Foam.
The word appears on the product page for the Nike Air Force 1: "foam midsole."1 On the Nike Pegasus 41: "ReactX foam."1 On the Adidas Stan Smith: "cushioning."1 On the New Balance 574: "ENCAP midsole."1
Four brands. Four ways of saying the same thing without saying what it is.
What it is: ethylene vinyl acetate. A copolymer of two petroleum-derived monomers -- ethylene and vinyl acetate -- produced by high-pressure free radical polymerisation, then expanded with chemical blowing agents into a closed-cell structure.2 The vinyl acetate content in running shoe applications typically ranges from 19 to 28 percent by weight, based on published formulation studies of expanded EVA midsole composites.3 The result is a thermoplastic petroleum plastic with a cellular architecture.
The word for this, on every product page, in every shoe shop, in every review, is "foam."
Not ethylene vinyl acetate. Not petroleum copolymer. Foam. A word that describes a sensation -- soft, light, yielding -- and says nothing about the molecule producing it.
In 2024, researchers at the University of California San Diego placed microplastic particles sanded from solid EVA sheet -- not foamed midsole material, but the same copolymer chemistry -- in active compost at 45 degrees Celsius, conditions specifically designed to promote degradation. After 200 days, particle counts remained unchanged. CO2 evolution: zero. Biodegradation: zero [composting study].4 The EVA was the study's negative control. It was chosen because it was the paradigmatic material that does not break down.
The foam in your shoe is made of a plastic that, under conditions engineered for decomposition, refused to decompose.
The Inventory
Strip the brand name. Strip the proprietary technology label. What is a sneaker midsole?
Ethylene. In conventional EVA, derived from steam cracking of petroleum naphtha or ethane. The backbone monomer. Approximately 72 to 81 percent of the copolymer by weight in footwear applications.3
Vinyl acetate monomer. Produced via the reaction of ethylene with acetic acid and oxygen, or from acetylene and acetic acid.5 The comonomer that gives EVA its flexibility. In footwear formulations, typically 19 to 28 percent by weight.3 Without it, you would have polyethylene -- rigid, inflexible, unsuitable for a midsole. Vinyl acetate is what makes the plastic feel like cushioning.
Chemical blowing agent. Most commonly azodicarbonamide, which decomposes at approximately 200 degrees Celsius, releasing gases -- primarily nitrogen, carbon monoxide, and ammonia -- that inflate the polymer into its cellular structure [polymer chemistry].6 Azodicarbonamide was added to the EU REACH Regulation candidate list of Substances of Very High Concern (SVHC) in 2012 on the basis of its classification as a respiratory sensitiser [occupational].6 The ZDHC (Zero Discharge of Hazardous Chemicals) programme, whose signatories include major footwear brands, has recommended avoiding it where possible.6
Cross-linking agents, antioxidants, UV stabilisers, processing aids. The formulation chemistry that does not appear on any product page, any certification label, or any consumer-facing disclosure.
The result, assembled: a foamed petroleum copolymer with a closed-cell architecture, containing residues of hazardous blowing agent chemistry, dressed in the word "foam."
A YAN product page audit conducted in February 2026 found the following.1
| Brand | Model | Midsole Language on Consumer Page | "EVA" Named? |
|---|---|---|---|
| Nike | Air Force 1 '07 | "foam midsole" | No |
| Nike | Pegasus 41 | "ReactX foam midsole" | No |
| Adidas | Stan Smith | "cushioning" | No |
| New Balance | 574 | "ENCAP midsole" / "EVA midsole foam" | Partial -- NB is an exception |
New Balance names EVA on some product listings and even discloses "approximately 3% bio-based content" for its 574 model.1 This is more than most. It is still not standard practice.
