Assessing Environmental Impacts in a LIFE-CYCLE Perspective
MICH A EL Z. H AUSCHILD TECHNICAL UNIV ERSIT Y OF DENMARK
Life-cycle assessments have important limitations, but efforts are under way to improve the hat are the environmental impacts of an armchair, a cellular phone, or a steak, if you account for all the activities needed to produce, maintain, consume, and eventually dispose of the product? Life-cycle impact assessment (LCIA) is the part of life-cycle assessment (LCA) in which the inventory of a product’s material flows is translated into environmental impacts and resource consumption. The environmental impacts may range from local (e.g., land use) to global (e.g., climate change). As an environmental analysis tool, LCA is focused on the product system, which comprises all the processes of a product and its components—from the cradle to the grave (Figure 1)—and sets the frame for LCIA. This article describes current LCIA methodology and the newest developments.
© 2005 American Chemical Society
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methodology.
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FIGURE 1
From the cradle to the grave The product system comprises all the processes that a product undergoes throughout its life cycle. Materials
Manufacturing
Transport
Raw materials/ chemicals
Use
Disposal
Energy Product Emissions
Waste
Life-cycle assessment LCA is widely used by industry during the development and marketing of products and is the cornerstone of the EU’s integrated product policy (IPP) currently under development. IPP aims to reduce the environmental impact of product consumption (1, 2). In addition, the International Organization for Standardization (ISO) has standardized an LCA framework that consists of four elements (3). Goal and scope define the intended use of LCA and set boundaries for the product system (Figure 1). These also define the temporal and technological scope and assessment parameters. The system is quantified in the functional unit, which is the function or service that determines the reference flow of products. For example, a packaging study might define the functional unit as a “packaging of 1000 L of milk in 1-L containers.” The relevant comparison may be between 1000 carton boxes and 40 returnable polycarbonate bottles, which can be used on average 25 times. LCA normally compares different ways of obtaining the same function. In order to ensure relevance and fairness, it is crucial that the product systems being compared actually provide the same function. This is guaranteed by carefully defining the functional unit. Inventory analysis collects input and output data for all the processes in the product system. These data are related to the reference flow given by the functional unit. The data for the different processes are typically aggregated over the life cycle and pre82A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / FEBRUARY 15, 2005
sented as total emissions of substance X or total use of resource Y. LCIA translates inventory data on input (resources and materials) and output (emissions and waste) into information about the product system’s impacts on the environment, human health, and resources. Interpretation evaluates all the LCA results according to the study’s goal. Sensitivity and uncertainty are also analyzed to qualify the results and the conclusions.
Restrictions on LCIA Focusing on a product system and a functional unit sets the boundary conditions for the impact assessment. The product system’s emissions may occur at different times and locations, depending on where in the life cycle the process is located. Because of international trade, production is often global and processes can take place anywhere. Also, the product’s life cycle—from resource extraction to final disposal—may take years. If parts of the product are landfilled, then emissions may continue for centuries. The spatial and temporal conditions of the product system are usually poorly resolved, and the emissions are often aggregated over the life cycle. LCIA thus operates within restrictions. Because knowledge about the geographical location and the temporal course of many processes is very limited, LCIA generally relies on steady-state models, which assume a linear relationship among emission loads,
steady-state concentrations and impacts, and modest or no geographical differentiation. Similar restrictions apply to the site-generic form of environmental risk assessment (ERA), in which the outcome is an estimate of the risk level in different environmental compartments associated with a certain (e.g., annual) use and chemical emission. In sitespecific ERA, the temporal course of the emission and the local environmental conditions are often known. This facilitates the use of nonequilibrium, unsteadystate models and results in nonlinear and dynamic modeling of the environmental concentrations from emissions and their associated risks. The inventory analysis compiles the functional unit’s input and output data. Typically, these data represent a small fraction of the daily emissions to air, water, and soil from the processes equivalent to the functional unit’s share of the total output from the processes. The data are typically determined from a mass balance over the processes and presented as mass loads (kilogram per functional unit). However, the mass loads normally lack information about the emissions’ temporal course or resulting concentrations in the receiving environment. LCIA thus has to operate on mass loads representing a fraction, often infinitesimal, of the processes’ full emission outputs. This restriction does not apply in ERA, in which the object of the assessment will typically be an activity’s full emission load.
