As Safe and Sustainable by Design (SSbD) becomes a central requirement in European chemical policy, understanding how to correctly apply the tools behind the SSbD framework has become essential. The Joint Research Centre (JRC) defines SSbD as a five-step process. JRC guidance suggests that:

• Steps 2 and 3 should follow established Risk Assessment (RA) principles, using tools such as EUSES,  ECETOC TRA, - following REACH methodologies - or any other tool capable of delivering the required regulatory-level exposure and safety evaluation.  

• Step 4 should rely on LCA/PEF methodologies, using USEtox (or EF-compliant toxicity models) or any other scientifically robust LCA method capable of delivering life-cycle impact results.

Other tools are acceptable at each step, provided they generate results that meet the expected purpose and scientific standards of that step.

Despite this intended separation of functions, an increasing number of SSbD practitioners are exploring whether USEtox could be used simultaneously for both risk assessment (steps 2–3) and sustainability assessment (step 4). This trend is understandable: both RA and LCA rely on multimedia fate modelling, which may give the impression that they can serve similar purposes.

However, merging these steps — or attempting to use USEtox in place of risk assessment — is scientifically incorrect and risks undermining the credibility and regulatory validity of SSbD assessments.

Shared Scientific Roots, but Completely Different Histories

Although USEtox far-field and EUSES both originate from the SimpleBox multimedia fate models, they were developed decades apart and for very different purposes.

EUSES was developed in the 1990s by RIVM to support EU chemical safety regulations. It estimates local, regional, and continental concentrations and evaluates safety using PECs, PNECs, DNELs, and RCRs. 

USEtox Far-Field was developed during the UNEP–SETAC consensus process (2005–2010) to harmonize toxicity characterization in LCA. It uses a continental box and optional global box to estimate long-term background impacts per kilogram of emission, expressed as DALY/kg (human) or PAF·m³·day/kg (ecotoxicity).

Thus, although they share historical roots, their objectives, architecture, and outputs diverge completely.

They Answer Two Different Questions

USEtox Far-Field answers the question: “How harmful is 1 kg of this emission at the continental or global scale?” This is a global impact question, without safety thresholds.

Risk Assessment answers: “Is exposure safe for workers, consumers, and ecosystems under real conditions?” This is a local and scenario-specific safety question.Because the questions differ, the results and model structures must differ as well.

What Each Tool Can and Cannot Do

In Life Cycle Assessment, and therefore in SSbD Step 4, toxicity impacts from different life cycle stages are summed together. USEtox is used to calculate single chemical score which are then aggregated (summed) from raw material extraction, chemical production, formulation, consumer use, and end-of-life emissions. Each emission is converted into the same metric (DALY/kg or PAF·m³·day/kg), which allows them to be mathematically added. This is intentional: LCA provides a global pressure indicator that represents the total potential impact of the entire product system.

In contrast, Risk Assessment is always performed for one chemical at a time, for one use at a time, and for one exposure scenario at a time. Risk Characterization Ratios (RCRs) are never summed across life cycle stages, nor across unrelated exposure contexts. A worker’s exposure at plant A cannot be added to a consumer exposure in another country. Each RCR is meaningful only within its own scenario.

The difference reflects the scientific purpose of each framework. LCA is additive, comparative, and system-wide. RA is scenario-specific, threshold-based, and local.

LCA answers a global question—what is the total environmental pressure created by this product? RA answers a safety question—are humans and ecosystems protected in this specific situation?

These approaches are complementary because they examine different dimensions of chemical performance: global environmental pressure (LCA) and local safe use (RA). Trying to merge them, or to use USEtox in place of RA, breaks both paradigms and leads to misleading conclusions.

SSbD requires both perspectives, used separately but coherently.

Beyond Today’s Boundaries: Can USEtox and EUSES Assess Planetary Chemical Pressure?

Although USEtox and EUSES were designed for different purposes, it is important to recognize that both models share a common scientific foundation in the SimpleBox multimedia fate framework. This means that, in principle, both are capable of calculating environmental concentrations, and both could—if adapted—contribute to assessing the global chemical pressure exerted on the planet.

Today, USEtox already provides a global pressure indicator and could, with further development, produce regional concentration estimates simply by applying the appropriate environmental volumes to its fate results. Conversely, EUSES could theoretically be extended to deliver a pressure-type indicator by calculating RCRs at the continental scale and by applying a multi-chemical cumulative RCR (treating the combined toxicity of all substances as fully additive). Such an approach would mirror the logic used in LCA, where impacts from different chemicals and life-cycle stages are summed to represent a system-level burden.

However, this would require significant methodological adaptations. Moreover, while cumulative RCRs provide a formal structure similar to LCA, the underlying toxicological assumption of full additivity among all chemicals, from all life cycle stages is scientifically questionable and not aligned with established mixture toxicology.

Both USEtox and EUSES could, in theory, be extended toward a unified “chemical pressure” perspective — but neither is designed for that purpose today.

Any future effort to bridge these domains must proceed with caution, scientific rigor, and a clear understanding of the conceptual limits of each model.

Conclusion — Why SSbD Needs Both Tools

A credible Safe & Sustainable by Design strategy requires:- Global impact assessment through LCA (USEtox) - Local safety assessment through RA (REACH/EUSES). Using each tool for its intended purpose ensures both scientific accuracy and regulatory relevance.