Assessing Mass Intensity as a Green Chemistry Metric: Why Expanding System Boundaries is not Enough
Abstract
Evaluating the environmental impacts of chemicals is crucial for a sustainable chemical industry. While Life Cycle Assessment (LCA) is recommended for evaluating environmental impacts, collecting chemical life-cycle data is often challenging. Hence, environmental performance is often approximated using simple “Green Chemistry Metrics”, particularly mass intensities like the “Process Mass Intensity” (PMI). However, despite their widespread use, mass intensities lack standardized system boundaries. Thus, this study systematically analyzes whether and with which system boundaries mass intensities can reliably approximate LCA environmental impacts. For this purpose, we evaluate Spearman correlation coefficients between sixteen LCA environmental impacts and eight mass intensities with varying system boundaries. The eight mass intensities include the (gate-to-gate) PMI and seven cradle-to-gate mass intensities considering parts of the upstream value chain, termed “Value-Chain Mass Intensity” (VCMI). For VCMI, we divide all value chain products into seven product classes and examine how including these classes in the system boundary affects the correlation. We find that expanding the system boundary from gate-to-gate to cradle-to-gate strengthens correlations for fifteen of sixteen environmental impacts. Additionally, the influence of product classes on the strength of the correlation varies depending on the environmental impact. These variations stem from a few key input materials that are represented differently across product classes, and each environmental impact is approximated by a distinct set of such materials. Consequently, a single mass-based metric cannot fully capture the multi-criteria nature of environmental sustainability. Furthermore, key input materials serve as proxies for environmental impacts because their consumption implies processes in the value chain. For instance, the input material coal implies a coal combustion process which emits carbon dioxide, making coal a key input material for approximating climate change impact. However, as processes change over time, the reliability of mass-based environmental assessment is highly time-sensitive, especially in light of the transition towards a defossilized chemical industry. We therefore question whether mass intensities should be used as a reliable proxy and suggest focusing further research on simplified LCA methods.
- This article is part of the themed collection: Green Chemistry 25th Anniversary Collection