An impact is a causal, directional relationship between an Activity and an environmental issue of concern, typically involving a significant Externality. This fundamental concept in Life Cycle Assessment represents the connection between what we do and the resulting effects on the environment, human health, or resource availability.
In the context of ISO 14040 and ISO 14044, impacts form the core of what LCA seeks to understand and quantify. Whilst activities generate exchanges (flows of materials, energy, or services), impact refers specifically to the consequences of those exchanges on aspects of the environment that we care about. These environmental issues of concern might include climate change, human toxicity, resource depletion, ecosystem quality, or any other area where human activities affect the world around us.
The relationship is both causal and directional, meaning that impacts trace a clear pathway from the activity to the environmental effect. For instance, the emission of carbon dioxide from combustion (an exchange from an activity) causes an impact on climate through the greenhouse effect. This directional nature allows LCA practitioners to attribute environmental burdens back to the activities that cause them.
Most impacts in LCA involve significant externalities. These are costs or effects experienced by society or the environment that are not reflected in the market price of goods and services. A factory producing steel might pay for raw materials and energy (internal costs), but the health effects of air pollution on nearby communities represent an externality. Understanding impacts, therefore, is essential for revealing the full environmental and social cost of activities and products beyond what conventional economic accounting captures.
Impact categories group related impacts together based on common environmental mechanisms or endpoints. Climate change, acidification, and eutrophication are all examples of impact categories, each representing a class of impacts that share environmental pathways and consequences.
