With the help of the Bureau of Mines, we have developed an environmentally oriented framework for characterizing material flows in the United States.1 Choosing metrics requires a grasp of the diversity and enormity of U.S. materials flows (Figure 1). Our framework considers primarily three components: inputs to the economy (including imports), outputs (including exports), and extractive wastes. We aim for comprehensiveness in this framework in the sense that we do not want to "lose" materials and would eventually hope to record the complete materials balance. Our choice of inputs and outputs as major categories derives from the simplest of materials-flow models. We group extractive wastes separately because they represent immense mobilizations of materials readily distinguished from commodities, products, and other wastes. We use previously published data for all the values indicated and generally adhere to existing classifications.
We segment inputs into energy, construction minerals, industrial minerals, metals, forestry products, and agricultural products. We class outputs as domestic stock,2 atmospheric emissions, other wastes, dissipation, and recycled materials. Imports and exports represent the masses of major individual commodities and classes of commodities crossing U.S. borders. Extractive wastes include residues from the mining and oil and gas industries. We account for water in Figure 1 but not in the material metrics because the weight and omnipresence of this resource would obscure what remains. We also omit consumption of atmospheric oxygen for biological respiration and in industrial processes.3 We do not explicitly consider manufactured chemical products, but do include the mass of feed stocks used for organic and inorganic chemical production.
Materials have the advantage of offering a single unit of measure, weight, that allows for direct comparison across a broad range of material types. Kilo-