Life Cycle Assessment: System Modelling Approaches and Impact Assessment Methods

Life Cycle Assessment (LCA) is a method of analysis that aims to evaluate the environmental impacts of products and services through a holistic approach. Its foundations were first laid in the 1960s, when energy crises led to work on the energy and material flows of production systems within the scope of resource efficiency analysis. This early work took the form of energy and resource analysis rather than a holistic approach, and remained limited in terms of system boundaries and methodology.

In the 1970s and 1980s, as environmental awareness grew and waste management problems came onto the agenda, the understanding that products should be assessed across their life cycle became widespread. In this context the need for LCA standardisation arose in the 1990s, and ISO published the ISO 14040 standard in 1997 and ISO 14044 in 2006. ISO 14040 defines the general framework, basic principles and theoretical concepts of the LCA concept, while ISO 14044 provides methodological guidance on the operational application of the concepts ISO 14040 defines.

The ISO 14025 standard, published in the same year as ISO 14044, aims for LCA results to be shared transparently. The Type III environmental labels defined within that standard — that is, Environmental Product Declarations (EPD) — are reports that express the environmental impacts arising across a product’s life cycle through numerical indicators and are subject to third-party verification. By defining the structure of EPDs, their content requirements, product category rules (PCR) and verification processes, ISO 14025 aims to ensure the reliability and transparency of LCA studies.

In 2012 the European Committee for Standardization (CEN) published EN 15804, a specific EPD standard based on ISO 14025 for the construction materials sector, one of the sectors in which LCA studies are most widely applied. Within EN 15804, methods and rules for construction material EPD studies are defined at points such as system boundaries, mandatory environmental impact categories and calculation rules.

Today LCA has gone beyond being merely a method of analysis and has become a fundamental tool for product comparisons, policy development processes and sustainable design decisions. Accordingly, different system modelling approaches and impact assessment methods are used depending on the purpose and scope of the study; this means that, even where the same data set is used, different system modelling approaches and impact assessment (LCIA) methods can give different results.

Principal system modelling approaches

System modelling approaches determine how the product system is represented and, in particular, how situations such as multi-output processes and recycling are handled. The most widely used approaches in this context include cut-off, APOS (Allocation at the Point of Substitution) and consequential approaches.

The cut-off approach defines system boundaries sharply and takes into account only the environmental burdens belonging directly to the product system under examination. In this approach recycled materials do not carry the environmental burdens of their previous life cycles, while future recovery benefits are not taken into account for waste leaving the system. For this reason the cut-off approach is widely used, particularly in product-based assessments and in environmental product declarations.

The APOS approach is based on environmental burdens in multi-output systems being shared between products within a defined set of rules. In this approach both physical and economic relationships can be taken into account. While APOS offers a more balanced representation between the cut-off and consequential approaches, it can produce variability in results depending on the allocation criteria selected.

The consequential approach focuses on assessing the marginal effects of a change made in the system on markets and production systems. The aim in this approach is to answer the question of which production processes a particular decision will increase or decrease “in the real world”. It does not focus only on the environmental impacts of the product; it includes in the calculation the relationship between that production and which indirect environmental impacts it caused, or which indirect environmental impacts it reduced, in the long term or in parallel. For this reason consequential LCA is preferred particularly in policy analysis and strategic decision support processes, but carries higher uncertainty because of the assumptions it contains. It can generally be used by decision-makers for policies rather than for specific product studies.

For example, when we take a packaging product made using recycled plastic within the scope of a Life Cycle Assessment: under the cut-off approach the recycled plastic used in that packaging enters the system “with zero burden”, carrying no environmental burden from its previous life cycle, and only the impacts of the recycling operation (collection, separation, processing) are taken into account. Under the APOS approach, the same plastic is neither entirely free of burden nor assessed on its own; it takes a certain share from the previous product system, and that burden is shared with the new product according to physical or economic relationships, so the total impact of the packaging is calculated in a more balanced but method-sensitive way. Under the consequential approach the focus changes completely; the use of recycled plastic is assessed through the extent to which it reduces the production of primary (virgin) plastic in the market, and that reduction is reflected in the system as a credit in the form of an “avoided burden”. For this reason the same product can give lower results under cut-off, mid-range results under APOS and, under the consequential approach, much lower or even negative impact results depending on the assumptions.

Impact assessment methods

Another important element causing divergence in LCA studies is impact assessment methods. These methods determine how inventory results are converted into environmental impact categories. One of the methods widely used in this context is the ReCiPe method. The ReCiPe method presents results at two different levels: midpoint and endpoint. The midpoint approach focuses on specific environmental impact categories such as climate change or acidification and produces results with lower uncertainty, while the endpoint approach reduces these impacts to higher-level damage categories such as human health, ecosystem quality and resource use. This makes the results more interpretable, while increasing the level of uncertainty because of the additional modelling assumptions.

Particularly in technical analyses and process improvement work, midpoint results are preferred because they contain lower uncertainty. The endpoint approach, by contrast, brings together the different impact categories obtained at midpoint level and makes a damage assessment at a higher level. This approach reduces complex environmental impacts to a simpler form that is more comprehensible for decision-makers.

The Environmental Footprint (EF) 3.1 method developed by the European Union is an impact assessment approach that aims to provide a higher level of standardisation and comparability in LCA studies. Developed by the European Commission, this method offers a standardised framework for particular impact categories, characterisation factors and normalisation-weighting steps, and is therefore widely used in policy- and regulation-focused work in particular.

At sector level, the EN 15804 standard developed specifically for construction materials makes it compulsory for LCA studies to be carried out within particular methodological rules. The “EN 15804 adapted” approach used in this context refers to versions of that standard brought into line with current impact assessment methods (EF 3.1, for example). This approach aims to increase sectoral comparability through requirements such as system boundaries being defined in a modular structure and particular impact categories being reported as mandatory.

In conclusion, the system modelling approaches and impact assessment methods used in LCA studies are directly determinative of the results obtained, and even where the same data set is used, different results — even results in opposite directions — can be obtained depending on different methodological choices. For this reason it is critically important, in interpreting LCA results, that the methodological framework used is clearly defined and that the results are assessed in that context.

Sources

ISO (2006). ISO 14040: Life cycle assessment – Principles and framework.

ISO (2006). ISO 14044: Life cycle assessment – Requirements and guidelines.

ISO (2006). ISO 14025: Environmental labels and declarations – Type III environmental declarations.

CEN (2019). EN 15804: Sustainability of construction works – Environmental product declarations.

Guinée, J.B. (2002). Handbook on Life Cycle Assessment. Hauschild, M.Z., Rosenbaum, R.K., Olsen, S.I. (2018).

Life Cycle Assessment: Theory and Practice. Huijbregts, M.A.J. et al. (2017). ReCiPe2016 method.

European Commission (2021). Environmental Footprint method. Ecoinvent (2023). System Model Documentation.