Sneaker Life Cycle Impact
The American term sneakers refers to footwear with a flexible sole made of rubber or synthetic material and an upper part made of leather or canvas. Sneakers were originally sporting apparel, but today are worn much more widely as casual footwear. 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. A life cycle measures the environmental impact of the raw materials, processing, and transport to the final market as well as waste disposal. 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 the 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 technically 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 broke down the various steps involved in both materials extraction and manufacturing. 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. These processes are energy-intensive — and therefore, carbon-intensive — compared with the energy that goes into making shoe materials, such as polyester and polyurethane.
"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 manufacturing."
Kirchain and his colleagues have published their results in the Journal of Cleaner Production.
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,
In response to environmental concern, 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 life cycle.
Life-cycle assessment is a technique to assess environmental impacts associated with all the stages of a product's life from-cradle-to-grave (i.e., from raw material extraction through materials processing, manufacture, distribution, use, repair and maintenance, and disposal or recycling). These can help avoid a narrow outlook on environmental concerns by compiling an inventory of relevant energy and material inputs and environmental releases, evaluating the potential impacts, and Interpreting the results to help make a more informed decision. Cradle-to-grave approach means 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 recycled.
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.
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.
In tallying the carbon emissions from every part of a running shoe’s life cycle, 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.
"We are often restricted to quantifying the environmental impacts of material production only, since the manufacturing data is either not readily available or proprietary," says Khanna, assistant professor of civil and environmental engineering at the University of Pittsburgh who did not participate in the research.
He adds that knowing the manufacturing contribution may help companies find more effective ways to reduce a product’s carbon footprint.
For further information see Sneaker Life Cycle.
Sneaker image via Wikipedia.