The EU Footwear Labelling Directive (94/11/EC) requires material disclosure by broad category -- leather, textile, or "all other materials" -- for three shoe components.7 It does not require naming the specific polymer. An EVA midsole falls under "all other materials" or is simply not labelled. By contrast, EU Regulation No 1007/2011 on textile fibre names requires garments to disclose specific fibre names from a defined annex: polyester must be labelled "polyester."8 A polyester shirt is labelled with its polymer identity. An EVA shoe is not. The disclosure asymmetry is regulatory, not accidental.
The Feedstock Trick
In August 2018, Braskem and Allbirds jointly announced what they described as the first commercially available bio-based EVA for footwear -- SweetFoam, produced from Braskem's I'm Green bio-based resin.9 The bio-based fraction derives from sugarcane. Specifically: sugarcane is fermented to bioethanol, bioethanol is dehydrated to ethylene, and this ethylene is chemically identical to petroleum ethylene. It is then polymerised with vinyl acetate to produce EVA [polymer chemistry].9
The ethylene is bio-based. The polymer is the same.
But EVA is a copolymer. It requires two monomers. And here is where the green headline stops.
The vinyl acetate fraction -- typically 19 to 28 percent of the copolymer in footwear applications -- remains petroleum-derived in every commercially available footwear bio-based EVA product.10 Braskem's own documentation identifies the bio-based content of I'm Green EVA at 45 to 80 percent, depending on grade, as measured by ASTM D6866.10 This range is consistent with the ethylene fraction being bio-based while the vinyl acetate fraction is not. The 45-to-80-percent bio-based content maps precisely to the ethylene content of a copolymer containing 20 to 40 percent vinyl acetate.10
Braskem does not claim its vinyl acetate is bio-based. No Braskem product specification, press release, or distributor document identifies the vinyl acetate fraction as bio-based.10
Bio-based vinyl acetate monomer does exist in the chemical industry. Gantrade markets Bio-VAM. Celanese offers ECO-B VAM via mass balance accounting. Wacker Chemie produces VINNECO dispersions using bio-acetic acid.11 These products serve coatings, adhesives, and sealants markets. Mass balance, the accounting method used by Celanese, means fossil and bio feedstocks are physically mixed in production but allocated separately via certified bookkeeping -- it is an accounting identity, not a molecular one.11 None of these bio-based VAM products have been adopted in footwear-grade EVA production. No footwear brand has disclosed using bio-based vinyl acetate in its midsole formulation.
The scoped claim: in commercially available footwear bio-based EVA, the vinyl acetate fraction remains petroleum-derived.
Veja discloses this arithmetic more transparently than most. The Condor running shoe's midsole is 62 percent bio-based -- 57 percent sugarcane, 5 percent banana oil -- according to Veja's own project transparency page.12 The remaining 38 percent is not bio-based. Veja does not label the 38 percent as "petroleum" on the page. But subtraction is not ambiguous: 100 minus 62 equals 38 percent of the midsole derived from non-renewable sources.
And even the 62 percent. Once polymerised, it is EVA. The sugarcane ethylene has become part of a copolymer whose polymer identity, whose cross-linked molecular structure, whose environmental persistence, is identical to petroleum EVA. "Bio-based" describes the origin of some of the carbon atoms. It does not describe the fate of the polymer they form.
FKuR, Braskem's European distributor, describes I'm Green bio-based EVA as a "drop-in solution" that "has the same properties as its fossil counterpart."10 The same properties. The same polymer. The industry says so.
This is the feedstock trick: the conflation of feedstock with fate. The consumer hears "sugarcane" and infers biodegradable. The chemistry delivers an identical immortal copolymer.
The Body and the Ground
Two pathways. One during use. One after.
