A determining product is a Product output of an Activity for which a change in demand will directly affect the production volume of that activity. This fundamental concept in Life Cycle Assessment establishes the causal relationship between market demand and production decisions, making it essential for consequential modelling approaches.
The defining characteristic of a determining product is its market-driven influence on activity levels. When demand for a determining product increases, the activity that produces it will typically scale up production accordingly. Conversely, when demand decreases, production volumes decline. This responsiveness distinguishes determining products from by-products, which are produced as secondary outputs whose quantities are largely determined by the production levels of other products from the same activity.
In the context of ISO 14040, determining products align with the concept of the reference flow, which measures the outputs from processes in a given product system required to fulfil the function expressed by the functional unit. However, the determining product concept extends beyond this by explicitly addressing the economic and market dynamics that drive production decisions.
The ecoinvent database uses the term "reference product" as a synonym for determining product. Whilst both terms describe the same concept, "determining product" more clearly emphasises the causal relationship between demand and production volume, making the term particularly valuable in consequential LCA modelling where understanding marginal changes and market responses is critical.
Understanding which products are determining products versus by-products is crucial for system modelling decisions, particularly when handling multi-output activities and applying system expansion or allocation procedures. This distinction affects how environmental burdens are attributed across different product outputs and how substitution effects are modelled in the life cycle.
