Fortunately, I recently had way too much fun at another west coast swing competition: 5280 Westival in Denver, Colorado. Unfortunately, my dance shoes are quickly reaching the end of their useful lives, and I may well need to purchase a new pair in the near future. So this week, I decided to do my due diligence and research the environmental footprint of one of the two styles of shoes I might purchase: boots. Specifically, I’m going to examine the Urban Premiere boots from Sway’D, shown below.
Image source: Sway’D shoes
SwayD is a major supplier of dance footwear for swing, and they often have a booth to display and sell their products at compeitions. The mission and values stated on their website does briefly touch on sustainability – “We weave a global community web, and we maintain positive roles in all the communities we touch through sustainability, empathy, tolerance, and love” – but as typical of these website, lacks any sort of detail.
From a brief Google search, it appears that the most research has been done into the carbon footprint of running shoes. One MIT study found that a typical pair of running shoes generates 14 +/- 2.7 kg CO2eq of carbon emissions. The study broke the total impact into five stages: materials, manufacturing (in China), use, end-of-life, and transport. The material processing and manufacturing processes account for 97% of these emissions, since running shoes are made of synthetic materials (lower material embodied carbon) that go through many manufacturing processes such as foaming and injection molding (relatively energy intensive).
However, the study notes that if the shoe were made of a natural material such as leather, the embodied carbon in the material would be much more significant. Since dance boots inevitably incorporate leather into the soles (it is much better and safer for dancing), I will attempt to modify this analysis accordingly. Since I know very little about the other four processes for the boots, I will assume for now that the values derived for manufacturing, use, end-of-life, and transport (total 10 kg CO2eq/pair) will be approximately the same for my boots as for the running shoes, and I will investigate the difference in materials more in-depth
The MIT study attributes 4.0 +/- 0.36 kg CO2eq per pair of athletic shoes to materials and processing, and it breaks down this impact by shoe part. My intuition suggests that my dance boots are much simpler than a pair of running shoes, since there are fewer moving parts and much less support. As far as I can tell, out of all the parts of a running shoe (shown below in this diagram from ASICS), my dance boots probably only have a sockliner, lasting, outsole, and possibly a heel counter, as well as an outer covering (which is much more extensive than a for running shoes). According to the MIT analysis, the socklining, other sole, and packaging together account for 20% of carbon emissions (0.8 kg CO2eq/pair). I will use this value as a starting point for my analysis, then examine the outsole and outer covering separately and add these emissions to derive the total.
Image source: ASICS, Anatomy of a Running Shoe
Outer covering: textile
The tag on the inside of my boot actually has a key noting the type of materials used (pictured below). Using this key to pictograms, I learned that the upper/outer covering is a textile.
I’m not a tailor, materials scientist, or other materials expert, but I believe the covering generally has two layers: a faux suede on the outside, and a synthetic foam-like polymer on the inside. I don’t want to tear apart my boots to get a super accurate measurement (and scraps were inevitably discarded as a result of manufacturing that will limit the accuracy of my measurement anyways), so to estimate the amount of material incorporated, I split the boot into several geometric shapes with the dimensions noted in the figure below: a cylinder for the leg/heel (blue), half of a cylinder for the toe (orange), a flat rectangle for the leather sole (pink), and two rectangles for the straps that wrap around the foot and leg of the boot (yellow). Note that I added 5 cm of length to each strap because I have previously trimmed the straps to a more appropriate length. The straps clearly have two layers of leather/suede; I assumed all of the other parts have one layer of faux suede on the outside and one of the synthetic material on the inside.
Artificial suede: With the measurements above, I estimate my boots have about 0.36 m2 faux leather per pair. If this can be approximated as an artificial leather made of polyurethane (about 3.7 kg CO2 per kg “pleather”, thickness approx. 1 mm, density 62 kg/m3), the artificial suede accounts for about 0.09 kg CO2 emissions per pair of boots.
Synthetic material: I estimate my boots have about 0.27 m2 of this synthetic material per pair. For now, I will assume this material is polypropylene. If the supposedly polypropylene material has a density of 946 kg/m3, the weave has a porosity (empty space) of about 45%, and the material is about 0.25 cm thick (from a measurement), this translates to about 0.35 kg of polypropylene material. This seems a bit high to me, given how light these shoes are, but I will use this number for now.
According to ecotextiles, polypropylene requires about 115 MJ/kg to manufacture the fiber, and 5,000 kcal/m of thermal energy and 0.5 kWh/m of electrical energy to weave the fibers into fabric. Since I don’t know the details of how all of this energy is generated, I will treat all of this energy as electricity and assume a general emission factor of 0.88 kg CO2eq/kWh (for the Chinese electric power generation grid mix).
With these assumptions, the synthetic material represents about 2.5 kg CO2 emissions per pair of boots.
The same pictogram indicates that the outsole of my boots are leather. A life cycle analysis of the carbon footprint of leather can be difficult, especially there has been some question if the impacts of raising the animals should be included, since leather is a co-product of milk and meat production. While there is not yet a standard methodology to calculate emissions associated with the leather industry, one study gives guidance that following ISO rules for LCA analysis, leather LCA should include processes from the slaughterhouse to the exit gate of the tannery (i.e. agriculture and animal farming are excluded). This actually makes a significant difference, since one estimate specific to the auto industry suggests that as much as 85% of the total carbon footprint of leather would be produced during cattle breeding and agricultural processes, were these included in the analysis. Similarly, another study of light leather production estimates about 53% of the carbon footprint would be attributed to agriculture and breeding, were these included in the analysis. For this analysis, I have decided to exclude the carbon attributed to agriculture and breeding.
One study analyzes the life cycle carbon footprint for producing light leather from sheep and goats, which is generally intended for manufacturing clothing and footwear. It estimated a total carbon footprint of 5.81 kg CO2eq/m2 leather.
I estimated the area of leather required to manufacture one pair of boots by actually measuring my current pair of black Sway’D Urban Premiere Boots. If I measure the sole of my shoe as a rectangle (not completely unreasonable, since scraps were likely generated in the manufacturing process), there’s about 0.016 m2 of leather in one shoe, or 0.032 m2 per pair. This represents about 0.19 kg CO2 per pair of boots.
With all of these factors combined, the total footprint looks like this:
- Manufacturing, use, end-of-life, transport: 10 kg
- Socklining, sole (other), packaging: 0.8 kg
- Outer lining (artificial suede and polypropylene textile): 2.6 kg
- Outsole (leather): 0.19 kg
- TOTAL: 13.58 kg CO2