Characteristics of LCIA LCIA transforms inventory data into information about the environmental impacts from the product system. At the same time, it reduces the inventory’s numerous data items into a limited collection of impact scores. This involves modeling the potential impacts of the inventory results and expressing them as impact scores that can be added within each category. The LCA community agrees that the key areas of protection are human health, natural resources, the natural environment, and the “manmade” environment (4). Current knowledge about the relationship between emissions and their effects on the environment is used to model the impacts to these areas of protection, as shown in Figure 2. For example, greenhouse gases (GHGs), such as CO2 and CH4, cause an impact early in the pathway by increasing the atmosphere’s ability to absorb infrared radiation. A later impact would then be increased atmospheric heat content, which propagates to the global marine and soil compartments. This, in turn, causes changes in regional and global climates and rising sea levels, which eventually damage areas of protection: human health and the natural and “manmade” environments. The fate processes shown in Figure 2 include the degradation and transport of the GHG to the troposphere, the stratosphere, and the global water and soil compartments. These fate processes would occur throughout the impact pathway,
FIGURE 2
The impact pathway underlying modeling of impacts at midpoint and damage level in a life-cycle impact assessment (LCIA) (a) An environmental mechanism or impact pathway is necessary to determine the impacts and damages in LCIA. (b and c) The uncertainties of the models and parameters as well as in interpreting the indicators in terms of damage to the areas of protection contribute to the overall uncertainty of the assessment. Both must be taken into account when choosing the optimal location of the midpoint indicator. The uncertainties may favor a choice (b) early in the impact pathway or (c) near to the damage level (adapted from Ref. 5).
(a)
(b)
Substance emission
Impact n Damage
Damage Areas of protection
Uncertainty
Impact pathway
Impact 2
Impact pathway
Midpoints
(c)
Uncertainty of interpretation
Impact 1 Fate process: transport and transformation
Uncertainty
Uncertainty of models and parameters
Overall uncertainty
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all the way from emission to damage. The severity of the resource consumption is related to the material’s scarcity, that is, the relationship between the current consumption and the availability and quality of the resource’s reserve. A distinction is normally made between nonrenewable resources, such as petrochemicals and metal ores, and renewable resources, which are primarily biotic. For the latter, consumption is mainly an issue if the extraction rate exceeds the natural regeneration rate (6).
A holistic perspective In principle, LCIA attempts to model a product system’s impacts, which may damage one or more areas of protection. This means that the assessment addresses not only the toxic impacts, as does ERA, but also the known impacts of air pollutants (climate change, stratospheric ozone depletion, acidification, photochemical ozone, and smog formation) or waterborne pollutants (eutrophication and oxygen depletion). LCIA also accounts for impacts from various land uses, noise, and radiation, as well as resource use and loss. Some LCIA methods even include human health impacts from occupational exposure (6). The product may also cause unknown impacts that currently lack characterization models. At present, most practioners restrict LCIA to environmental impacts and disregard social impacts or costs. The latter are covered by life-cycle costing (LCC), which developed independently from LCA methodology and is not covered by ISO standards. Nonetheless, LCC methods are compatible with LCA (7). Omitting both costs and social impacts, which could affect human health or, indirectly, other areas, from LCIA may be seen as inconsistent with the defined areas of protection. However, researchers are now developing a methodology to assess social impacts for a life-cycle perspective that will supplement environmental LCIA (8).
