their leased and sold products as assets, these companies are designing new products with reuse, remanufacture, and recycling in mind. Even before these newly designed products have come to market, the change has resulted in cost savings (Murray, 1994; Richards and Frosch, 1994). At the 3M Corporation, a simple measure—waste mass divided by the total output mass (the sum of product, by-product, and waste masses)—is used to encourage materials-use efficiency (Richards and Frosch, 1994). Enterprising entrepreneurs are using recycled materials in innovative new products, such as textiles.
The industrial ecology perspective is beginning to influence designers of manufacturing processes to seriously consider waste streams. Designers of products are beginning to view their creations as transient embodiments of matter and energy with added value that can be recaptured and recreated within a continuing flow of materials extending beyond the point of sale. Products and the materials they contain are being designed so that they can be reused at the end of their lives.
The whole industrial process can be thought of as a closed cycle in which the manufacturer has overall custody for the material used. In this system, the manufacturer must consider the entire material and energy stream, from materials input and manufacturing through the life of the product and its eventual reuse or disposal. This concept has begun to be embodied in law (as in Germany), making manufacturers responsible for their products through to final disposition.
From a systems point of view, it is not clear whether it is most useful to consider the flow of materials at the level of an individual plant, a manufacturing firm, a group of firms engaged in producing a product, an industrial sector, or industrial activities as a whole. Suboptimization (i.e., optimization of a particular process or subsystem instead of the larger system in which the process or subsystem is embedded) may be less efficient than optimization of the larger-scale system. For example, a larger, more complex, more diverse system may offer a wider variety of opportunities for reuse of materials. More complete system closure may be attained by considering all of industry as the system to be optimized and closed rather than by considering a single firm, process or product.
Some of the complexity of the flow of materials through the U.S. industrial system is suggested by the case of lead (Figure 1). This attempt to follow the flow of materials through "industrial metabolism" (Ayres, 1989) shows that the recycling of spent consumer products already plays a major role in the U.S. lead industry.
Some firms have already begun to design their products and processes with a view to closing material loops as much as possible. However, if a product is the transient embodiment of materials to which a firm has already added value, then closing the loop on those value-added materials raises an important question for