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Your running shoes may be leaving a heavier footprint on the planet than you imagined, according to a study published in the April 2013 issue of the Journal of Cleaner Production. A life-cycle assessment by MIT’s Materials Systems Laboratory found that an average set of trainers generates 30 pounds of carbon dioxide, equivalent to leaving a 100-watt light bulb burning for a week. What surprised researchers wasn’t the volume of the shoes’ emissions, however, but rather the bulk of their origin. Led by Randolph Kirchain and Elsa Olive, the team of scientists discovered that more than two-thirds of a running shoe’s carbon impact can come from manufacturing processes, more than the acquisition of raw materials or its actual use combined.