Development of the Method and U.S. Normalization Database for Life

Jul 18, 2006 - Normalization is an optional step within Life Cycle Impact. Assessment (LCIA) that may be used to assist in the interpretation of life ...
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Environ. Sci. Technol. 2006, 40, 5108-5115

Development of the Method and U.S. Normalization Database for Life Cycle Impact Assessment and Sustainability Metrics J A N E B A R E , * ,† T H O M A S G L O R I A , ‡ A N D G R E G O R Y N O R R I S §,| U.S. Environmental Protection Agency, 26 W. Martin Luther King Drive, Cincinnati, Ohio 45268, Five Winds International, 137 Newbury Street, Boston, Massachusetts, 02116, Sylvatica, 147 Bauneg Hill Road, Suite 200, North Berwick, Maine 03906, and Harvard School of Public Health, 677 Huntington Avenue, Boston, Massachusetts 02115

Normalization typically is set within the reference boundaries of total emissions or resource use for a given area, which may be global, regional, national, or local. The total emissions or resource for a given area are calculated on a per capita basis or similar measurement for a more consistent measure of intensity. The normalization results are then used with a baseline scenario, such as a given alternative product system. Normalization can be viewed as a method of putting the characterization results in context or “analysis of significance” (2). The main aim of normalization is to relate the environmental burden of a product (or service) to the burden in its surroundings. “Normalization relates the micro world of an LCA study to the macro world in which the product/service is embedded” (3). As with all approaches to normalization, normalization of LCI and LCIA results adjusts the characterization results to a common unit. The basic calculation of normalization results is of the following:

Ni ) Si/Ai

Normalization is an optional step within Life Cycle Impact Assessment (LCIA) that may be used to assist in the interpretation of life cycle inventory data as well as life cycle impact assessment results. Normalization transforms the magnitude of LCI and LCIA results into relative contribution by substance and life cycle impact category. Normalization thus can significantly influence LCAbased decisions when tradeoffs exist. The U. S. Environmental Protection Agency (EPA) has developed a normalization database based on the spatial scale of the 48 continental U.S. states, Hawaii, Alaska, the District of Columbia, and Puerto Rico with a one-year reference time frame. Data within the normalization database were compiled based on the impact methodologies and lists of stressors used in TRACIsthe EPA’s Tool for the Reduction and Assessment of Chemical and other environmental Impacts. The new normalization database published within this article may be used for LCIA case studies within the United States, and can be used to assist in the further development of a global normalization database. The underlying data analyzed for the development of this database are included to allow the development of normalization data consistent with other impact assessment methodologies as well.

Introduction The International Organization for Standardization (ISO) 14042 refers to normalization as “calculating the magnitude of indicator results relative to reference information” with the aim to better understand the relative magnitude for each indicator result of a product system under study (1). Normalization can be used by LCA practitioners to check for inconsistencies of Life Cycle Inventory (LCI) and Life Cycle Impact Assessment (LCIA) results, provide and communicate information on the relative significance of the indicator results, and prepare for additional procedures, such as grouping, weighting, or life cycle interpretation. * Corresponding author phone: (513) 569-7513; fax: (513) 5697111; e-mail: [email protected]. † U.S. Environmental Protection Agency. ‡ Five Winds International. § Sylvatica. | Harvard School of Public Health. 5108

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where i ) the environmental problem (or impact category), N ) the normalized results, S ) product system results prior to normalization, and A ) results of the reference area. The results in each impact category are divided by an estimate of the total impacts in that category for a chosen system or region over a chosen time period (3) and thus are termed “relative contribution approach.” Relative contribution approaches are considered external normalizations because they require an external database of the other contributors within the temporal and spatial region. The advantage of external normalization is that weighting may occur independent of the case at hand (4). This relieves study commissioners from conducting weighting processes for every case study, and allows for a consistent framework for multiple cases. Disadvantages of external normalization include the need for a compiled database at the appropriate spatial and temporal scale, the lack of consensus on the normalization database, and the introduction of additional uncertainty as it relates to the database described. Although it is not necessary, one of the most common uses of normalization is as a preparation step for the weighting process (5). As such, it is valuable to consider how normalization should best be structured to allow it to act as an appropriate precursor for weighting. Over the past few years, it has become customary to conduct normalization at the national level on an annual basis for practical reasons (i.e., generally inventory data are collected on a national basis), but there is no scientific reason these spatial and temporal scales need to be chosen. Ideally, detailed temporal and spatial modeling would be conducted within the characterization phase that would allow the aggregation of impacts across spatial and temporal scales. A reference spatial and temporal scale would then be cited as the basis of the normalization database. Theoretically, an external normalization could be conducted with different spatial and temporal scales, but, in actuality, no known normalization databases exist in which the temporal and spatial scales are different for different impacts. The database presented within this paper was compiled to be consistent for use with TRACI (6) (and its incorporated stressors) and as such may be used at varying levels of spatial detail for the various impact categories. The underlying data are provided so that practitioners may use these data for additional purposes, such as application to other impact assessment methodologies (e.g., the CML 2001 (7), Eco10.1021/es052494b CCC: $33.50

