An exchange is a causal, directional relationship between a Human activity and another human activity or between a human activity and the natural, social or economic environment or between environmental mechanisms. This fundamental concept in Life Cycle Assessment describes how materials, energy, and impacts flow through systems and across system boundaries.
The concept of exchange is central to understanding how Life Cycle Assessment models the interactions within and between production systems and their surrounding environment. ISO 14040 defines the system boundary in terms of these exchanges, which are the mechanism through which activities connect to form product systems and through which those systems interact with the broader environment.
Exchanges can be categorised into three distinct types based on their endpoints. First, exchanges may occur between human activities, such as when one production process supplies a product to another. These intermediate exchanges represent the interconnected nature of economic and industrial systems. Second, exchanges may occur between human activities and the environment, which are termed elementary exchanges. These include unprocessed inputs from nature, emissions to air, water and soil, physical impacts, and working hours under specified conditions. Third, exchanges may occur between environmental mechanisms themselves, such as the atmospheric transport and chemical transformation of pollutants.
The directional nature of exchanges is critical to LCA modelling. Each exchange has a clear origin and destination, allowing practitioners to trace flows through complex systems and quantify the environmental burdens associated with specific activities. This causal directionality distinguishes exchanges from simple inventory data by establishing the relationships that allow for comprehensive environmental accounting throughout a product's life cycle.
The terms "flow", "input", and "output" are used synonymously with exchange in LCA practice, though "exchange" more precisely captures both the directional and causal aspects of these relationships that are essential to rigorous life cycle modelling.
