An impact category is a class of Impacts possessing some common quality or qualities. This organisational structure in Life Cycle Assessment allows practitioners to group and evaluate environmental impacts that share similar characteristics or affect the same aspect of the environment.
According to ISO 14040, an impact category represents environmental issues of concern to which Life cycle inventory analysis results may be assigned. This definition emphasises the fundamental role of impact categories in bridging the gap between the raw inventory data collected during an LCA study and the meaningful environmental insights needed for decision-making.
Impact categories serve as the foundation for Life Cycle Impact Assessment (LCIA), the phase of LCA where the environmental significance of inventory results is evaluated. Each impact category corresponds to a specific environmental concern, such as climate change, acidification, eutrophication, human toxicity, or resource depletion. The categories are defined based on shared environmental mechanisms or endpoints that are affected by different emissions or resource uses.
The process of assigning inventory results to impact categories involves characterisation, where individual elementary flows (such as emissions of carbon dioxide, methane, or nitrogen oxides) are translated into contributions to their relevant impact categories using scientifically derived characterisation factors. A single elementary flow may contribute to multiple impact categories. For example, nitrogen oxide emissions contribute to both acidification and eutrophication impact categories.
The selection and definition of impact categories in an LCA study depends on the goal and scope of the assessment, the geographic context, and the availability of suitable characterisation models. Different LCIA methodologies may define impact categories differently or focus on different sets of environmental concerns, reflecting various perspectives on environmental priorities and scientific understanding.