During use: outsole abrasion. Forster, Wilson, and Tighe (2023), at the University of New England in Australia, measured microplastic deposition from shoe outsoles during two public trail running events in New South Wales. The finding: 0.3 plus or minus 0.1 to 0.9 plus or minus 0.2 microplastic particles per linear metre per runner from shoe outsole abrasion, with deposition increasing on sloped and rocky surfaces [consumer-use migration].13 The main polymer types identified were polyurethane, polyethylene terephthalate, and polyamide -- outsole and upper materials, not EVA.13 The Wasser 3.0 research compilation reports that soft rubber outsoles produce 77 to 343 percent more microplastic particles than hard rubber outsoles [consumer-use migration].14 Every step is a deposition event. The shoe is not waiting to become waste. It is shedding while being worn.
A distinction is required. Outsole abrasion data measures rubber and polymer compounds worn against pavement. The midsole, sealed between the upper and the outsole, contributes to environmental microplastic load through a different mechanism: fragmentation during degradation over decades in landfill, not abrasive wear during walking. Both mechanisms deposit persistent polymer particles into the environment. They operate through different materials and different timescales.
The toxicity of what sheds. Kim et al. (2022), at Konkuk University in South Korea, tested leachates from microplastic fragments of four shoe types -- slippers, trekking shoes, running shoes, and sneakers -- on three aquatic organisms. Sneaker sole fragments demonstrated the highest toxicity: growth inhibition in algae, immobility and mortality in water fleas, and severe abnormalities in zebrafish embryos [ecotoxicology, aquatic model organisms].15 The key toxic chemicals identified in leachates included benzothiazole and carbon disulfide -- rubber-curing chemicals characteristic of outsole compounds, not EVA-specific additives [ecotoxicology].15
After use: the persistence. A sneaker midsole loses cushioning performance during use. Verdejo and Mills (2003) measured a 15 percent decrease in energy absorbed by EVA foam over 270 kilometres of machine-simulated running. Hamill and Bates (1988) documented 7.3 percent loss in shock-absorbing capability after 420 kilometres.16 The cellular structure wrinkles. The closed cells lose air. The runner notices reduced bounce and replaces the shoe.
But compression is not biodegradation. The cells collapse. The polymer chains remain. An EVA midsole that no longer cushions a foot is still a fully intact synthetic copolymer. It has lost a mechanical property. It has not lost a single carbon-carbon bond.
The Allemann et al. (2024) study, published in Nature's Scientific Reports, tested this directly. EVA microplastic particles -- sanded from solid EVA sheet (not foamed midsole material, but the identical polymer chemistry), size-selected between 350 micrometres and 5 millimetres, mixed with fresh compost at a 1:12 mass ratio -- were incubated at 45 degrees Celsius for 200 days [composting study].4 Zero particle count reduction. Zero CO2 evolution. Zero biodegradation [composting study].4
In the same study, under identical conditions, a bio-based thermoplastic polyurethane (TPU-FC1) degraded from 4,221 plus or minus 694.7 particles to 135 plus or minus 34 particles per half gram of compost -- a 97 percent reduction [composting study].4
One material degraded. EVA did not. EVA was chosen because it does not. The researchers used it as the non-biodegradable control.
The foamed cellular structure of shoe midsole EVA -- with its blowing agent residues and cross-linked architecture -- may behave somewhat differently from solid sheet under degradation conditions. But the polymer chemistry is identical. And the composting conditions -- active compost, 45 degrees Celsius -- are more favourable for degradation than landfill conditions. If EVA does not degrade under conditions designed to degrade it, it will not degrade in a landfill.
The claim that EVA persists for "300 to 1,000 years" appears across dozens of industry blogs, consumer guides, and secondary sources. No primary study could be located for this specific range -- it is a ghost number, widely repeated, never sourced.17 What can be stated with confidence: EVA, a polyolefin copolymer, is expected to persist for centuries in landfill environments. The Allemann et al. finding -- zero degradation after 200 days under conditions engineered for decomposition -- is consistent with a persistence window measured in centuries, not decades.