 2006 American Chemical Society Published on Web 07/18/2006

indicator 99 (8), Environmental Design of Industrial Products (EDIP) 97 (9), or IMPact Assessment of Chemical Toxics (IMPACT) 2002+ (10).

TRACI - The Tool for the Reduction and Assessment of Chemical and Other environmental Impacts The U.S. normalization database to be presented here has been constructed using the TRACI framework, impact categories, and stressors. The TRACI tool is a collection of life cycle impact assessment models that provide relative comparison of the potential harm to human health and the environment due to releases of substances to the air and water. TRACI includes impact indicator models for the following ten impact categories: acidification, ecotoxicity, eutrophication, global warming, human cancer, human noncancer, human health criteria, ozone depletion, smog formation, and fossil fuel use (6). TRACI’s list of impact categories has been more fully described in another paper, but it is worth repeating that this list is not all-inclusive. While TRACI does cover many of the categories in other impact assessment methods, it does not include impacts such as genetically modified organisms, invasive species, pathogens, radiation, human casualties due to accidents, or resource depletion issues other than fossil fuels. It also does not assess the positive impacts which can be a result of human behavior (e.g., wetland restoration). The set of impact categories listed here can be viewed as a minimal set, but could benefit from further expansion in the future. The normalization data presented here have been developed to support the impact categories which are found within TRACI. These include mostly chemical emissions and a few resource use issues. These data may be used to support decisions that are made at various levels, but as an example, if an LCA were conducted on a specific object and its alternative (e.g., insulated wire), then the practitioner would need to collect the inventory data to address the impacts of each of the emissions and resource uses related to this product and its alternative. The normalization database could then be used to put each of the impact categories into perspective. TRACI currently provides location-specific modeling for acidification, eutrophication, human health cancer and noncancer, human health criteria, and photochemical smog formation. The level of spatial differentiation available within TRACI may be seen in Table 1. This level of differentiation was determined based upon the available input parameters and base models as well as the probabilistic analyses conducted to determine the effects of maintaining this differentiation. In all cases which allow spatial differentiation, the user may choose not to differentiate if the inventory data are available at a much more coarse level. Prior Practice. Previously compiled databases have generally been compilations of emissions and resource use flows for a geographic region (often national, continental, or global) during a selected time period (often annual) for specific environmental themes (11-14), and several examples of the use of normalization within case studies involving various weighting exercises exist (15, 16). To assist in U.S. case studies and to further the development of a global normalization database, this compilation of data comprising a U.S. normalization database is being published. Normalization Method. To develop normalized values for each of the impact categories in TRACI, we first selected the reference system. It is admissible, though it has not been common practice in LCA to date, to use different reference systems for different impact categories (4). However, for this baseline version of the U.S. normalization database, the same reference system for each impact categorysannual emissions

TABLE 1. TRACI Categories and Spatial Coverage impact category

U. S.

acidification (H+ eq/kg emission) ecotoxicity (2,4-D eq/kg emission) eutrophication (nitrogen eq/kg emission) ozone depletion (CFC-11 eq/kg emission) global warming (CO2 eq/kg emission) human health cancer (benzene (C6H6) eq/ kg emission) human health criteria (PM2.5 eq/kg emission) human health noncancer (toluene (C6H5CH3) eq/ kg emission) photochemical smog (g NOx eq/kg emission) fossil fuel use (MJ surplus energy/ MJ of extracted energy)

