In addition, as our societies mature and populations increase, the available resourcesbecome scarcer, and greater attention must be paid to more effective utilization ofthese resources relative to this materials cycle.Furthermore, energy must be supplied at each cycle stage; in the United Statesit has been estimated that approximately one-half of the energy consumed bymanufacturing industries goes to produce and fabricate materials. Energy is a resourcethat, to some degree, is limited in supply, and measures must be taken toconserve and more effectively utilize it in the production, application, and disposalof materials.Finally, there are interactions with and impacts on the natural environment atall stages of the materials cycle.The condition of the earth’s atmosphere, water, andland depends to a large extent on how carefully we traverse this materials cycle.Some ecological damage and landscape spoilage undoubtedly result during the extractionof raw materials phase. Pollutants may be generated that are expelled intothe air and water during the synthesis and processing stage; in addition, any toxicchemicals that are produced need to be disposed of or discarded. The final product,device, or appliance should be designed so that during its lifetime, any impacton the environment is minimal; furthermore, that at the end of its life, provisionis made for recycling of its component materials, or at least for their disposal withlittle ecological degradation (i.e., it should be biodegradable).Recycling of used products rather than disposing of them as waste is a desirableapproach for several reasons. First of all, using recycled material obviates the needto extract raw materials from the earth, and thus conserves natural resources andeliminates any associated ecological impact from the extraction phase. Second, energyrequirements for the refinement and processing of recycled materials are normallyless than for their natural counterparts; for example, approximately 28 timesas much energy is required to refine natural aluminum ores as to recycle aluminumbeverage can scrap. Finally, there is no need to dispose of recycled materials.