An activity is the fundamental unit of action in Life Cycle Assessment, defined simply as doing or making something. The term derives from the Latin actio, meaning "doing or making", which captures its essence as any form of transformation or action that occurs within or beyond the economy.
ISO 14040 uses the term "process" as a synonym for activity, defining it as a "set of interrelated or interacting activities that transforms inputs into outputs". However, the broader LCA definition encompasses not only industrial and economic processes but also natural phenomena and social actions, regardless of their commercial significance.
Activities are divided into two primary categories. The first comprises Human activities, which are performed by humans, machines, or animals in human care. These include production activities such as manufacturing and agriculture, consumption activities such as eating and exercising, market activities that distribute products, and the accumulation of stocks. The second category encompasses Environmental mechanisms, which are natural processes not performed by humans or their controlled systems, such as atmospheric dispersion of pollutants, soil degradation, or ecosystem responses to human impacts.
This comprehensive definition is essential for Life Cycle Assessment because it allows practitioners to model the complete chain of cause and effect from human actions through environmental mechanisms to ultimate impacts. The classification system recognises that understanding environmental consequences requires accounting for both direct human activities and the subsequent natural processes they trigger. An illustrative example of this breadth can be found in the International Standard Industrial Classification of All Economic Activities, which systematically categorises human economic activities whilst recognising that LCA must extend beyond purely economic considerations.
