The useful life of the product is also very important, if the metal product lasts N units of time and takes X units of energy to produce then the relative energy investment is X/N. If a plastic razor lasts N/50 units of time and takes Y units energy then 50Y/N is your energy investment. If Y is less than X/50, then the more environmentally object is the plastic razor.
Don't forget that, if recycled, Steel is almost infinitely useful. That is to say, if you put 2lb of steel into a recycling process, you get 2lb of steel out. The adjustment for oxidization only applies marginally because, with razors, one is recycling stainless steel which oxidizes at a rate so slow that it could be called, for the sake of argument, none at all.
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