Towards Credible and Comparable Accounting of Environmental Attributes: Applicability and Limitation on Non-Proportional Allocation as Mass Balance Model

There is a growing demand for industrial materials that claim environmental attributes based on life cycle thinking. To track and manage such environmental attributes, Chain of custody (CoC) models are growingly applied. Among CoC models, the mass balance model–credit method (MB-CR) and book and claim (B&C) model implement non-proportional allocation of environmental attributes. Though there is a case that applying these models could avoid additional environmental burdens that would otherwise occur, the eligibility of applying non-proportional allocation requires careful consideration. This study aims to clarify the requirement and limitation for applying MB-CR and B&C models and to justify the cases of applying those models. A key requirement is environmental rationality, defined as the ability to contribute to avoiding additional environmental burdens, particularly during transition phases where the target environmental attribute is not abundant. A key limitation is technical feasibility, which constrains allocation within what is physically achievable for industrial materials. This study contributes to establishing a scientifically grounded and systematically structured methodology for non-proportional allocation of environmental attributes. Applying MB-CR and B&C models under the requirement and limitation ensures that the non-proportional allocation aligns with the avoidance of environmental burden while maintaining credibility, transparency, and feasibility in environmental claims of industrial materials.