×

urban/ rural

east/ west

U. S. census region

state

×

×

×

×

×

×

× × × × ×

×

×

×

×

×

×

×

×

×

×

×

from U. S. sourcesswas selected. This selection of annual emissions and resource uses from the United States is consistent with most previously compiled normalization databases, and could also be justified as a typical U.S. frame of reference. It should be noted that this perspective may have some biases which are inherent to the scale chosen, e.g., the U.S. tends to be one of the largest consumers of fossil fuels globally, but because of past regulations has done a better job of controlling persistent toxic pollutants than many other countries on the globe. While these facts are important to keep in mind when viewing data which have been normalized, the U.S. annual perspective is still a valid one, and probably one which is most familiar with the proposed audience of this work. Estimates were sought for total annual domestic U.S. release (or consumption) flows for each of the substances for which TRACI contained impact characterization factors. We sought data for the most recent year available for all impact categories to establish a base year. Although more current data were available for many of the impact categories, the most current data for the National Emissions Inventory for stationary hazardous air pollutants (HAPs) were for the year 1999, establishing the base-year. Therefore, the external normalized values for a given impact category reflect an estimate of the total impact in a category for the activities which occur within the U.S. (or given region) over one year for the year 1999. In the case of fossil fuel consumption, these fossil fuels may be from domestic or foreign sources, but the normalization inventory reflects activities which occur on U.S. soil, regardless of the location of the extraction of the fuel. The external normalized value for each impact category was calculated by multiplying the reference system’s inventory flows (either the emissions or resource use quantities) with the corresponding TRACI characterization factors, and summing the results for that impact category. It is important to note that these normalization data reflect the emissions and resource uses in 1999. While it may seem apparent that it would be better to reflect a more up-to-date time frame, even this would have its limitations in trying to make decisions which have their greatest impacts in the future. With each passing year, agricultural practices are becoming more efficient and control technologies are capturing more of the pollutants which would have been released in prior years. Developments in fields such as nanotechnology may greatly impact what future pollution VOL. 40, NO. 16, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Normalization Parameters of U.S. Dataset system basis system treatment magnitude scales temporal scale spatial scale

geographic region (U.S.) all in-system activities total national and per capita per year the smallest spatial delineation defined by the characterization factors.

scenarios look like. Several of TRACI’s impact indicator models provide characterization factors that vary regionally: for example, there are state-level characterization factors for acidification, eutrophication, smog formation, and criteria pollutants. For the substances associated with each of those impact categories, we sought annual emission totals for each state. The total U.S. normalized value is equal to the sum of the statelevel characterized inventories. Thus, the state-level emissions were multiplied by the state-specific characterization factors to yield U.S. normalized values for that impact category. For example, to calculate the normalized value for the spatially influenced impact category (i)

NV(i) ) Σx,sex,s × CF(i)x,s where NV(i) ) normalized value for impact category (i) within the temporal scale of one year, ex,s ) emission or resource depletion of stressor (x) for the spatial scale (s) within the temporal scale of one year, and CF(i)x,s ) characterization factor for impact category (i) for stressor (x) within spatial scale (s). Normalized life cycle inventory results compare the life cycle impacts from the studied product system to those of the normalization reference system. Results are expressed in terms of a ratio of the product system impacts to the reference system impacts. When the product system being studied is on a magnitude scale relevant to a single person rather than a nation (e.g., 1 week’s diaper use for a baby, or the packaging required for 1000 fast food sandwiches), then it is more appropriate to normalize with respect to this same magnitude scale. For use in such cases, we provide normalization data on a per capita basis, to complement the national-scale data. Therefore, the normalized value for each stressor was divided by the U.S. or regional population. An adjusted 1999 population estimate based on the 2000 census data was used to represent the total U.S. population (17, 18). Normalization Parameters. The system boundaries for a normalization database can be specified to various temporal or spatial scales. Ideally, the parameters selected for a normalization would be consistent with the experience of those who intend to utilize the normalization database, especially for weighting purposes. In the U.S. it would be typical for respondents to be most knowledgeable and thus, most focused, on U.S. boundaries. Traditionally, within LCIA circles, normalization databases have been calculated based on a one-year time frame. The parameters chosen for the U.S. normalization dataset are shown in Table 2. These parameters were used for each impact category. However, more detail on the technical approach and the data sources used to determine the annual emissions estimates follows in the discussion of each individual impact category. The normalization results are also presented below for each of TRACI’s impact categories. The relative contribution of each substance contributing >0.01% of the total normalized value is also displayed. Cross Cutting Remarks (Which Include Multiple Impact Categories). The TRI database was one of the significant data sources for the normalization database presented here. Unfortunately, some of the substances listed within the TRI database are expressed in vague terms (e.g., mercury, mercury 5110

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compounds, copper, copper compounds, lead, lead compounds). In these cases the actual speciation and composition of every emission is unknown. Here, the highest TRACI characterization factors were used to represent the class when the normalized value was determined. In actuality, if the compositions and speciations were known, the normalized value for that substance (or class of substances) and the total normalized value may be lower than is shown in the results.

