Uncertainty caused by life cycle impact assessment methods: Case studies in process-based LCI databases

Life cycle impact assessment (LCIA) transforms inventories of environmental flows to environmental impacts in life cycle assessment (LCA) studies. Many impact assessment methods have been developed for impact categories such as global warming, acidification, and ecotoxicity. These impact assessment methods provide different characterization factor values and impact units for the same impact category. LCA studies often report results for one or at most a few impact assessment methods and overlook the uncertainty caused by selecting and using different methods. In this study, we systematically evaluated uncertainties that result from considering different LCIA methods; cases from the US LCI and the ecoinvent database were used to demonstrate the uncertainties. Results showed that the discrepancies of the total impact results were caused by differences in 1) the total emission values included in the inventory; 2) the coverages of substances in methods, and 3) the differences in the characterization factor values of the substances. Case studies showed large uncertainties for all impact categories except for global warming. For most of the categories studied, the maximum values were 10,000 times larger than the minimum values. These results should be taken into consideration for impact assessments that involve categories with large uncertainties. The information provided by this paper can help researchers and software developers to understand the uncertainty caused by choosing different LCIA methods and the importance of communicating such information to LCA practitioners. The results can also help decision-makers to build guidance that is related to build and choose LCIA methods for various topics.