represents the contribution per quantity of an ele-
mentary flow to a specific environmental impact (category). It is calculated using
(scientifically valid and quantitative) models of the environmental mechanism
representing as realistically as possible the cause–effect chain of events leading to
effects (impacts) on the environment for all elementary flows which contribute to
this impact. The unit of a CF is the same for all elementary flows within an impact
category. It is defined by the characterisation model developers and may express the
impacts directly in absolute terms (e.g. number of disease cases/unit toxic emission)
or indirectly through relating them to the impact of a reference elementary flow (e.g.
CO 2 -equivalents/unit emission of greenhouse gases).

10.2.3.2 How Is It Calculated?

The modelling of a characterisation factor involves the use of different models and
parameters and is typically conducted by experts for a particular impact category
and its underlying impact pathway or environmental mechanism. Various
assumptions and methodological choices are involved and this may affect the output
as reflected in the differences in results that may be observed for the same impact
category when applying different LCIA methods. This must be considered when
interpreting the result of the LCIA phase. The first step when establishing an impact
category is the observation of an adverse effect of concern in the environment,
leading to the conclusion that we need to consider such effects in the context of
decisions towards more sustainable developments. Once accepted as an effect of
concern, the focus will be on how to characterise (quantify) the observed effect in
the framework of LCA.
The basis and starting point of any characterisation model is always the estab-
lishment of a model for the environmental mechanism represented by a cause–effect
chain. Its starting point is always the environmental intervention (represented by
elementary flows), essentially distinguishing two types based on the direction of the
relevant elementary flows between technosphere and ecosphere:

• An emission into the environment (=elementary flow from the technosphere to
the ecosphere),
or
• A resource extraction from the environment (=elementary flow from the eco-
sphere to the technosphere).