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, according to a new MIT-led life cycle assessment.
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
How the study was
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
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
A typical pair of running shoes comprises 65 discrete parts
requiring more than 360 processing steps to assemble, from sewing and cutting
to injection moulding, 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 says, will help shoe designers
identify ways to improve designs and reduce shoes’ carbon footprint. He adds
that the findings may also help industries assess the carbon impact of similar
consumer products more efficiently.
“Understanding environmental footprint is resource
intensive. The key is, you need to put your analytical effort into the areas
that matter,” Kirchain says. “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
Kirchain and his colleagues have published their results in
the Journal of Cleaner Production.
The sum of a shoe’s
In 2010, nearly 25 billion shoes were purchased around the
world, the majority of them manufactured in China and other developing
countries. As Kirchain and his co-authors write in their paper, “An industry of
that scale and geographic footprint has come under great pressure regarding its
social and environmental impact.”
In response, companies have started to take account of their
products’ greenhouse-gas contributions, in part by measuring the amount of
carbon dioxide associated with every process throughout a product’s lifecycle.
One such company, ASICS, an athletic equipment company based in Japan,
approached Kirchain to perform a lifecycle assessment for a running shoe
manufactured in China.
The team took a “cradle-to-grave” approach, breaking down
every possible greenhouse gas-emitting step: from the point at which the shoes’
raw materials are extracted to the shoes’ demise, whether burned, landfilled or
The researchers divided the shoes’ lifecycle into five major
stages: materials, manufacturing, usage, transportation and end-of-life. These
last three stages, they found, contributed very little to the product’s carbon
footprint. For example, running shoes, unlike electronics, require very little
energy to use, aside from the energy needed to infrequently wash the shoes.
The bulk of emissions, they found, came from manufacturing.
While part of the manufacturing footprint is attributable to a facility’s
energy source, other emissions came from processes such as foaming and
injection molding of parts of a sneaker’s sole, which expend large amounts of
energy in the manufacture of small, lightweight parts. As Kirchain explains it,
“You have a lot of effort going into the molding of the material, but you’re
only getting a very small part out of that process.”
“What stood out was this manufacturing burden being on par
with materials, which we hadn’t seen in similar products,” Olivetti adds. “Part
of that is because it’s a synthetic product. If we were looking at a leather
shoe, it would be much more materials-driven because of the carbon intensity of
An improved design
In tallying the carbon emissions from every part of a
running shoe’s lifecycle, the researchers were also 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, would also
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.”