Chapter 6 Interpretation
6.4 Quality Control
Because of the intensive use of data in a life cycle assessment, the interpretation phase includes double-checking and verification at critical points, looking at impor-tant data sets and key assumptions. This section presents a series of procedures to ensure the validity of LCA results. Some checks are done at specific life cycle assessment phases, and some are performed throughout the study. The main point of quality control is to verify the consistency of results and look into anything that is unexpected. In the case of the slightest unexpected or surprising result, never let it go—either a mistake has been made (which is usually the case) or you have the
0
Carcinogenic Noncarcinogenic
Respiratory inorganics Ionizing radiation
Ozone layer depletionRespiratory organicsAquatic ecotoxicity Terrestrial ecotoxicity Terrestrial acidity/eutrophication
Land occupation Aquatic acidification
Aquatic eutrophication Global warming
Nonrenewable energyMineral extraction 0.01
0.02 0.03 0.04 0.05
Desktop Laptop
FIGURE 6.6 Normalized endpoint impact scores for the desktop and laptop PC scenarios, based on the IMPACT 2002+ impact assessment method.
0.0E+00 5.0E–05 1.0E–04 1.5E–04 2.0E–04 2.5E–04 3.0E–04
Desktop Laptop
Human health (DALY)
Others
Airborne radon-222 Airborne dioxins
Airborne aromatic hydrocarbons Airborne arsenic
Arsenic, ion in water Airborne nitrogen oxides Airborne sulfur dioxides Particulates, < 2.5 µm airborne
FIGURE 6.7 Contribution of different pollutants to the human health damage, using the IMPACT 2002+ impact assessment method.
Interpretation 157
opportunity to learn something interesting and new. The analyst has no option but to understand and explain the discrepancy.
6.4.1 COntrOlsat every phaseOf lCa
6.4.1.1 Goal and Scope Definition: System Modeling
A properly conducted study requires a transparent and understandable representa-tion of the system. For this, a systematic flow chart depicts each scenario and system-atically numbered modules (see Figure 8.1) to avoid forgetting processes.
6.4.1.2 Inventory Analysis: Unit Control
Errors too often come from careless mistakes, especially when different units are involved in large data sets. To minimize errors, the analyst must systematically check units for each calculation, always carefully accounting for the factor of 1000 when converting between such units as grams, kilograms, tons, megajoules, and gigajoules. Moreover, it is not always legitimate to add two quantities that have the same units. For example, although the normalized scores for human health and eco-systems are both expressed in person-years per FU, they cannot be summed directly without implicitly or explicitly assuming weighting factors for the total normalized impacts of these two categories.
6.4.1.3 Inventory Analysis: Mass Balance
One way to verify inventory results is to check the mass balances of certain elements.
The carbon balance is most commonly calculated (see the example in Section 8.2.4) but balances of nitrogen, phosphorus, and heavy metals can also be checked.
6.4.1.4 Inventory Analysis: Energy and CO2 Balances “by Hand”
Several key steps are needed to establish consistent energy and CO2 balances for each unit process and across the entire system. First, for each major foreground unit process, we check the assumptions, intermediary flows, energy use per reference unit, CO2 emissions per reference unit, and finally the contribution of each intermediary flow to the energy and CO2 emissions per FU (Tables 4.2 and 4.3). Particular attention should be paid to the electricity mix chosen. Indeed, electricity mixes have very different energy efficiencies and CO2 emission factors depending on their region or country of origin (Section 4.2.2). Because this choice can greatly influence the LCA results, it is important to clarify the assumptions under which calculations have been made.
When calculating the CO2 balance, do not forget to account for the use stage and the end of life, since a large part of CO2 emissions occur during the use stage through combustion and during the waste disposal stage through combustion or eventual decomposition. This aspect is often overlooked when using an ecoinvent data set in which combustion is not included and can thus result in substantially underestimat-ing CO2 emissions.
LCA software programs (e.g., SimaPro) are extremely useful in performing the large sets of calculations necessary for a full LCA, but can also lead to errors when not properly handled. It is thus advisable to check that this software provides the same results as those obtained “by hand” or spreadsheet calculations for a few key
158 Environmental Life Cycle Assessment substances or flows. If differences concerning energy, CO2, or NOx exist, they must be understood and explained.
6.4.1.5 Inventory Analysis: Comparing CO2 and Energy
One way to check the consistency of the inventory results is to compare energy con-sumption and CO2 emissions for each submodule and for the entire FU.
First, the ranking of scenarios based on CO2 emissions should in most applica-tions be equivalent to the ranking based on nonrenewable primary energy consump-tion. If they are different, either an error has been made or something new can be learned from this difference.
The ratio of CO2 emissions to nonrenewable primary energy usage (gCO2/MJ) is calculated for each life cycle stage. The calculated ratios are then checked against the values of the stage’s dominant processes and materials, using the typical values in Figure 4.2 to check orders of magnitude. This step helps the analyst check whether the results are totally absurd, or whether certain major stages in the life cycle were not taken into account.