The four steps of LCIA Selection of impact categories and classification. The first step is to define the categories representing the product system’s relevant environmental impacts. In most studies, existing impact categories are simply adopted. Next, the inventory’s substance emissions are assigned to the relevant impact categories, according to their contribution to the environmental problems of each category. Figure 3 shows environmental impact categories that are often modeled in LCIA. Characterization models the impact from each emission according to the impact pathway (Figure 2) and expresses an impact score in a common unit for all contributions within the category (e.g., kg CO2equivalents for all GHGs contributing to climate change). A characterization factor is derived, which expresses each substance’s specific impact (e.g., kg CO2-equivalents/kg substance). Characterization is performed by multiplying the emission with 84A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / FEBRUARY 15, 2005
the relevant characterization factor. The impacts from emissions of different substances can then be summed within each impact category; this translates the inventory data into a profile of environmental impact scores and resource consumptions. Normalization puts the different impact scores and resource consumptions onto a common scale and facilitates comparisons across impact categories. LCA is often used for comparative studies—“is alternative A preferable to alternative B?” Comparisons across impact categories are necessary when there are trade-offs, such as when improvements in one impact score are obtained at the expense of another score. Normalization relates the impact scores and resource consumptions to a common reference. Often, the impact from society’s total activities is used as the reference. Normalization then expresses the product system’s relative share of the total societal impact for each category and for each resource consumption. Valuation, which is used here for the ISO terms “weighting” and “grouping and ranking”, reflects the relative importance assigned to the various environmental impact and resource consumptions. Grouping and ranking qualitatively express the relative importance of the impact categories, whereas weighting applies factors to the impact scores to aggregate them into one figure. One-score results are easy to communicate, but the loss of information about the environmental impacts is substantial. Thus, aggregation of impact scores should be done with caution (6). Some valuation is needed to compare LCAs when trade-offs occur. Normalization expresses relative magnitudes of the impact scores and resource consumptions, whereas valuation expresses their relative importance. According to the ISO standard, the first two steps of the impact assessment are mandatory and the normalization and valuation steps are optional (3). Because preferences and stakeholder values are applied, the valuation step cannot be performed objectively. The ISO standard for LCIA does not permit valuation in studies of comparative assertions that are publicly disclosed (9). The ISO standard refrains from standardizing detailed methodological choices. However, several well-documented LCA methodologies have been developed to fill this gap over the past decade (6, 10, 11–16). Figure 3 shows an output from an LCIA methodology.
Best estimates in LCIA LCA characterization modeling for toxic and ecotoxic chemicals is inspired by ERAs, but important differences exist. ERA is often performed in a legislative context to guard against unacceptable environmental risk, not to provide the best estimate of the actual risk. Therefore, a conservative approach is often followed, and a detailed ERA is conducted only if a preliminary assessment indicates a risk. LCIA, on the other hand, attempts to address all the relevant environmental impacts of a product. Therefore, a conservative esti-
mate of a substance’s ecoFIGURE 3 toxicity is unwanted in the context of LCA (17). To avoid Impact profiles for two refrigerator designs an unintentional bias while assessing different impacts, Blue represents a design with R134a used as a refrigerant and blowing agent in foam, and red LCIA applies a best estimate is a pentane–isobutane alternative. All impacts are normalized and expressed as a “milli-perfor fate, exposure, and effect son equivalent” (mPE). One PE is the annual impact caused by an “average” person and is calof substances. culated for each category by dividing society’s total annual impact by the number of inhabitants. By avoiding conservative Decisions based on comparisons across impact categories require some valuation because cateestimates, LCIA potentially gories may carry different importance. Profiles were calculated with EDIP97 (Environmental Deconflicts with a fundamensign of Industrial Products) LCIA methodology (6 ). tal principle of sustainable development (18). In the Climate change 1992 Rio Declaration on Environment and DevelopStratospheric ozone ment, the precautionary formation principle states that where threats of serious or irreversPhotochemical ozone formation ible damage exist, lack of full scientific certainty shall not be used as a reason for Acidification postponing cost-effective measures to prevent enviNutrient enrichment ronmental degradation (19). In other words, the precauChronic ecotoxicity tionary principle sanctions a in water conservative approach if the damage could be irreversHuman toxicity via water ible or serious. LCIA still aims for a best Human toxicity estimate of risk on the basis via air of scarce knowledge. Conflict with the precautionary O 50 100 150 200 250 300 350 principle can be partially Impact scores (mPE) overcome in the valuation step if higher importance is assigned to those impact categories for which prevalidity is typically based on their derivation from cautionary considerations are justified. In a later commonly accepted environmental models that section of this article, a stakeholder-based approach are adapted to operate within the restrictions posed to LCIA is presented with value choices integrated by the LCA. into the entire assessment.