Acidification Acidification refers to processes that increase the acidity (hydrogen ion concentration, H+) of water and soil systems. The acidification model in TRACI makes use of the results of an empirically calibrated atmospheric chemistry and transport model to estimate total North American terrestrial deposition of expected H+ equivalents as a function of the emissions location (6). The majority of the annual emissions data used in calculating the normalized values for the acidification impact category were obtained from the National Emissions Inventory and Data: Criteria Pollutants (19). This report presents EPA’s latest data quality reviewed, final estimates of criteria pollutants, which includes ammonia, sulfur dioxide, and nitrogen oxides. The 1999 Toxics Release Inventory (TRI) database was used as a supplementary data source. National data for hydroflouric acid and hydrochloric acid were obtained from the 1999 TRI Explorer. The 1999 TRI data were reported in pounds per year converted to kilograms per year (20). Emissions reported in the TRI are derived from a subset of economic sectors and from a subset of facilities whose releases or uses of the TRI substance exceed EPA reporting thresholds. Thus, TRI does not provide estimates of total U.S. releases; however, it provides the best available estimate of a subset of national releases for these substances. The acidification model provides characterization factors for eight substances; however, normalized values were limited due to lack of publicly available data. Compilation of the data is limited based on the availability of information within the sources, in this case nitric oxide and nitrogen dioxide are included within the category of nitrogen oxides as opposed to having individually reported emissions estimates, and sulfur dioxide is listed, but sulfur oxides are not a separate item within the source database. As stated in the Normalization Method section, U.S. annual emissions were used as the reference system. The annual emissions are found in the Supporting Information. Acidification is a regionalized impact category within TRACI, meaning that users may use state-specific characterization factors in cases where they know the state of origin for the life cycle inventory data. TRACI provides regionalized characterization factors for ammonia, NOx, and SO2; however, TRACI provides only average characterization factors for the remaining pollutants. Further, the normalized values were then divided by the U.S. population to obtain per capita normalized values. The normalized values are measured in H+ equivalents per year per capita. The results may be found in the Supporting Information. The U.S. normalized value for acidification based on 1999 emissions data is 2.08 × 10+12 H+ equiv/yr. This may also be expressed in a per capita basis as 7.44 × 10+03 H+ equiv/yr per capita. Three substances or groups combined to provide greater than 99% of the total acidification normalized value. These substances or groups were ammonia (40%), sulfur dioxide (39%), and nitrogen oxides (20%).

Ecotoxicity TRACI characterizes the potential ecological harm of a unit quantity of substance released into the environment in ecotoxicity potentials (ETPs) measured in 2,4-dichlorophe-

noxy acetic acid (2,4-D) equivalents for 161 substances released to air and water (6). Two data sources were considered for the emissions of each of these substances annually: NEI and TRI (19, 20). Both databases were downloaded for comparison, and for every substance, the TRI data source provided more inclusive and comprehensive data. TRACI characterization factors are available for both emissions to air and water, and so these results are reported independently below. As for releases to water, TRI was used as the only data source, since NEI only contains air emissions, not water emissions. After the data source was selected for each substance, the TRACI characterization factor was used to determine the normalized value for each individual substance, and then a table was constructed to represent those substances whose normalized value contributed >0.01% of the total normalized value. The normalized values were then divided by the U.S. population to obtain per capita normalized values. The normalized values are measured in kg of 2,4-D equiv/yr per capita. Results may be found in the Supporting Information. The total U.S. normalized value for 1999 for ecotoxicity is the sum of the air and water normalized values: 2.03 × 10+10 2,4-D equiv per yr (air) and 2.58 × 10+08 2,4-D equivs per yr (water). When added together, the air emissions dominate the normalized value for a total of 2.06 × 10+10 2,4-D equiv per yr. This can also be expressed as 7.38 × 10+01 2,4-D equiv per yr per capita. The largest contributions were determined to be copper, nickel, zinc, and chromium, however, it is recognized that the emissions were often not speciated (e.g., chromium) or differentiated (e.g., copper compounds), and these calculations may not have overestimated the normalized value since the most potent value was used when the emissions data were unclear about speciation or differentiation.

