Unfortunately tanning is a toxic business. Many people worldwide suffer from toxins that have their origin in the tanning industry. Workers in the shoe industry become sick, the environment around tanneries is often polluted and local communities, living close to tanneries, suffer from toxic water and food when the solid waste and the waste water are not treated correctly. Even consumers of leather shoes can be affected by toxins in shoes, such as hexavalent chromium, which can cause contact dermatitis.
In this factsheet we focus on the chromium tanning process (because 80 to 85 percent of all leather worldwide is chromium tanned). Other stages of the process, as well as other tanning methods, are therefore not described or explained, or not in full detail. Some other possible processes in the preparatory and finishing stage are not mentioned.
By: Change Your Shoes Campaign, 2015
Leather - a short introduction
Leather is made from animal skins. Cattle and sheep are the major source of hides and skins for leather, pig and goat skins play a less important role. In 2010, the production of bovine hides and skins was 354.6 million pieces worldwide and the production of lambskin was 537.2 million pieces (FAO, 2013). In order to produce leather, which resists decay and heat, hides and skins are first cleaned and then tanned. Tanning is the process by which hides and skins are preserved. It is the reaction of collagen fibres in the hide with tannins, chromium, alum, or other chemical agents. For the finishing process of the leather (which includes mechanical processes like cutting and smoothing, and chemical treatments to colour, soften, lubricate and apply a surface finish to the leather), further chemical are used. In 2008, around 50% per cent of all footwear was made of leather (UNIDO, 2010).
Typical processes for leather tanning & finishing.
Source: International Labor Organization, Geneva. © 2011.
There are hundreds of different leather types and many, many different chemicals to choose from when producing leathers. Basic leather properties such as softness or fullness are determined by the work in the beamhouse and by tanning. Retanning, which applies retanning agents and dyes to the material, provides the physical strength and properties desired depending on the end product. For each end product the tanning process is different and the type and amount of waste produced may vary enormously (L.A.H.M. Verheijen, January, 1996 ). In general, all leathers undergo three fundamental sub-processes in their production: preparatory stages, tanning, and crusting. Further sub-process, such as surface coating, can be added into the leather processing sequence. But the types of operations and sub-processes that leathers undergo vary.
Preparatory stage (pre-tanning)
In the preparation stage, hides and skins are prepared for tanning. After slaughter, hides and skins must be temporarily preserved by curing them with salt (other methods of preserving hides and skins are temperature control, drying, use of biocides, etc.). Preservation of rawstock has the objective of rendering the flayed pelt resistant to putrefaction to allow transport and storage (Covington A. D., 2009). During the preparatory stages, many of the unwanted raw skin components like flesh, hair or subcutaneous material are removed. Different pre-treatment options exist for the skin. Curing prevents putrefaction of the protein substance (= collagen) from bacterial growth.
The steps in the production of leather between curing and tanning are collectively referred to as beamhouse operations. They normally include:
- Soaking (= preserved raw hides regain their normal water content, dirt and salt are removed)
- unhairing (= removal of hair, usually by chemical means. This term can also include the removing of wool or bristles)
- liming (= generic term for alkaline treatment to cause hydrolysis and swelling in the pelt. This is often done by slaked lime [Ca(OH)2])
- deliming (= the step after liming when the pH is lowered for bating)
- bating (= generic term for the use of proteolytic enzymes [e.g. pancreatic material, bacterial fermentation, dog dung, etc.] to degrade non-structural proteins in the skin)
- and pickling (= acidification of pelt in brine for preservation or preparation prior to tanning).
The leather manufacturing industry processes raw skins and hides for various purposes. Raw skins are transformed into stable finished products by a series of chemical and mechanical processes (Sharphouse, 1989). Within these processes, tanning is important because animal hides decompose unless the tanning process is carried out. Tanning can be performed by different methods (see below). Up until the end of the nineteenth century, almost all leather was made by vegetable tanning - a traditional craft process which uses chemicals occuring naturally in plants for the tanning process. It is usually accepted that the faster method of chrome tanning started commercially in 1884 (Covington A. D., 2009).
It is estimated that today more than 80% - 85% of leathers in the world are tanned by chrome tanning. Chrome tanning uses chromium III sulphate ([Cr(H2O)6]2(SO4)3) to cross-link the collagen sub-units of the hide. Chromium III salts are typically prepared from chromium VI compounds. The chemistry of chromite processing highlights the potential dangers in burning or incinerating waste products from chrome tanning, such as leather dust and trimmings; the fly ash will contain Cr VI and therefore constitute an environmental danger (Covington A. D., 2009).
Chromium III salts are typically prepared from chromium VI compounds. The pH must be very acidic when the chromium is introduced to ensure that the chromium complexes are small enough to fit in between the fibres and residues of the collagen. Once the desired level of penetration of chrome into the substance is achieved, the pH of the material is raised again to facilitate the process. This step is known as basification. In the raw state, chrome-tanned skins are blue and therefore referred to as "wet blue." (http://en.wikipedia.org/wiki/Tanning#cite_note-8)
The hexavalent chromium (Cr VI) occurring in leather products is caused by impurities of the used chromium III sulphate and/or because of an oxidation of Cr III to Cr VI during further processing in aqueous medium, such as neutralisation, colouring or after tanning (BfR, 2006). It is also known that Cr III converts to Cr VI during the aging processes.
