An input or output is a causal, directional relationship representing material, energy, or service flows in a Life Cycle Assessment model. This term is synonymous with Exchange.
An exchange represents the fundamental building blocks of LCA models, capturing how materials, energy, and services move between different parts of a system. These relationships are both causal (meaning one activity causes the flow) and directional (the flow moves from one point to another with a clear origin and destination).
Exchanges occur in several distinct contexts within LCA modelling. They can represent flows between human activities, such as when one production process supplies materials to another. They also capture flows between human activities and the natural, social, or economic environment, such as when raw materials are extracted from nature or emissions are released to air, water, or soil. Additionally, exchanges can occur between environmental mechanisms themselves, which is particularly relevant when modelling environmental processes as part of impact assessment characterisation models.
The ISO 14040 series of standards, which provide the framework for Life Cycle Assessment, uses the terms "input" and "output" throughout to describe these flows crossing system boundaries and moving between unit processes within product systems. An input enters an activity from another activity or from the environment, whilst an output leaves an activity to go to another activity or to the environment.
The term "flow" is also used as a synonym for exchange, particularly in the context of intermediate flows (flows between unit processes) and elementary flows (flows with mass or energy properties crossing the boundary between the technosphere and the environment).