And the bio-based version? The polymer is chemically identical.9 A systematic search was conducted across PubMed, Scopus, and Google Scholar using the terms "bio-based EVA" AND ("biodegradation" OR "composting" OR "decomposition"), "sugarcane EVA" AND "environmental fate", and "biobased ethylene vinyl acetate" AND "persistence", covering publications through January 2026. The search returned zero peer-reviewed studies comparing bio-based EVA and petroleum EVA decomposition rates. The chemistry predicts identical persistence. The empirical data, specific to bio-based EVA decomposition, does not exist -- because no one has conducted the study.
TIMELINE OF PERSISTENCE
- Time on foot: 300-500 miles running; 12-24 months casual wear
- Time for cushioning to degrade: 270-420 km (mechanical, not chemical)16
- Time under composting conditions (measured): zero degradation after 200 days at 45 degrees C [composting study]4
- Time in landfill (estimated): centuries
- Difference if bio-based: none (polymer chemistry identical; no comparative study exists)
The Inversion
The softness that makes the midsole comfortable is the closed-cell structure. The closed-cell structure is the foamed plastic architecture that will persist for centuries. The cushioning system is the persistence system.
Shoe outsoles shed microplastic particles with every step -- 0.3 to 0.9 per linear metre [consumer-use migration].13 The midsole, once discarded, fragments over decades into microplastic that enters soil and waterways. The feature that makes you buy the shoe -- the soft, light, yielding "foam" -- is the petroleum plastic that outlives you.
OEKO-TEX Standard 100 tests footwear against a list of over 1,000 harmful substances at point of sale, according to OEKO-TEX's own standard criteria.18 It tests what a shoe contains. It does not test what a shoe becomes. It does not test decomposition rate. It does not test microplastic release during use. It does not test environmental persistence.18 A shoe made of a polymer that will persist for centuries can pass every existing certification -- because no existing certification asks how long the material lasts.
The regulatory architecture tests composition. It does not test fate.
The Counter-Position
Three defenses deserve engagement.
"Bio-based EVA reduces production carbon footprint." This is true. Braskem's 2025 updated lifecycle assessment, conducted under ISO 14040/44 and ISO 14071 standards and independently audited by a third party, claims bio-based EVA enables a net benefit of approximately 4.5 kg CO2e per kg when replacing fossil-based alternatives [lifecycle assessment, cradle-to-gate].19 The carbon improvement at production stage is real.
It is also scoped. The assessment is cradle-to-gate -- it measures carbon from raw material extraction through manufacturing.19 "Cradle-to-gate" explicitly excludes end-of-life. It measures the carbon cost of making the material. It does not measure what the material does for the next several centuries.
A production-stage carbon benefit is legitimate for what it claims. It does not change the polymer's molecular structure. It does not change the polymer's environmental persistence. It does not change the decomposition timeline from centuries to anything shorter. The case does not dispute the 4.5 kg CO2e/kg benefit. The case notes that this benefit answers a production question and is presented as if it answers a fate question. Reduced production carbon is not reduced persistence.
"The dose of microplastic from walking is negligible." The Forster et al. data provides per-metre particle counts for outsole abrasion [consumer-use migration].13 Daily exposure depends on distance walked, shoe type, and surface. Individual per-step exposure may be small. What is not small: the cumulative environmental load from billions of steps, on billions of shoes, across decades. Microplastic particles from shoe soles -- whether outsole rubber or midsole EVA fragments -- are persistent. They accumulate in soil and waterways. Individual dose is uncertain. Population-scale accumulation is not.
"EVA is inert and non-toxic." EVA is not classified as hazardous under CLP/GHS regulations [regulatory classification].20 The polymer matrix, in isolation, does not migrate. This defense addresses the pure polymer. It does not address the formulated product -- EVA foam containing blowing agent residues, cross-linking agents, antioxidants, and processing aids. The Kim et al. (2022) leaching study found sneaker sole fragments produced the highest toxicity among four shoe types tested, with benzothiazole and carbon disulfide identified in leachates [ecotoxicology, aquatic model organisms].15 That study tested outsole rubber, not EVA specifically. But it demonstrates that shoe sole microplastic particles are not inert in aquatic environments -- the formulated product behaves differently from the pure polymer.
