Selection Of Impact Indicators

The selection of indicators reflects the goal and scope in terms of choice of system borders, the interpretation of the precautionary principle and the intended means of integration towards a total impact value. In LCIA impact indicators may be chosen anywhere along the cause-effect chain. For example, emissions and use of resources may be used directly as impact indicators and evaluated against what is normal or against national emission goals and so on. Or indicators may, as in CBA, be chosen late in the cause-effect chain to reflect those issues that are observable and known to ordinary people, such as excessive mortality or fish kill. In RA and EIA the selection of indicators is largely made according to praxis and not dealt with as an explicit procedural step, as in LCA.

In RA the indicators are mostly a ratio between two numbers. The numerator is an estimated concentration that may occur in a certain compartment (prognosticated environmental concentration, or PEC) of the environment. The denominator is an estimation of a 'no effect level' (prognosticated no effect concentration, or PNEC) or an 'acceptable' level arrived at by some informal process. A ratio greater than unity is an indicator of risk. (Many substances, such as carcinogens, may not have a finite 'no effect level', which implies that any measurable concentration indicates risk.)

In EIA the indicators are seldom emissions but may be concentrations in the environment or observable changes, like decline in tree growth and decreased biodiversity. The position of indicators along the cause-effect chain may vary in EIA between different impact issues and is often determined from what is available and practicable.

In the LCIA standard (ISO 14042) (ISO 2000) the selection of impact categories and category indicators is required to be consistent with the goal and scope, justified and reflect a comprehensive set of environmental issues related to the product system being studied. It is also required that the category indicator names be accurate and descriptive, that references be given and that the environmental mechanism linking the emission or resource use to the category indicator be described. It is further recommended that the indicators be internationally accepted, represent the aggregated emissions or resource use of the product system on the category endpoint(s), avoid double counting and be environmentally relevant. It is also recommended that value choices and assumptions made during the selection of impact categories and category indicators be minimized. The selection of category indicators at the same level in the cause-effect chains may help to avoid double counting.

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