A typical pair of running shoes accounts for 30 pounds of carbon dioxide emissions, equivalent to keeping a 100-watt light bulb on for a week, according to a new MIT-led life-cycle assessment. 

You might assume that the best way to shrink the carbon footprint of any consumer product would be to make it out of more sustainable materials. But more than two-thirds of the carbon dioxide emissions can come from manufacturing processes, with less than a third from acquiring or extracting raw materials.

A team led by Randolph Kirchain and Elsa Olivetti of the Materials Systems Laboratory examined the steps involved in making running shoes. They found that a typical pair comprises 65 discrete parts requiring more than 360 processing steps, from sewing and cutting to injection molding, foaming, and heating. For these small, light components, such processes are energy-intensive—and, therefore, carbon-intensive—compared with the production of shoe materials, such as polyester and polyurethane. The team also determined that much of a sneaker’s carbon impact comes 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.

The group’s results, Kirchain says, will help shoe designers identify ways to improve designs and reduce shoes’ carbon footprint. They may also help industries assess the carbon impact of other consumer products more efficiently. 

“Material substitution strategies alone may not be sufficient in reducing the environmental impact of products,” says Vikas Khanna, an assistant professor of civil and environmental engineering at the University of Pittsburgh, who did not participate in the research. “For example, switching to renewable material sources alone may not be sufficient for products that involve high manufacturing energy requirements.”