A unit process is the smallest Activity considered in the Life cycle inventory analysis for which input and output data are quantified. This definition represents a modification of ISO 14040 clause 3.34, where the term "element" has been replaced with "activity" to better align with current LCA terminology and practice.
In ISO 14040, the original definition states that a unit process is the "smallest element of a product system for which data are collected when performing a life cycle assessment". By adopting the term activity instead, the definition becomes more consistent with how LCA practitioners describe the fundamental building blocks of product systems.
Unit processes serve as the foundational data units in Life Cycle Inventory analysis. Each unit process represents a discrete transformation or activity within a product system, such as the production of a specific material, a transportation step, an energy generation process, or a waste treatment operation. For each unit process, practitioners collect and quantify all relevant inputs (raw materials, energy, auxiliary materials) and outputs (products, co-products, emissions to air, water and soil, and waste).
The level of detail at which activities are divided into unit processes depends on the goal and scope of the LCA study, data availability, and the required precision. A unit process must be sufficiently detailed to allow for meaningful data collection and analysis, yet practical enough to be manageable within the scope of the study. Multiple unit processes are linked together through intermediate exchanges to form complete product systems that model entire product life cycles from raw material extraction through manufacturing, use, and end-of-life treatment.
