Washington: A typical pair of running shoes generates 30 pounds of carbon dioxide emissions, equivalent to keeping a 100-watt light bulb on for one week, a new MIT-led lifecycle assessment has revealed.
But what`s surprising to researchers isn`t the size of a shoe`s carbon footprint, but where the majority of that footprint comes from.
The researchers found that more than two-thirds of a running shoe`s carbon impact can come from manufacturing processes, with a smaller percentage arising from acquiring or extracting raw materials.
This breakdown is expected for more complex products such as electronics, where the energy that goes into manufacturing fine, integrated circuits can outweigh the energy expended in processing raw materials. But for "less-advanced" products - particularly those that don`t require electronic components - the opposite is often the case.
So why does a pair of sneakers, which may seem like a relatively simple product, emit so much more carbon dioxide in its manufacturing phase?
A team led by Randolph Kirchain, principal research scientist in MIT`s Materials Systems Laboratory, and research scientist Elsa Olivetti broke down the various steps involved in both materials extraction and manufacturing of one pair of running shoes to identify hotspots of greenhouse-gas emissions.
The group found that much of the carbon impact came from powering manufacturing plants: A significant portion of the world`s shoe manufacturers are located in China, where coal is the dominant source of electricity. Coal is also typically used to generate steam or run other processes in the plant itself.
A typical pair of running shoes comprises 65 discrete parts requiring more than 360 processing steps to assemble, from sewing and cutting to injection molding, foaming and heating. Olivetti, Kirchain and their colleagues found that for these small, light components such processes are energy-intensive - and therefore, carbon-intensive - compared with the energy that goes into making shoe materials, such as polyester and polyurethane.
The group`s results, Kirchain said, will help shoe designers identify ways to improve designs and reduce shoes` carbon footprint.
He added that the findings may also help industries assess the carbon impact of similar consumer products more efficiently.
Kirchain and his colleagues have published their results in the Journal of Cleaner Production.