Page 194 - Materials Chemistry, Second Edition

180 R.K. Rosenbaum et al.

directly linked to the original cause). Contrary to a frequent misconception, that does
not mean that the total uncertainty (i.e. including all its sources, not just parameter and
model uncertainty) of an indicator increases when going further down the cause–effect
chain, because the increase in parameter and model uncertainty is compensated by an
increase in environmental relevance. If the latter is low (as is the case for indicators
placed early in the cause–effect chain) the relationship of an indicator to an envi-
ronmental issue is assumed but not modelled and thus hypothetical and therefore
uncertain. A detailed discussion on these issues can be found in Chap. 11.
To select the impact indicator, developers must therefore strike a compromise
between choosing an indicator of impact:
1. Early in the environmental mechanism, giving a more measurable (e.g. in the
lab) result but with less environmental relevance and more remote from the
concerns directly observable in the environment

Versus
2. 2. Downstream in the environmental mechanism, giving more relevant but hardly
verifiable information (e.g. degraded ecosystems, affected human lifetime)

This has led to the establishment of two different types of impact categories,
applying indicators on two different levels of the environmental mechanism: mid-
point impact indicators (representing option 1 from above) and endpoint impact
indicators (representing option 2).

10.2.3.6 Midpoint Impact Indicators

When the impact assessment is based on midpoint impact indicators, the classifi-
cation gathers the inventory results into groups of substance flows that have the
ability to contribute to the same environmental effect in preparation for a more
detailed assessment of potential impacts of the environmental interventions,
applying the characterisation factors that have been developed for the concerned
impact category. For example, all elementary flows of substances that may have a
carcinogenic effect on humans will be classified in the same midpoint category
called “toxic carcinogen” and the characterisation will calculate their contribution to
this impact. Typical (and emerging) midpoint categories (including respective
sub-categories/impact pathways) are:
• Climate change
• Stratospheric ozone depletion
• Acidification (terrestrial, freshwater)
• Eutrophication (terrestrial, freshwater, marine)
• Photochemical ozone formation
• Ecotoxicity (terrestrial, freshwater, marine)
• Human toxicity (cancer, non-cancer)