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Down and Dirty: Why Sneakers Leave a Large (Carbon) Footprint
Green Technology Featured Articles
May 28, 2013

Down and Dirty: Why Sneakers Leave a Large (Carbon) Footprint

By Cheryl Kaften
TMCnet Contributor

It turns out that running shoes leave a nasty footprint—and it won’t help to wipe your feet at the door.

A typical pair of running shoes generates 30 pounds of carbon dioxide (CO2) emissions—equivalent to keeping a 100-watt light bulb on for one week, according to the just-released findings of a lifecycle assessment led by the Massachusetts Institute of Technology (MIT (News - Alert)).


But what’s surprising to researchers isn’t just the size of a sneaker’s footprint; it is where most of that carbon comes from. The researchers found that more than two-thirds of a running shoe’s CO2 impact is built in during the manufacturing process, with a smaller percentage originating from acquired or extracted raw-material components.

The researchers divided shoes’ lifecycle into five major stages: materials, manufacturing, usage, transportation and end-of-life. The last three stages, they found, contributed very little to the product’s carbon footprint. 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 CO2 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. 


Running shoes (photo courtesy of MIT News).

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 to the energy that goes into making shoe materials, such as polyester and polyurethane. The group’s results, Kirchain says, will help shoe designers identify ways to improve designs and reduce the carbon footprint of their products. He adds that the findings also may help industries to assess the carbon impact of similar consumer products more efficiently. “Understanding [an] environmental footprint is resource-intensive. The key is, you need to put your analytical effort into the areas that matter,” Kirchain explained. “In general, we found that if you have a product that has a relatively high number of parts and process steps, and that is relatively light [weight], then you want to make sure you don’t overlook manufacturing.”

In tallying the carbon emissions from every part of a running shoe’s lifecycle, the researchers were able to spot places where reductions might be made. For example, they observed that manufacturing facilities tend to throw out unused material. Instead, Kirchain and his colleagues suggest recycling these scraps, as well as combining certain parts of the shoe to eliminate cutting and welding steps. Printing certain features onto a shoe, instead of affixing them as separate fabrics, also would streamline the assembly process. Kirchain and Olivetti view their results as a guide for companies looking to evaluate the impact of similar products. “When people are trying for streamlined approaches to [lifecycle assessments], often they put emphasis on the materials impact, which makes a lot of sense,” Olivetti says. “But we tried to identify a set of characteristics that would point you to making sure you were also looking at the manufacturing side — when it matters.”

Vikas Khanna, assistant professor of Civil and Environmental Engineering at the University of Pittsburgh, commented that focusing on the carbon impact from a product’s manufacturing is a needed, though difficult, adjustment for the lifecycle business. “We are often restricted to quantifying the environmental impacts of material production only, since the manufacturing data is either not readily available or proprietary,” said Khanna, who did not participate in the research. He remarked that knowing the manufacturing contribution may help companies to find more effective ways to reduce a product’s carbon footprint. “It is important to keep in mind that material substitution strategies alone may not be sufficient in reducing the environmental impact of products,” Khanna said. “For example, switching to renewable material sources may alone not be sufficient for products that involve high manufacturing energy requirements."

Kirchain and his colleagues have published their results in the Journal of Cleaner Production.




Edited by Alisen Downey


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