A mixed-unit input-output model for life cycle assessment: Development, uncertainty, and application

Material flows of heavy metals and other toxic chemicals are of great concern for companies, regulators, researchers and society at large. Understanding how and why we use these toxic chemicals can help us to use them more efficiently. In this thesis I develop a mixed-unit, input-output life-cycle assessment model (MUIO-LCA) capable of tracking cadmium, lead, nickel, and zinc flows among roughly 500 sectors of the U.S. economy.; Flows of cadmium, lead, nickel and zinc commodities are modeled using data provided by the U.S. Geological Survey (USGS) by adding sectors to national input-output accounts developed by the U.S. Bureau of Economic Analysis (BEA). First an aggregate, summary level model based on the BEA 12 by 12 sector monetary input-output tables is introduced and used to explain the method. Then a detailed model is constructed by augmenting the detailed BEA 1997 Benchmark Input-Output make and use tables. The 1997 Benchmark Input-Output Accounts consist of 483 commodities, 491 industries, and 13 final demand sectors. Flows of cadmium, lead, nickel, and zinc were added as an additional 46 commodities, 20 industries, and 10 final demand sectors.; Flows of an additional 103 materials are modeled with use factors developed from USGS production and consumption data. These materials include 48 metals, 38 minerals, coal combustion by-products, agricultural products, and wood products.; An assessment of the uncertainty in model is presented. This includes a qualitative discussion of the error in input-output life-cycle assessment models as well as a first-order quantitative analysis of uncertainty in model parameters. Because of the many data points involved in the creation of the mixed-unit make and use tables, definition of distributions for each data point was not possible. Uncertainty in the direct and total requirements matrices was estimated by performing a Monte Carlo simulation where each non-zero value in the make and use table was allowed to vary uniformly +/-10%. Trials for which the total commodity output or total industry output varied from the original values by more than 20% were rejected. Trials for which an entry in the total commodity output calculated from the make and use tables disagreed by more than 20% were also rejected. Uncertainty in the use factors for the additional 103 materials was estimated by calculating these factors using consumption data as well as production data for the period 1997 to 2004.; To demonstrate MUIO-LCA the material use associated with the production of an average automobile is modeled. Results are compared to those of a comprehensive, process-based inventory of the material content and supply chain material use for a generic U.S. family sedan performed by the USAMP LCA Project (Sullivan '98). Results for the lead content of an average car were compared to an inventory provided by the Clean Car Campaign (Trumble '98). In both cases model results were generally in agreement with the independently performed studies. Comparison to these case studies highlighted the need for care in interpreting the results of the MUIO-LCA model. MUIO-LCA provides information about commodities consumed directly and throughout the supply chain of a product. While MUIO-LCA results can provide guidance on the possible material content of a product itself, the model includes tangential use of materials consumed in processes but not included in products. Thus MUIO-LCA results include but can be greater than the material content of the product itself. In certain cases MUIO-LCA results were less than those estimated in the comparison studies. Most of these differences could be explained by differences between the average commodity mix and commodity mix consumed by the automobile industry within certain commodity sectors.; An exploratory examination of the material intensity of the 483 monetary transaction commodities in the MUIO-LCA model is performed using the total requirements matrix. Results are summa...