What Would Change My Mind
A standardised, independently verified burial or composting study comparing bio-based EVA and petroleum EVA decomposition rates over five or more years under controlled conditions. If bio-based EVA demonstrated measurably faster biodegradation -- not just different feedstock carbon accounting, but different environmental fate -- the feedstock trick framing would require revision.
The polymer chemistry predicts identical persistence. But prediction is not measurement. The study does not exist. The question is open.
Until it is answered, the evidence supports a single conclusion: bio-based EVA is the same plastic with a different origin story.
The Levers
Tier 1: No-Cost / Low-Friction (Start Now)
Read "foam" as "plastic." In every footwear context, whenever you encounter the word "foam" describing a midsole, the material is almost certainly ethylene vinyl acetate -- a petroleum copolymer. The word is a euphemism. Now you know what it replaces.
Read "bio-based EVA" as "same plastic, different feedstock certificate." It is not a biodegradability claim. It is a production carbon accounting claim. The polymer identity is unchanged. The persistence is unchanged.
Extend the life of your current shoes. A worn pair is not generating new production emissions. Wear them until they fail. Premature replacement -- driven by fashion, not function -- multiplies the environmental load.
When shoes wear out, do not put them in textile recycling bins. The material complexity of a sneaker -- typically 40 or more materials bonded with adhesives -- makes disassembly uneconomic. Shoes placed in textile recycling are overwhelmingly exported, not recycled.
Tier 2: At Natural Replacement Point
When you are already replacing, investigate midsole chemistry. The question: is this EVA?
At least one brand using a genuinely different midsole material exists -- Vivobarefoot x Balena (developing BioCir flex, citing ASTM D6400-04 and EN 13432 composting standards, currently in pilot testing for 2026 launch).21
The friction is real. These brands are direct-to-consumer only. No high street availability. The price premium over mass-market EVA sneakers (50-120 GBP) is substantial. The range of styles is limited.
And the infrastructure gap is real. The composting infrastructure required to industrially compost a shoe does not exist at consumer scale -- the same structural gap documented in Article 023, "The Waterproof Gap" (which found that industrial composting infrastructure for footwear remains unavailable to consumers, making compostability claims functionally meaningless at point of disposal). The pattern recurs: in Article 018, "Wellington Boots," the word "rubber" concealed vulcanisation chemistry and synthetic additives comprising 15 to 85 percent of the boot by weight. Here, the word "foam" conceals a petroleum copolymer. Different footwear categories, same linguistic mechanism -- a sensory word standing in for a polymer identity the consumer never learns.
The honest assessment: the technology is ahead of the infrastructure. The alternatives are real, purchasable, and represent material innovation. They are also expensive, niche, and dependent on claims that lack independent verification.
Tier 3: Systemic
Demand polymer disclosure on footwear labels. A midsole labelled "foam" when the material is ethylene vinyl acetate copolymer is not informing the consumer. It is naming a sensation and concealing a polymer. EU Regulation No 1007/2011 requires "polyester" on a shirt label.8 The EU Footwear Directive permits "other materials" on a shoe.7 Close the gap.
Demand biodegradation testing as a condition of "bio-based" marketing claims. If a brand markets feedstock origin -- "made from sugarcane," "plant-based materials" -- it should be required to disclose environmental fate. Production carbon accounting is not persistence data. If you advertise the source, disclose the decomposition.
The product that should exist: An affordable, widely available sneaker with a non-persistent midsole. A shoe whose cushioning material biodegrades within a human lifetime under realistic disposal conditions. It does not exist. That absence is not a market gap. It is the material consequence of an industry built on a petroleum copolymer hidden behind a four-letter word.