Eutrophication TRACI characterizes the potential impairment of U.S. water bodies caused by various emissions to air and releases to water. The substances released to the air that contribute to eutrophication are phosphorus, NOx, nitrogen dioxide, nitric oxide, and ammonia. Additionally, other releases to water that contribute to eutrophication include phosphorus, phosphate, nitrogen, nitrate, chemical oxygen demand (COD), biological oxygen demand (BOD), and ammonium (6). The eutrophication model provides characterization factors at the state and regional levels; therefore, state annual emissions for these substances were collected. The primary data sources used in obtaining the state annual emissions estimates were the National Emissions Inventory (NEI) (19) for air emissions, and the USDA for water emissions (21). Normalized values for eutrophication were calculated for air releases and water releases. The results may be found in the Supporting Information. The total U. S. normalized value for 1999 for eutrophication is the sum of the air and water normalized values: 1.4 × 10+09 N equiv per yr (air) and 3.58 × 10+09 N equiv per yr (water). Both air and water emissions result in similar potential for impacts for the total normalized values of 5.02 × 10+09 N equiv per yr and 1.80 × 10+01 N equiv/yr per capita. The leading contributors were phosphorus, nitrogen oxides, ammonia, and nitrogen.

Global Warming The impact category of global warming (also known as global climate change) refers to the potential change in the earth’s climate caused by the buildup of chemicals that trap heat that would have otherwise passed out of the earth’s atmo-

sphere. Simulations have been conducted to try to quantify the potential endpoint effects of these disturbances (22). The primary source of data for the global warming annual emissions was the USEPA Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2002 (23-29). The inventory represents the most comprehensive and up-to-date inventory of the U. S. economy, and is consistent with IPCC guidelines (30-32). The emissions included within this source include most anthropogenic sources and sinks of emissions, but do not include nonanthropogenic sources and sinks of emissions. Only substances for which national or global emissions data were available are included. The total U.S. normalized value for global warming is 6.85 × 10+12 CO2 equiv per yr or 2.45 × 10+04 CO2 equiv per yr per capita. Carbon dioxide contributed the largest majority (83%) of the total normalized value for global warming. Emissions of methane (9%) and nitrous oxide (6%) were the next largest contributors.

Human Health Cancer TRACI’s human health cancer category is based on the human toxicity potential (HTP), a calculated index that reflects the potential harm of a unit of substance released into the environment, including both the inherent toxicity of a compound and its potential dose. The models in TRACI characterize cancer HTPs for air and surface-water emissions. For human health cancer, emissions are evaluated in terms of benzene equivalents. The human health cancer model provides HTPs for 146 substances, with individual HTPs calculated for air and water (6). For releases to air, there are two major EPA data sources: National Emissions Inventory and Data:Criteria Pollutants (19) and 1999 TRI Explorer (20). Emissions reported in the TRI are derived from a subset of economic sectors and from a subset of facilities whose releases or uses of the TRI substance exceed EPA reporting thresholds. Thus, TRI does not provide estimates of total U.S. releases; however, it provides the best available estimate of a subset of national releases for these substances. For this effort, both databases were downloaded and the emissions were compared to determine which set of data was more comprehensive. Based on the above analysis, it was decided that the TRI database would provide more comprehensive coverage for all air emissions. Water emissions were estimated using the TRI database since it was the best available source for this information. After the data source was selected for each substance, the TRACI characterization factor was used to determine the normalized value for each individual substance, and then a table was constructed to represent those substances whose normalized value contributed >0.01% of the total normalized value. The normalized values were then divided by the U.S. population to obtain per capita normalized values. The results may be found in the Supporting Information. The total U.S. normalized value for 1999 for human health cancer is the sum of the air and water normalized values: 7.03 × 10+07 benzene equivalents/year (air) and 1.76 × 10+06 benzene equiv/yr (water). The total normalized values are 7.21 × 10+07 benzene equiv/yr and 2.58 × 10-01 benzene equiv/yr per capita. The most significant contributors were reported to be arsenic (35%), chromium (13%), and lead (11%) in air, and lead (74%), allyl trichloride (12%), and arsenic (8%) in water; however, it is recognized that the emissions were often not speciated (e.g., chromium) or differentiated (e.g., lead compounds), and these calculations may not have overestimated the normalized value since the most potent value was used when the emissions data were unclear about speciation or differentiation. VOL. 40, NO. 16, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Human Health Noncancer TRACI’s human health noncancer category is based on the human toxicity potential (HTP), a calculated index that reflects the potential harm of a unit of substance released into the environment, including both the inherent toxicity of a compound and its potential dose. The models in TRACI characterize noncancer HTPs for air and surface-water emissions. For human health noncancer, emissions are evaluated in terms of toluene equivalents. The human health noncancer model provides 292 HTPs for air and water emissions. For releases to air, there are two major EPA data sources: National Emissions Inventory and Data:Criteria Pollutants (19) and 1999 TRI Explorer (20). Emissions reported in the TRI are derived from a subset of economic sectors and from a subset of facilities whose releases or uses of the TRI substance exceed EPA reporting thresholds. Thus, TRI does not provide estimates of total U.S. releases; however, it provides the best available estimate of a subset of national releases for these substances. For this effort, both databases were downloaded and the emissions were compared to determine which set of data was more comprehensive. Based on the above analysis, it was decided that the TRI database would provide more comprehensive coverage for all air emissions. Water emissions were estimated using the TRI database since it was the best available source for this information. After the data source was selected for each substance, the TRACI characterization factor was used to determine the normalized value for each individual substance, and then a table was constructed to represent those substances whose normalized value contributed >0.01% of the total normalized value. The normalized values were then divided by the U. S. population to obtain per capita normalized values. The results may be found in the Supporting Information. The total U. S. normalized value for 1999 for human health noncancer is the sum of the air and water normalized values: 3.69 × 10+11 toluene equiv/yr (air) and 4.24 × 10+10 toluene equiv/yr (water). The total normalized values are 4.11 × 10+11 toluene equiv/yr and 1.47 × 10+03 toluene equiv/ yr per capita. Given the previously mentioned caveats about the lack of differentiation and speciation within the emissions inventory, lead dominated the total air contribution (79%) and lead also dominated the total water contribution (99%).