Potential impacts, not real effects Emissions that represent fractions of the total emissions from the processes are aggregated over time and space in the life-cycle inventory. The impacts are calculated by LCIA and hence represent the sum of impacts from past and future emissions. Furthermore, these emissions impact ecosystems differently, depending on where the processes are located. In reality, environmental effects arise at a specific point in time and space as a consequence of the total impact to the ecosystem. Because of missing information about emissions to the ecosystem from processes outside the product system and background concentrations of other substances, interpreting the modeled LCIA impacts in terms of real environmental effects is difficult. Instead, LCIA impacts are used as environmental performance indicators for comparing and optimizing the system or product. Product systems are fictitious entities that we cannot monitor in the real world, and LCIA characterization models are difficult to validate. Their
Impacts at midpoint and damage level
LCA began in the mid-1980s and was developed through international working groups under the Society of Environmental Toxicology and Chemistry (SETAC) (20–23). But LCIA is still expanding, and some of the central current discussions are reviewed next. Traditional characterization methods model the effect on an indicator located between emission and damage in the impact pathway at the point where it is judged that further modeling involves too much uncertainty (a “midpoint”, see Figure 2; 5, 6, 10, 14, 15). An alternative school of characterization modeling states that the LCA’s purpose is to reveal relevant damages to areas of protection. Consequently, this is what LCIA must model. Characterization modeling must include the entire impact pathway, because the damages are located at the end (Figure 2; 12, 13, 16). Proponents of the midpoint school state that damage modeling is highly speculative for several impacts, particularly for modeling those at the later part FEBRUARY 15, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 85A
of the impact pathway. It often involves value choices outside the valuation step, and these can be difficult to communicate transparently to the user. Some impacts are lost because the damage is not modeled at all (24). Furthermore, some LCA commissioners may not need comparisons or aggregations across impact categories before they act. In those cases, the midpoint indicator scores, which are less uncertain, are preferable to the damage scores. On the other hand, proponents of the damage school find that the increased uncertainty in characterization modeling is justified by a reduced uncertainty in interpreting results. A valuation is needed only for areas of protection, whereas the midpoint approaches must evaluate a higher number of midpoint-based impact scores. This evaluation must somehow interpret the potential to cause damage to areas of protection (24). For example, the midpoint approach would entail a semiquantitative analysis of the unmodeled parts of the impact pathway, in which the severity and reversibility of the impacts on endpoints, their geographical extent and expected duration, and the models’ uncertainties are considered (11). The damage approaches try to model these aspects quantitatively. Obviously, the midpoint valuation of results at midpoint level introduces additional uncertainty to the midpoint approaches. Thus, researchers must consider different types of uncertainty when they choose the position of the midpoint impact indicator in the valuation, the statistical uncertainty of the models and parameters used to model the indicator, and the uncertainty of interpreting indicator results in terms of damage to the areas of protection (Figure 2). The midpoint and damage schools of thought are not incompatible. They both model relevant impact indicators but disagree on whether the additional uncertainty in damage modeling is justified by the improved interpretation of the results. This trade-off will vary between the different categories of impact as illustrated in Figure 2. Reliable damage modeling is still a long way off for a global impact category such as climate change. And the midpoint approach still chooses the indicator rather early in the impact pathway (at the level of radiative forcing). However, it seems within reach for some of the more regional impact categories, such as acidification and photochemical ozone formation. As more and better environmental models become available, the optimal indicator point will move toward the areas of protection. And, as larger parts of the impact pathway are included in the characterization modeling, the midpoint approach will become more like the damage approach. Until they converge, the two approaches will complement each other (24). Work is under way to make the two approaches compatible (16, 25).
Getting the exposure right The impacts caused by an emission depend on the quantity of substance emitted, the emission’s inher86A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / FEBRUARY 15, 2005
ent properties, and the properties of the emitting source and the receiving environment. Traditional characterization modeling only includes the first two aspects and assumes a global set of standard conditions for the emission. This is not a problem for global-impact categories, but it makes a difference for regional or local impacts. Global standard conditions can disregard large and unknown variations in the exposure of sensitive environments. Sometimes, differences in sensitivities in the receiving environment can have a stronger influence on the impact than the properties of the substance on which the modeling is based (15, 26). LCA is a tool for pollution prevention; this is one reason why it neglects local exposure variations. On the other hand, if the decisions based on LCA are expected to improve the environment, then the modeled impacts in LCIA must be in accord with the actual impacts caused by the product system. Therefore, spatial differentiation may be important (4). A more pragmatic reason for disregarding exposure variation is ignorance about the location of processes in the product system. However, it is possible to know at least the country of emission for many processes. Exposure modeling can thus be differentiated at this level by developing site-dependent characterization factors that are based on the country or region of emission and on the substance’s properties. For example, variations in acidification impact can be as high as three orders of magnitude among different European countries (27), so even spatial differentiation at the level of countries represents a real improvement. Several groups have developed site-dependent characterization for LCIA (28, 29). Recently, methods supporting site-dependent characterization of a range of nonglobal impact categories were published for processes in Europe (5) and in the United States (15).
