The Impact Assessment Phase3
As shown in Fig. 15.4, there are several steps in a life cycle impact assessment.
Sometimes LCA stops at the end of the inventory step. There may be two explanations for this. One is that all emissions and resource depletion improve, when compared to a reference alternative. In this case an impact assessment is not required.
The other approach is of a more philosophical nature. The environment is regarded as starting at the end of the stack or wastewater pipe, and emissions are regarded as the environmental impacts.
Ecologists tend to define the environment in terms of physical, chemical, or biological properties within the environment. In LCIA, such definitions are called the category indicators, numerical entities belonging to impact categories. For example, the indicator “moles of H+” belongs to the impact category “acidification.”
Selection of impact categories, category indicators, and models
Assignment of LCI results to impact categories (classification)
Calculation of category indicators (characterization)
Category indicator results (LCIA profile)
Calculating the magnitude of category indicators relative to reference value(s) (Normalization)
Grouping Weighting Data quality analysis
Once the indicator is defined, a model can be developed that predicts the indicator value as a function of an emission. Such models are normally simple linear models defined by characterization factors. If an emission is multiplied by a characterization factor, an indicator value is obtained.
The sum of indicator values obtained when multiplying all emissions assigned to that impact category by their respective characterization factors is called the category indicator result. The indicator “moles of H *" may, for in stance, be the sum of contributions from sulfur dioxide, nitrogen oxides, and hydrogen chloride, and there is a characterization factor for each of them relative to the indicator.
The category indicator results may then be analyzed further by normalization, grouping, and weighting. These procedures are optional in the standard, as they are not as well known as the preceding steps and they contain more subjective elements. The aim of these steps is to clarify the results by comparing them to some reference.
An indicator result could, for instance, be compared to the total indicator value in an area, or to the average indicator value per inhabitant per year. Other common methods are to compare with national reduction targets and with damage costs from emissions and resource depletion. Sometimes these are weighed across impact categories made by expert panels. However, some studies using this technique would not meet the requirement of transparency for the weighting process set by ISO.
When making an LCA for a product or technical concept, all these steps are not determined from the start at each occasion. It is common to use readymade lists of characterization and weighting factors. In this way, the impact assessment is speeded up. Examples from such lists are shown in Table 15.2.
The ecoscarcity method was developed by Ahbe et al.4 It is essentially based on an “Ecofactor” which is calculated for each emission equal to (F/Fk) ■ 1/Ffc ■ X, where F is the emission in the area, Fk is the emission target or the critical load in the area (country), and K is a constant used for practical purposes to obtain easy-to-handle magnitudes of the figures.
The effect category method4 is similar to the ecoscarcity method, but it uses equivalent emissions for environmental themes, such as global warming and acidification. Instead of relating C02 alone to a critical load, all emissions of greenhouse gases are transformed to C02 equivalents, in terms of global warming potential and the total C02 equivalent emissions are compared to the critical load of greenhouse gases.
The EPS enviro-accounting method, version 964>5 and the previous version, estimates the willingness to pay (WTP) of OECD inhabitants in EURO for avoiding changes in five safeguard subjects:
• human health,
• ecosystem production capacity,
• natural resources, and
• aesthetic values.
The value 0.064 for C02 given in Table 15.2 represents the sum of all impacts on human health, biodiversity, etc.
TABLE 15.2 Combined Characterization and Weighting Factors Obtained with Different Methods and Different Evaluation Principles
The Tellus method4 is also based on the willingness to pay, but in this case the WTP concerns cleaning equipment. The highest cleaning cost per mass unit of an emission is used to represent society’s WTP for not emitting that substance.
The various weighting methods often give different results. This is natural as they express different weighting principles. The ecoscarcity and effect categories methods compare the LCI results with current emission policy in the area and give a high weighting to those substances that are subject to regulations. The EPS enviro-accounting method has a more global, long-term — damage-oriented approach, and puts a considerable weight on resources and greenhouse effects. The Tellus method provides some indication of what the cleaning cost could be. If weighting is done, it is recommended that several weighting methods be used.4 Recent advances in weighting are described by Bengtsson.6
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