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What is bio-based carbon?

Explaining the meaning of this term and its increasing relevance to the modern footwear industry.

by Nicola Pichel-Juan

Image © bonetta |

The word ‘carbon’ is often used today in relation to climate change, global warming and sustainability. Organisations typically talk about ‘reducing the carbon emissions’ of the products they make or source, as well as the emissions associated with their own operations and those of their wider supply chains. This article will discuss what exactly carbon is and how it contributes to global warming. We will also consider the different sources or types of carbon – something that is increasingly being evaluated in the development of new materials for the footwear industry.

Carbon is crucial to all life on earth and is the fourth most abundant element in the observable universe. It is present in the earth’s atmosphere in the form of carbon dioxide (CO2), which is a colourless and odourless gas. Human activity since the industrial revolution, predominantly in the form of extracting and burning fossil fuels such as oil, gas and coal, has dramatically increased the amount of CO2 in the atmosphere from approximately 280 parts per million (ppm) in the 18th century to around 365 ppm in 2002 and 420 ppm today. This is a key contributing factor in climate change and global warming.

While reducing the emissions of CO2­ (and other greenhouse gases such as methane) 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 element of lifecycle assessment studies and is used when analysing the impacts of different materials and products.

Types of carbon

The carbon cycle

‘Bio-based’ carbon (also known as ‘modern’ or ‘recent’ carbon) comes from biomass such as plants and animals. Plants absorb carbon dioxide from the atmosphere and use it to make many compounds through the process of photosynthesis. The plant compounds are consumed 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. This is a natural cycle that occurs relatively quickly over a period of years or decades.

‘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, which means that more heat is trapped, and the planet gets warmer.

Although the warming effect from bio-based and fossil carbon is the same, bio-based carbon does not result in additional carbon being added into the atmosphere. Increasingly, organisations are testing (and even certifying) that their materials and products are bio-based – that is, contain this type of carbon.

Understanding and assessing carbon content

There are two different measures for calculating bio-based carbon content. The first method considers such carbon to be a proportion of organic carbon in the material being assessed, and this particular method tends to be predominant in the USA.

The second method evaluates bio-based carbon as a proportion of all the carbon in the material being assessed (both organic and inorganic), and is generally the preferred method in Europe. However, it should be noted that for materials which contain no inorganic carbon, both calculations would yield the same result.

What is the difference between organic and inorganic carbon?

Total organic carbon is carbon that originates at some point in time from living things (biological materials and organic compounds) of both bio-based and fossil origin.

Total inorganic carbon is carbon originating from non-living things, such as ores and minerals.

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. This involves measuring and comparing the amount of radioactive carbon/carbon 14 (‘C14’) in a material 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, which 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. Therefore, the amount of C14 present in a material can be used to determine its bio-based and fossil carbon content.

For any organisation which would like to analyse the bio-based carbon content of its materials, SATRA is able to arrange testing to ASTM D6866-18 and the equivalent ISO 16620-2 methods.

While a high bio-based carbon content will demonstrate that a material is made from natural ingredients, it should be noted that it does not mean that the material will necessarily be biodegradable or compostable – particularly within the parameters set by existing specifications. These credentials would therefore require additional testing.

Relevance for the footwear industry

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Testing for bio-based content can be very beneficial to provide credible data to support any claims being made for such a material

Over recent years, a large number of new and ‘sustainable’ materials have been launched and marketed to the footwear industry, often as alternatives to leather. However, what has not always been made clear is that in some cases the materials contain large amounts of plastics – either as backing materials or binders – to achieve the required strength. While these materials may well have a sustainable story – for instance, in terms of repurposing waste streams (enhancing the circular economy), the use of ingredients derived from fossil fuels must also be taken into account.

More recently, 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 also 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.

How can we help?

Please email for further information on how to understand the environmental impacts of your products, or to arrange bio-based carbon testing.

Publishing Data

This article was originally published on page 14 of the December 2023 issue of SATRA Bulletin.

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