Human Health Criteria Particulate matter can be emitted directly from various industrial and residential sources or they can form when gases such as sulfur oxide and SO2, NOx, and VOCs interact with other compounds in the air. Exposure to coarse particles (PM10) is primarily associated with the aggravation of respiratory conditions, such as asthma, whereas fine particles (PM2.5) can cause increased cardiovascular hospital admissions, increased restricted activity days, increased cases of chronic bronchitis, and even premature death. TRACI uses PM2.5 equivalents to characterize the human health impacts of criteria emissions. Annual emissions estimates were collected for four substances (PM10, PM2.5, NOx, and SO2). The primary source for annual emissions data for the criteria impact category was the National Emissions Inventory (NEI) and Data: Criteria Pollutants, summary tables (19). The report presents EPA’s latest data quality reviewed, final estimates of criteria pollutants, which includes, PM, sulfur dioxide, and nitrogen oxides. The NEI data were reported in short tons per year; 1999 data were used. The criteria model provides state level characterization factors. The results may be found in the Supporting Information. The 1999 U.S total normalized values for human health 5112

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criteria were 2.13 × 10+10 PM2.5 equiv/yr and 7.63 × 10+01 PM2.5 equiv/yr per capita. The largest contributors were PM10 (60%), PM2.5 (31%), sulfur dioxide (8%), and nitrogen oxides (1%).

Ozone Depletion The ozone depletion impact category in TRACI calculated the potential contribution of substances to the destruction of the ozone layer. National emissions data were obtained for 19 of the 65 substances in the United Nations Environment Program’s model (33). Even though this may seem to be poor coverage, in actuality, emissions of ozone depleting substances are significantly down because of the Montreal Protocol, and this national emissions data set is expected to fairly accurately represent 1999 emissions. The primary source of the U.S. annual emissions data for ozone depleting substances was the Inventory of U. S. Greenhouse Gas Emissions and Sinks: 1990-1999 (28). The report estimates 1999 anthropogenic emissions of greenhouse gases in the United States, including halogenated compounds. Halogenated compounds are ozone-depleting substances. Additional databases used to estimate the emissions of ozone depleting substances included the 1999 Toxics Release Inventory (TRI) (20). The results may be found in the Supporting Information. Only substances for which national emissions data were available are presented. The total U.S. normalized values for ozone depletion are 8.69 × 10+07 CFC-11 equiv/yr and 3.11 × 10-01 CFC-11 equiv/yr per capita. The major contributors were CFC-12 (58%), carbon tetrachloride (20%), CFC-11 (8%), Halon 1301 (7%), HCFC-22 (4%), and HCFC-141b (1%).

