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.

Supplementary files

Article information

Article type
Paper
Accepted
14 Jul 2025
First published
17 Jul 2025
This article is Open Access
Creative Commons BY license

Green Chem., 2025, Accepted Manuscript

Assessing Mass Intensity as a Green Chemistry Metric: Why Expanding System Boundaries is not Enough

S. Eichwald, H. Ostovari, H. Minten, J. Meyer-Waßewitz, D. Förtsch and N. von der Assen, Green Chem., 2025, Accepted Manuscript , DOI: 10.1039/D5GC02994E

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

Read more about how to correctly acknowledge RSC content.

Social activity

Spotlight

Advertisements