System boundaries define the set of criteria that specify which unit processes are included within a product system and which are excluded from the Life Cycle Assessment. They establish the limits of the system being studied, determining what flows and activities will be accounted for in the analysis and what will be considered outside the scope.
According to ISO 14040, system boundaries are fundamental to defining the product system, as they determine the completeness and comprehensiveness of the LCA study. The boundaries separate the analysed system from the environment and from other systems, establishing clear delineation between what is modelled within the study and what lies beyond its scope.
Several types of boundaries must be considered when defining a product system. Geographical boundaries specify the physical locations included in the study, such as particular countries, regions, or production sites. Temporal boundaries define the time period covered by the data and analysis. Technological boundaries determine which production technologies and processes are included. Together, these boundaries shape the scope and applicability of the LCA results.
The selection of system boundaries is closely linked to the functional unit and goal of the study. Boundaries should be chosen to ensure that all significant processes contributing to the function being assessed are captured within the system. However, practical considerations often require establishing cut-off criteria for excluding processes or flows that contribute negligibly to the overall results, based on mass, energy, or environmental significance.
System boundaries directly influence the comparability of LCA studies. For meaningful comparison between different product systems, consistent boundary definitions must be applied. Transparency in documenting boundary choices and any associated cut-off criteria is essential for ensuring that LCA results can be properly interpreted and used in decision-making contexts.