Photochemical Smog Formation In TRACI, the photochemical smog model characterizes the influence of anthropogenic emissions of NOx and volatile organic compounds (VOCs) as precursors to smog (measured in NOx equiv). Although there are hundreds of smog substances in the TRACI model, most of the substances fall under the broad category of VOCs. EPA provides annual national and state-level emissions estimates for NOx and VOCs. The primary source for annual emissions data for the criteria impact category was the National Emissions Inventory (NEI) and the Data: Criteria Pollutants, summary tables (19). The report presents EPA’s latest estimates of criteria pollutants, including VOCs, SO2, and NOx. The report does not include any VOCs from nonanthropogenic sources (e.g., vegetation, especially trees), which may be a significant contributor to smog formation. The report does not separate the data into individual VOCs; therefore, data for generic VOCs were used to compute the U.S. normalized values for photochemical smog. The photochemical smog model provides characterization values at the state level. Therefore, annual emissions data were collected for this category. As stated in the overall Normalization Method, the state-level normalized values were summed to reach the national normalized value for each substance. The results are found in the Supporting Information. The total U.S. normalized value for photochemical smog is 3.38 × 10+10 NOx equiv/yr and 1.21 × 10+02 of NOx equiv/yr per capita. The largest contributors were nitrogen oxides (61%) and VOCs (39%).

Fossil Fuel Depletion For the fossil fuel depletion category, TRACI characterizes the potential depletion of three fossil fuels: coal, natural gas, and oil. Annual U.S. consumption estimates were collected for each fossil fuel. These fossil fuels may be from domestic or foreign sources, but the normalization inventory

reflects activities which occur on U.S. soil, regardless of the location of the extraction of the fuel. The estimates were converted to energy units (mega-joules). The annual consumption estimates for coal, natural gas, and oil were obtained from the U.S. Department of Energy’s (DOE’s) Energy Information Administration (EIA) State Energy Data Report (33). The report includes U.S. annual estimates for coal, natural gas, oil, nuclear, hydroelectric, and waste (waste energy). The latest report estimates energy consumption for the year 1999. The results may be found in the Supporting Information. Oil contributes to more than half of the normalized value. The normalized value for fossil fuel consumption is 4.04 × 10-02 MJ per yr per capita. The total U.S. normalized value for fossil fuel depletion was 1.14 × 10+07 MJ surplus/yr and 4.08 × 10-02 MJ surplus/ yr per capita. The largest contributors were oil (57%), natural gas (36%), and coal (8%).

Discussion When conducting a comprehensive impact assessment it is very difficult to determine the significance of the results relative to each other since the basis and even the units for each impact category are different. The normalization database provides a consistent perspective on data by placing each impact category on the same basis (e.g., annual emissions or resource use). While it must be recognized that this still does not mean that all of the impact categories are “equally significant” after a normalization exercise, the results of the normalization exercise would be useful in a weighting exercise in which the weighting panel were told to consider all current and future impacts which would result from the annual U.S. emission rates for pollutant categories (e.g., global warming, human health cancer, smog formation, ecotoxicity) or consumption rates for each resource use category (e.g., fossil fuel depletion). Within the weighting exercise, it must be recognized that the determination of weighting factors is not a scientific process, but is entirely subjective (i.e., different people bring different perspectives to the table) and would rank the relative significance of different impacts based on their prior knowledge, history, experiences, and preferences.

Results Individual spreadsheets are available for each impact category, and summary tables and diagrams are provided within this report. References are noted along with their corresponding expected levels of coverage. Tables 3 and 4 summarize the results of the normalization calculations for each of TRACI’s impact categories. Publicly available data sources were used to populate the databases of emissions, and then a selective process was used to determine the most comprehensive database. Stressors were then normalized based on TRACI’s characterization factors at either the national or state level. Population data for calculating per capita normalization were also used on a national or state level based on the emissions data. In the table below, and in the supplemental tables and graphs (Supporting Information), the normalization data represents a compilation of the U.S. annual emissions or resource use for 1999. The data are subject to the limitations of the publicly available databases from which they were constructed.