One example of the importance of such a step is when petrochemical materials and fuels are considered. Figure 4.2 shows that when the whole life cycle is accounted for, including precombustion, usage, combustion, and waste treatment, petrochemi-cal materials and fuels have ratios around 60 gCO2/MJ. Thus, an emissions-to-energy ratio of 6–10 gCO2/MJ for diesel or fossil fuels means that the inventory has only accounted for the fuel emissions before combustion. A value of 30 gCO2/MJ for a plastic means that emissions during the elimination stage were neglected. It is, therefore, important to check this ratio to avoid omissions of life cycle stages and other calculation errors.
6.4.1.6 Inventory Analysis: Comparison of Inventory Results with Other Studies
Similar studies should obviously be taken into account when available. If the inven-tory results differ from the results of previous studies, these differences and their causes must be highlighted and explained, such as in a comparative table. The ori-gins of these differences must be identified, such as underlying assumptions and FUs, system boundaries, reference flows, coefficients of energy consumption, and CO2 emissions per flow unit.
6.4.1.7 Impact Assessment: Toxicity Check
Because the calculation of human toxicity and ecotoxicity is still under development and can vary between impact assessment methods, the impact analysis should be per-formed using several different impact assessment methods, with careful consideration for the contribution of each pollutant. The results obtained by different methods often give different orders of magnitude, and these differences must be explained. It is also crucial to test the robustness of results using a sensitivity analysis (Section 6.5.1).
6.4.1.8 Impact Assessment: Rules for Proper Use of LCA Software
When using LCA software, keep checking that all expected emissions and extrac-tions are considered in the given impact assessment method. Sometimes an emission
Interpretation 159
is not properly included simply because its name is not strictly identical between the inventory results and the impact assessment method (e.g., “nonmethane volatile organic compounds” vs. “nonmethane hydrocarbons,” or “PM” vs. “particles” vs.
“particulate matter”). Since a given chemical can have hundreds of different names, checks should be based on the chemical’s Chemical Abstracts Service (CAS) number when possible.
6.4.1.9 Project Management: Recommended Use of Spreadsheets
Because of the large amounts of data considered, some rules must be followed when using spreadsheets to minimize errors and increase transparency. First, no cell con-taining a formula should contain a number. All data required for calculation should be entered in separate cells, including unit conversion factors and constants. For example, 1000 g/kg should be in a separate cell and documented as a unit conversion factor rather than entering “1000” as part of a formula. If a given constant is later updated (i.e., a new toxicity study updates the effect factor of particulates on human health), this update will then automatically be reflected in all the formulas in which this constant is used. Second, the units should be clearly indicated for each variable and the units of the final results should be checked. Finally, each calculation must be documented, including the assumptions made, explanation of the variables, and origins of values.
6.4.1.10 Project Management: Rules for Project Documentation
Many LCA projects and data end up as unusable by anyone other than the creator, because the associated computer files are too poorly annotated. To enable further use of these projects, some precautions should be taken.
First, the main results of the report should include enough information for a reader to trace these back to their original spreadsheet(s). This may be a table or separate document listing the files that contain the data for each figure of the report.
Each of these files should be clearly documented, including a descriptive name, the creator, and the date of the last major change.
Similarly, to allow use of the individual processes in future studies, each process should be given an understandable name, with the author and source of information specified. Processes should also be entered in such a way that they can be verified by matching the data format of the original data source.
In summary, the data obtained during the study must be sufficiently and clearly annotated, and the assumptions must be well defined and expressed in a transparent manner for a later use by someone outside the project.
6.4.2 CritiCalOr peer reviewtO CheCkfOra COMprehensiveand COnsistent study
Once these more practical controls are complete for each life cycle assessment phase, you should check that the assumptions, methods, and data are consistent with the objectives of the study and that the results are comprehensive enough to support a conclusion based on the objectives listed in the goal and scope defini-tion phase.
160 Environmental Life Cycle Assessment Called a “critical review,” this ensures that the methods used to perform the life cycle assessment are consistent with ISO 14040, that they are valid from a technical and scientific point of view, and that the data used are appropriate and reasonable regarding the objective of the study. It ensures that the interpretations reflect the identified limitations and goals of the study and that the study report is transparent and consistent. A critical review can be carried out internally or externally, but is always performed by an expert independent of the study. ISO 14040 (pp. 9–11) pro-vides more details on the procedure, and the SETAC code of practice (SETAC 1993) provides key elements of the issues to be discussed in a critical review.
The critical review task should be budgeted for as approximately 5%–10% of the total LCA cost. It is preferable to involve the reviewers as soon as possible to be able to take their comments into account before the end of the project. In addition to the ISO and SETAC guidelines listed above, Klöpffer (2005) helps detail the mandate and the modalities of such a review.
6.5 OVERVIEW OF UNCERTAINTY,