Overview: The impact assessment is carried out on the basis of the inventory analysis data. The inventory flows are classified according to their potential impact on the environment, human health, or resources in so-called impact categories. These categories provide indicators of potential environmental impacts and not necessarily contributions to the actual effects. Actual effects may be dependent, for example, on the concentrations of contaminants in the environment and the exceedence of thresholds due to multiple sources at specific times and locations. In contrast to Environmental Impact Assessment, the Life Cycle Impact Assessment is hence (typically) not site-specific and only sometimes site-dependent. An integrated use of both methods in a joint "tool-box" is under scientific development.
A well known impact category is the Climate change (formerly "Global warming ") potential. All emissions which produce a potential contribution to the greenhouse effect are assigned to this category . The most well-known emission in this category , due to the current discussion, is Carbon dioxide . In classifying the inventory data according to their potential environmental impact categories , an aggregation in each impact category takes place. The number of the data are therefore considerably reduced and the results can be better interpreted by referring directly to the environmental impacts . Since the inventory data are related to the quantitative reference of the product or process, this relation also exists in the life cycle impact assessment . The impact assessment results and also data obtained from the inventory analysis can be used jointly for the (later) interpretation phase of the LCA study.
Key elements of impact assessment: LCIA consists of mandatory and optional elements. The mandatory elements are:
- Selection of impact categories, category indicators and characterization models
- Assignment of LCI results to the selected impact categories (classification)
- Calculation of category indicator results (characterization)
The optional elements are:
- Weighting, and
- Additional LCIA data quality analysis.
Areas of protection: The first step within the framework of an impact analysis is the selection of impact categories in connection with the scope and goal definition. The impact assessment categories should link the potential impacts and effects on what is referred to as the "areas of protection" of the LCA, i.e. the entities that we want to protect by performing and using the LCA. Today, there is acceptance that the protection areas of life-cycle assessment are:
- natural resources
- natural environment
- human health
- and often also: man-made environment
Classification: In the classification step the inventory data are assigned to categories according to their impact. For instance, carbon dioxide emissions contribute to the greenhouse effect and are hence assigned to the impact category Climate change. If a substance contributes to several impact categories, it has to be taken into account in all of these categories. Such a case is, for example, nitrogen oxide that causes both eutrophication and acidification .
Characterisation: Classification is followed closely by characterisation. Every substance is assigned a potential impact in the impact category under study. The potential impact of a substance is given relative to a dominant factor in the category , e.g. for the Climate change potential this is typically 1 kg of carbon dioxide emissions. These relative impacts (the characterisation factors of a substance) are than multiplied with the amount of each emission and the resulting impact values are summed for the respective impact category .
Illustration of intermediate results: After characterization and before the optional elements, it is common practice to represent the inputs and outputs of the product system by
- a compilation of the LCIA category indicator results for each of the different impact categories referred to as an LCIA profile,
- a set of those inventory results that are considered relevant elementary flows but have not been assigned to impact categories e.g. due to lack of environmental relevance, and
- a set of data that does not represent elementary flows (e.g. highly specific product flows of minor quantities, which were not considered due to cut-off criteria or waste flows, for additional documentation purposes).
Optional steps in LCIA: In addition to the mandatory elements of LCIA, there are optional elements and information, as listed below, which can be used depending on the goal and scope of the LCA:
- normalization: calculating the magnitude of category indicator results relative to reference information;
- grouping: sorting and possibly ranking of the impact categories;
- weighting: converting and possibly aggregating indicator results across impact categories using numerical factors based on value-choices; ! data prior to weighting should remain available;
- data quality analysis: better understanding the reliability of the collection of indicator results, the LCIA profile.
The optional LCIA elements may use information from outside the LCIA framework. The use of such information should be explained and the explanation should be reported. The application and use of normalization, grouping and weighting methods shall be consistent with the goal and scope of the LCA and it shall be fully transparent. All methods and calculations used shall be documented to provide transparency.