Bio-based carbon in footwear production

Explaining the meaning of this term and how materials with such credentials are being increasingly used within the footwear industry.

by Nicola Pichel-Juan

We probably see and hear the word ‘carbon’ every day in relation to climate change, global warming and sustainability. Organisations talk about reducing their own carbon footprint, as well as those of the products and services they provide. This article will discuss what carbon actually is and how it contributes to global warming.  We will also consider the different origins of carbon – ‘bio-based’ and ‘fossil-based’ – and why this is an important consideration when choosing materials.

Back to school

Carbon is element number 6 on the periodic table. It is crucial to all life on Earth and is the fourth most abundant element in the observable universe. Carbon is present in our planet’s atmosphere in the form of carbon dioxide (CO₂), which is a colourless and odourless gas. Human activity since the industrial revolution, mostly from extracting and burning fossil fuels such as oil, gas, and coal, has dramatically increased the amount of CO₂ in the atmosphere (see figure 1). This is a key contributing factor in climate change and global warming.

While reducing greenhouse gas emissions (including carbon) is crucial to mitigate the worst effects of climate change, it is important to recognise that not all carbon is equal. This is a key aspect of lifecycle assessment studies and should be used when analysing the impacts of different materials and products.

Bio-based or fossil carbon?

Bio-based carbon (sometimes known as ‘modern’ or ‘recent’ carbon) originates from biomass such as plants and animals. Essentially, it comes from something that was recently alive. Leather, cotton, natural rubber and cork are examples of bio-based materials that have traditionally been used in the manufacture of footwear. The carbon contained in these materials comes from a natural cycle that is kept in equilibrium over a period of a few years or decades.

Plants absorb carbon dioxide from the atmosphere and use it to produce glucose through the process of photosynthesis. These plants are eaten by animals and humans, who then respire (breathe out) the bio-based carbon dioxide. Eventually, plants and animals die and decompose, releasing further amounts into the atmosphere.

In contrast, fossil carbon has been formed over a much longer period of time. Living organisms (plants and animals) die, their remains become buried, and through heat and pressure over millions of years, fossil fuels such as oil and coal are created.  Extracting and burning these fuels releases huge amounts of additional fossil carbon into the atmosphere.

This additional carbon traps more heat, causing the planet to get warmer. Plastic materials that have been commonplace in footwear for the last few decades are derived from fossil carbon. These materials are used in such components as synthetic rubber outsoles, ethylene vinyl acetate (EVA) midsoles and polyurethane (PU) upper materials.

Therefore, although the warming effect from bio-based and fossil carbon is the same, bio-based carbon does not result in additional carbon being released into the atmosphere. Organisations are increasingly testing (and even certifying) that their materials and products are bio-based – that is, they contain a certain proportion of bio-based carbon.

Measuring bio-based carbon

There are two different measures for calculating bio-based carbon content:

1. Bio-based carbon as a proportion of all organic carbon in the material being assessed. This method tends to be used in the USA.
2. Bio-based carbon as a proportion of all the carbon in the material being assessed (both organic and inorganic). This method tends to be used in Europe.

It should be noted that for materials which do not contain any inorganic carbon, both calculations would yield the same result.

Determining the bio-based carbon content of a material involves similar processes to those used to establish the age of historical artefacts through carbon dating. The amount of radioactive carbon/carbon 14 (‘C14’) in a material is measured and compared to a reference sample with a known level of C14. The amount of C14 in plants and animals when they die is roughly equivalent to the level of C14 in the earth’s atmosphere at that time.

However, C14 is unstable, with a ‘half-life’ of 5,700 ±30 years. This means that over the course of 5,700 years, only half of the C14 present will remain. Due to their age, fossil fuels (and, therefore, items produced from fossil fuels) will contain little or no C14. This means that the amount of C14 present in a material can be used to determine its bio-based and fossil carbon content.

SATRA can arrange testing to analyse the bio-based content of materials according to ASTM D6866-18 and the equivalent ISO 16620-2 methods.

There is no universally agreed standard in terms of how much bio-based carbon a material must contain in order to be certified as ‘bio-based’. One popular scheme in Europe requires a minimum of 30 per cent of total carbon content and 20 per cent of bio-based carbon content for a material to achieve the lowest level of bio-based certification, with the highest level of certification being reserved for products with more than 80 per cent of bio-based content. This is based on the European calculation method.

In America, the US Department of Agriculture (USDA)’s ‘BioPreferred Program’ specifies different thresholds for minimum bio-based content, depending on the item type. For example, packaging must have a minimum bio-based content of 25 per cent (according to the US calculation method). The bio-based carbon percentage must then be declared on any product labelling which carries the claim that the item is a ‘USDA Certified bio-based product’.

The trend towards bio-based footwear materials

Different categories of bio-based materials are being used within the footwear industry today. Firstly, there are materials that have traditionally been used to manufacture footwear, such as leather, cotton, natural rubber and cork. There are also materials that repurpose waste, often from the food industry – examples including materials that contain pineapple, apple, grape, and mango.

Sugar cane, algae and castor oil are also successfully being used as inputs to the manufacture of midsoles and outsoles. A final category could be ‘laboratory-grown’ materials such as mycelium, although it could be argued that these are not yet being manufactured and used at scale.

It is important to understand exactly how ‘bio-based’ some of these materials truly are. A number of ‘sustainable’ materials that were launched and marketed as alternatives to leather actually contained large amounts of plastic derived from fossil fuels, either as backing materials or binders, with the plastic required to give the material strength.

More recently, however, there has been a definite shift towards the development of solely bio-based materials that do not contain ingredients derived from fossil fuels. Testing for bio-based content can be very beneficial to provide an organisation with credible data to support any claims being made for such a material.

Leather, traditionally used in the manufacture of footwear, will have a very high bio-based carbon content, as it is produced from the skins or hides of animals. Nevertheless, some leathers may have tannages, dyes or finishes/coatings derived from fossil fuels that will add fossil-based carbon.

When assessing the sustainable credentials of a material, there are typically trade-offs and compromises that must be made, as very few materials are likely to ‘tick all the sustainable boxes’. It is not sufficient to simply make generalisations that one material type is more sustainable than another. SATRA always recommends that a thorough assessment is carried out to understand the likely impacts of an item throughout its entire lifecycle and supply chain. The ability to gain an understanding of the bio-based content is one element of such an assessment.

Publishing Data

This article was originally published on page 10 of the December 2025 issue of SATRA Bulletin.

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