Don’t Forget Metrics: How Do We Prove Chemistry Is Actually Green?

In the recent RSC paper, “Modernising the 12 Principles of Green Chemistry: It’s Time,” the original 12 principles, first set out in 1998, are revisited by Lipshutz and Handa for a chemistry landscape that looks very different today. Among the perspective’s proposed additions is Principle 10, “Don’t Forget Metrics”, framed as a practical reminder, yet it carries a deeper provocation for the chemical sciences.

In the first article in this series, I explored Principle 11 – Innovate Following Nature’s Lead – and made the case for looking to biology, not just chemistry, when designing greener routes: asking not only “can we make this” but “how would Nature make it?” But that question is only half the challenge. Even the most elegant biocatalytic or bio-inspired route still must prove its credentials, rather than being assumed green by virtue of its origins. That is precisely where Principle 10 comes in.

This article looks to the question of measurement itself, not whether green chemistry is being practised, but whether it can be proven. Metrics, in this context, are not mere reporting tools; they are instruments of design, decision-making, and accountability. Too often, however, they are relegated to the end of process development, applied retrospectively as a kind of sustainability “badge” to validate a route that has already been fixed. This tendency risks reducing green chemistry from a guiding philosophy to a compliance exercise.

The temptation is understandable. Process chemists frequently work under pressure to deliver robust, scalable routes within tight timelines. Once a process is locked, applying metrics such as E-Factor or process mass intensity (PMI) offers a convenient way to benchmark improvements against legacy routes. A lower PMI or reduced waste profile can then be presented as evidence of greener credentials. But this post hoc application is inherently limited: it answers the question, “how green is this process?”  without asking the more important one: “how could this process have been greener?”

Embedded in Every Stage of Process Design

Embedding metrics throughout the design and development lifecycle changes transforms their role. Instead of retrospective scorekeepers, they become predictive, iterative tools. Early-stage route scouting can be guided by estimated PMI or solvent use, steering chemists toward inherently efficient transformations from the outset. During optimisation, metrics expose trade-offs — yield versus solvent burden, step count versus reagent toxicity — enabling more nuanced decisions. At scale-up, they provide a framework to assess whether changes genuinely improve sustainability, or simply shift the burden elsewhere.

This points to a broader truth: chemical processes are complex, multi-variable systems, and no single metric can capture their full environmental impact. Using  multiple metrics together is therefore not just advantageous, it’s essential.

The Power of Using Multiple Metrics

At first glance, using both PMI and E-Factor might seem redundant. The appeal of the E-Factor lies in its simplicity: an ideal value of zero aligns  directly with the principle of eliminating waste. PMI, meanwhile, offers a more holistic view of material efficiency, incorporating all inputs including water. The ACS GCI Roundtable has advocated for PMI over E-Factor on the grounds that management attention is often drawn more readily to efficiency than to waste generation. Whether that reasoning is pragmatic or simply sobering,  it makes a useful point: what we choose to measure reflects what we choose to value.

There is no reason, though, not to value both. Using PMI and E-Factor together ensures that process efficiency and waste remain equally visible, encouraging chemists and stakeholders to assess process performance through multiple lenses.

The blind-spot shared by both is a  lack of environmental comprehension, an inevitable shortcoming when quantities, not hazards, are being measured. Chemistry is rarely “green” or “non-green” on the basis of waste volume alone; toxicity, energy intensity, and operational risks matter just as much. To better reflect the complexity of real-world sustainability decisions, deployment of complementary integrated metrics – like iGAL and Life Cycle assessments (LCA) – is vital to help close that gap. These integrated approaches offer multidimensional insight; however, their comprehensiveness comes at a cost. These methods are more complex, less intuitive, and demand  specialist expertise, detailed datasets, and often involve subjective weighting.  Thankfully, simplified models and free tools are gradually lowering these barriers.  Although arguably the wrong tool for evaluating early stage  route selection, these integrated methods are invaluable for benchmarking shortlisted manufacturing routes, identifying burden shifting, and capturing genuine sustainability gains.

Deploying a suite of metrics enables chemists  to interrogate their processes from multiple different angles, revealing insights that a single measure would leave hidden. Metrics, in this sense,  are not a scoreboard, but a diagnostic toolkit.

Use Wisely

That said, metrics can mislead if used selectively or without context. A favourable comparison against a poorly optimised legacy route may create a false sense of progress. Similarly, focusing on a single metric can encourage “gaming,” where improvements in one area mask deterioration in another. This pattern is not unique to chemistry; in the UK food sector, for example, products favourably marketed as “low fat” or “high protein” may, in fact, still fall into the category of ultra-processed foods, creating a misleading narrative of their health benefits. The issue is not the metrics themselves, but how they are framed. Used as marketing tools rather than components of a holistic assessment, isolated metrics risk obscuring more than they reveal.

The same pitfall exists in pharmaceutical manufacturing. Treating green chemistry metrics as badges — applied selectively and highlighted for advantage — risks undermining the credibility of sustainability claims. The real objective is not to “win” a metric, but to genuinely reduce environmental impact across the product lifecycle, which demands transparency, consistency, and a willingness to engage with complexity rather than resolve it prematurely.

Making Accountable, Data-Driven Decisions

Encouragingly,  the industry already offers examples of metrics  integrated effectively. At CatSci, green chemistry metrics are embedded throughout our development workflows. By quantifying progress from route selection through to optimisation, it becomes possible to track improvements in real time and support data-driven decision-making. This approach not only enhances sustainability outcomes but also fosters a culture where environmental considerations are intrinsic to the work, not bolted on afterwards.

Such practices illustrate the true potential of Principle 10. Metrics provide a shared language for sustainability, aligning scientists, engineers, and stakeholders around measurable goals. They support innovation by making trade-offs explicit and enhance accountability by grounding claims in evidence, rather than assumption.

Ultimately, modernising the principles of green chemistry is not about adding complexity, but about sharpening their relevance in a world in which sustainability is both a scientific and societal imperative. “Don’t forget metrics” is a reminder that what we measure shapes what we value, and, by extension, what we design. Treated as an afterthought, metrics offer only superficial reassurance. Embedded from the outset, they can drive meaningful change.

The challenge ahead is as much cultural as it is technical:  moving from metrics as a final checkpoint to metrics as a continuous guide; from single indicators to complementary systems; from convenient narratives to honest evaluation. In that sense, “don’t forget metrics” is less a reminder and more a call to action.