Life cycle assessment (LCA) is a systematic approach to analyze and examine the impacts of products or services on the environment. Not only these impacts, but their whole life cycle (i. e., from resource extraction to end-of-life) is analyzed. LCA is based on the modeling of interconnections between the single processes of a product system, in order to identify the material and energy flows within the system. From this model, the so-called ‘elementary flows’ — i. e., material flows between the product system as a whole and the natural environment — can be derived. Finally, the impacts or damage to the environment can be assessed. The core methodology of LCA was developed during the 1990s, with the SETAC ‘Code of Practice’ as a first milestone (Consoli et al, 1993), and today is described in two international standards, ISO 14040 and 14044, which are part of the ISO 14000 family of standards on environmental management.

According to ISO 14040, a life cycle assessment study consists of four phases: goal and scope definition, inventory analysis, impact assessment, and finally interpretation (Fig. 3.2). The double arrows in Fig. 3.2 indicate that an LCA is seen as an iterative process; additional information gained during a study can require a backshift to a previous stage to include further aspects. The first phase of an LCA study, goal and scope definition, defines and specifies the objects and the research questions. It also defines the system

boundaries of the product system, e. g. as to space and time. A central task of LCA is the comparison of different products with the same function to extract particularly environmentally friendly goods or services. To fulfill this target, a clearly defined functional unit has to be determined. According to ISO 14040: ‘the functional unit defines the quantification of the identified functions of the product’, which means that the function has to be specified in terms of a quantity of a product or service, e. g. per kg of a product or per km of a transport service.

The second phase, life cycle inventory analysis (LCI), consists of three parts: creation of a flow model, data collection and calculation of the results. In a flow model, processes of the product system and their connections are described. The product system itself is defined by ISO 14040 as a ‘collection of unit processes with elementary and product flows, performing one or more defined functions and which models the life cycle of a product.’ For each unit process, data for input and output flows have to be gathered, together with other important information as to the process itself. The result of the LCI analysis is the quantification of the elementary flows resulting from the delivery of the functional unit, i. e. the product or service. The third step — the life cycle impact assessment, LCIA — transfers the results of the inventory analysis in a quantification of potential damage to the environment. To do so, the concept of impact categories is used. An impact category represents a certain environmental problem, e. g. climate change. It is quantified by a category indicator, which is based on an understanding of the underlying causes of an environmental problem as described in a respective scientific model (‘characterization model’). As the last phase of LCA, the interpretation transfers the results from the LCI analysis and LCIA to a clearly understandable message for the intended audience of an LCA study. The phase of interpretation also includes procedures of quality assurance and sensitivity analysis.

Details of the methodology of life cycle assessment can be found in textbooks as well as on internet platforms (Baumann and Tillmann, 2004).