Values in LCIA Traditionally, value choices in LCIA are relegated to the valuation step to keep the LCIA as sciencebased and objective as possible (obviously, the goal and scope definition of the LCA also contain value-based choices). In the valuation step, the relevant values are applied on the basis of the defined goals and the study’s most important stakeholders. Social science research has challenged this traditional perception (30). Even the science-based first steps of LCIA, in which impact categories are defined and emissions classified and characterized, can strongly depend on the ethical perspective. If most LCIA developers fail to recognize this, it is because they share the same ethical perspective and science background. An economist might take a different view. Some economic schools of thought adopt a perspective that optimizes the individual’s current status, putting less weight on the possibilities for future generations. LCIA scientists normally reject discounting of
future perspectives (23). However, many economists argue that there is no reason why future environmental impacts should not be discounted as future economical implications are. Environmentalists and environmental scientists often adopt an egalitarian attitude that strives for inter- and intragenerational equity and that makes it logical to adopt a precautionary view that protects future generations. The perception that no part of an LCIA is value-free has been implemented throughout the Eco-Indicator 99 LCIA methodology, which allows the user to choose between three ethical perspectives that represent different archetypal attitudes (12). In addition to the individualist and egalitarian perspectives is the hierarchical perspective, which holds a strong belief in preventing environmental problems through regulation. Varying ethical perspectives create diverse viewpoints, so LCIA methodology provides three different characterization factors for the same substance.
Toward a recommended practice The choice of LCIA method can make a large difference in the conclusion, particularly for toxic substances (31). However, ISO has refrained from standardization of the detailed methodologies. To address this problem, the United Nations Environment Programme (UNEP), together with SETAC, launched the Life Cycle Initiative in April 2002 to “develop and disseminate practical tools for evaluating the opportunities, risks, and trade-offs associated with products and services over their entire life cycle to achieve sustainable development” (8). One element of the initiative is to identify recommended practice(s) for LCAs within the framework laid out by the ISO standards and make the data and methodology available and applicable worldwide. The goal is to recommend specific LCIA characterization methods and factors for each environmental impact category within three years on the basis of expert consensus. The recommendations will address the midpoint level, but the relationship to the damage level will also be clarified. Some LCA applications may need different LCIA methodologies, and recommended practices may also vary because of differences worldwide in impact pathways and environmental conditions (8).
Outlook One of the strengths of LCIA is its ability to include most of the environmental impacts of the product system. However, the impacts of toxic chemical emissions and land use are poorly represented in current LCIA approaches. Little consensus exists on assessment methodology for these. Characterization factors are only available for a few hundred chemicals, no matter which methodology is chosen. As a consequence, these types of impacts are often excluded from LCIA. Method development activities are currently addressing these problems. LCIA methodology
development teams, EU authorities, and industrial users are involved in a major European research program, and they plan to propose a joint methodology that will support the calculation of thousands of characterization factors with available substance data for toxic substances (32). UNEP has also targeted LCIA’s poor representation of the impacts of toxic substances and land use and plans to recommend methodology and characterization factors in 2006. LCA and LCIA were initially seen as tools for assessing all environmental problems, but it has become clear that the strengths and the limitations of LCIA are two sides of the same coin. The extension in time and space of complex product systems limits LCIA’s ability to predict and validate actual effects. Moreover, because many environmental impacts require a best-estimate model, applying the precautionary principle in traditional LCIA is difficult. In the future, LCIA must focus on the characteristic strengths of LCA and leave other issues to the relevant analytical tools. Therefore, ERA, environmental impact assessment, and LCIA will remain complementary, not competing, methods. Michael Z. Hauschild is an associate professor in the department of manufacturing, engineering, and management at the Technical University of Denmark. Address correspondence about this article to
[email protected].
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