This paper presents the first U.S. normalization database available for use in sustainability, life cycle impact assessment, process design, or pollution prevention. All of these applications require quantitative data to guide decision making which impacts the current and future generations. Whether the application has a short term or long term focus may determine the relative importance and attention given to a particular impact category, but each of these applications benefit from the use of normalization data to put the information into perspective. For example, within sustainability applications, the focus may be greater on issues of resource depletion (e.g., fossil fuel depletion) or pollutants which have longer term consequences (e.g., global warming, or persistent bioaccumulative chemicals which contribute to human health toxicity and ecotoxicity). Similarly, based

TABLE 3. Summary of Normalized Values for TRACI Impact Categories for 1999

impact category

normalized value air

normalized value water

total normalized value

normalized unit

acidification ecotoxicity eutrophication global warming human health cancer human health noncancer human health criteria ozone depletion photochemical smog fossil fuel depletion

2.08 × 10+12 2.03 × 10+10 1.44 × 10+09 6.85 × 10+12 7.03 × 10+07 3.69 × 10+11 2.13 × 10+10 8.69 × 10+07 3.38 × 10+10 NA

NA 2.58 × 10+08 3.58 × 10+09 NA 1.76 × 10+06 4.24 × 10+10 NA NA NA NA

2.08 × 10+12 2.06 × 10+10 5.02 × 10+09 6.85 × 10+12 7.21 × 10+07 4.11 × 10+11 2.13 × 10+10 8.69 × 10+07 3.38 × 10+10 1.14 × 10+07

H+ equiv/yr 2,4-D equiv/yr N equiv/yr CO2 equiv/yr benzene equiv/yr toluene equiv/yr PM2.5 equiv/yr CFC-11 equiv/yr NOx equiv/yr surplus mega-Joules of energy/yr

TABLE 4. Summary of Normalized Values for TRACI Impact Categories for 1999 on a Per Capita Basis

impact category

normalized value air per capita

normalized value water per capita

total normalized value per capita

normalized unit per capita

acidification ecotoxicity eutrophication global warming human health cancer human health noncancer human health criteria ozone depletion photochemical smog fossil fuel depletion

7.44 × 10+03 7.29 × 10+01 5.15 × 10+00 2.45 × 10+04 2.52 × 10-01 1.32 × 10+03 7.63 × 10+01 3.11 × 10-01 1.21 × 10+02 NA

NA 9.24 × 10-01 1.28 × 10+01 NA 6.30 × 10-03 1.52 × 10+02 NA NA NA NA

7.44 × 10+03 7.38 × 10+01 1.80 × 10+01 2.45 × 10+04 2.58 × 10-01 1.47 × 10+03 7.63 × 10+01 3.11 × 10-01 1.21 × 10+02 4.08 × 10-02

H+ equiv/yr/capita 2,4-D equiv/yr/capita N equiv/yr/capita CO2 equiv/yr/capita benzene equiv/yr/capita toluene equiv/yr/capita PM2.5 equiv/yr/capita CFC-11 equiv/yr/capita NOx equiv/yr/capita surplus mega-Joules of energy/yr/capita

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on the intention of the application, the focus may be spatially local or spatially global. In all cases, these applications benefit from the perspective of the normalization data presented here. The normalization database was built entirely with publicly available data, and thus is subject to the limitations of the databases. Two of the largest limitations are the lack of speciation for substances such as chromium, and the aggregation of individual substances into a larger set (e.g., VOCs). The results reported within this database show that a small number of substances are responsible for the vast majority of the potential impacts in every category. This information could be used to further streamline and prioritize the search for inventory data in future case studies. This normalization exercise is the first published U.S. normalization database with the broad range of impact categories included here. The limitations of the data have been presented within this paper, while recognizing that this represents the best data available at this time. The data presented within this paper can be used in normalization and weighting exercises, to set the boundaries on a screening exercise, or to help provide perspective at a macro level. Future work at the macro level is planned.

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Supporting Information Available The results of the normalization analysis have been presented within this paper. Supporting Information is provided in table and figure format to allow a better view of the data which make up this analysis. This Supporting Information includes tables of the inventories for each of the impact categories as well as tables and charts of impact assessment results for each impact category. The tables provide a listing of each of the substances which contribute to each impact category, except in those cases where the number of substances is very largesin these cases only those substances contributing >0.01% of the total normalized value for that impact category are shown. Similarly, for those categories with numerous substances, for the sake of representation, only substances contributing 1% or more of the total normalized value for that impact category are shown within the pie chart. The tables and charts included are subject to the limitations listed within the paper. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review December 13, 2005. Revised manuscript received May 25, 2006. Accepted June 13, 2006. ES052494B

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