Chromium emissions may occur from chromate reduction, handling of basic chromic sulphate powder and from the buffing process. No air emissions of chromium occur during soaking or drying. At plants that purchase chromic sulphate in powder form, dust containing trivalent chromium may be emitted during storage, handling and mixing of the dry chromic sulphate (EPA).
Vegetable tanning is the oldest tanning method and it uses tannin. Tannins can be present in all plant constituents (e.g. they can be found in chestnut and oak wood, Sumac leaves and Mimosa bark) because they are a defence mechanisms against insects. Vegetable tanning may also use other ingredients found in plants. These can be used in the form of ground plant material, although nowadays it is often dried extracts of plant materials. Tannins bind to the collagen proteins in the hide. The leather is normally brown in colour. It is not stable in water.
Some reasons for the popularity of chrome tanning in the industry compared to vegetable tanning are the following:
- Chrome tanning is faster than vegetable tanning. The tanning reaction itself typically takes less than 24 hours, whereas vegetable tanning takes several weeks.
- Chromate tanning produces soft, stretchable leather with high thermal stability.
- Chrome tanning can creates water-resistant leather .
- Vegetable tanning may exhibit poor light fastness depending on the type of vegetable tanning.
After tanning, there are some more steps before the leather is finished. In general, they are post-tanning - which comprises retanning, dyeing, and fatliquoring and usually applies to chrome tanning. Drying, crusting and finishing - the crust that results after retanning and drying is subjected to a number of finishing operations. The purpose of these operations is to make the hide softer and to mask small mistakes. (L.A.H.M. Verheijen, January, 1996 ). For some leathers, a surface coating is applied. Tanners refer to this as finishing. This may include oiling, padding, polishing, impregnation, etc. According to the United Nations Industrial Development Organisation (J. Buljan, 2000), it is barely possible to find two tanneries following exactly the same finishing procedure or, more particularly, the same finishing formula even when they use the same raw material in order to produce the same type of finished leather. The operational differences in finishing grain leather and splits are considerable.
Hexavalent chromium has many negative impacts
Chromate has different effects on the organism depending on the oxidation state of the chromium. Chromium commonly occurs in two forms. Trivalent chromium (chromium III), a naturally occurring element that is relatively stable, and hexavalent chromium (chromium VI). Chromium VI has a different path of cellular uptake than Cr III and is more toxic. Hexavalent chromium [Cr(VI)] is known to be highly toxic, mutagenic and carcinogenic to humans and animals. It is usually produced by an industrial process. Its negative health effects depend on the route of exposure. For example, inhaling chromium VI can cause damage to the respiratory system, whereas dermal exposure generally does not, but can cause severe skin irritation (see below).
Mostofa, 45 years, a tannery worker is working in the dye vats at a tannery in Hazaribagh, Dhaka.
Dermal contact with hexavalent chromium compounds can cause allergic dermatitis. Chromium VI is one of the most common skin sensitizers and often causes a skin sensitizing effect. Wearing leather goods (containing hexavalent chromium) can be a means of exposure. This may happen, for example, when wearing shoes on bare feet. But wearing socks does not definitely prevent chromium VI from penetrating the skin. The key to the hexavalent chromium’s mutagenicity and carcinogenicity is the ability to penetrate the cell membrane (K. Kolomaznika, 2008).
Primary irritant dermatitis is related, as mentioned above, to the direct cytotoxic properties of chromium VI, while allergic contact dermatitis is an inflammatory response caused by the immune system. Sensitized individuals will exhibit an allergic dermatitis response when exposed to chromium above a threshold level (Polak, 1983).
The Agency for Toxic Substances and Disease Registry in the US lists the following symptoms in characterization of chromium allergic dermatitis: dryness, erythema, fissuring, papules, scaling, small vesicles and swelling (MacKie, 1981) (Adams, 1990).
Chromium pollution by tannery operations are ranked within the top ten pollution problems by the Blacksmith Institute (Blacksmith Institute, 2011). According to the information collected in Blacksmith’s inventory of sites, South Asia, and in particular India and Pakistan, has the highest number of tanning industries, with South America also at risk of large populations being exposed to chromium contamination. They estimate the ‘population at risk’ at identified sites to be 1.8 million people.
Processing one metric ton of raw hide generates 200 kg of final leather product (containing 3 kg of chromium), 250 kg of non-tanned solid waste, 200 kg of tanned waste (containing 3 kg of chromium), and 50,000 kg of waste water (containing 5 kg of chromium) (S. Hüffer, 2004). Thus, only 20% of the raw material is converted into leather, and more than 60% of the chromium is in the solid and liquid waste. During the production of leather goods, especially shoes, manipulation waste is produced, which makes about 15–20% of the entry material leather (K. Kolomaznika, 2008).
Hexavalent chromium can therefore be present in tannery waste water and solid waste with a considerable impact on the environment. Unfortunately the higher cost associated with the treatment of effluents sometimes leads to illegal dumping to save costs.
As a wide variety of chemicals is used during the tanning process, waste water from this industry can have very different chemical makeups. Other pollutants than chromium of concern within the tanning industry include azodyes, cadmium compounds, cobalt, copper, antimony, barium, lead, selenium, mercury, zinc, arsenic, polychlorinated biphyenls (PCB), nickel, formaldehyde resins and pesticide residues (M., 2010). In many countries, tannery sites are clustered together and are, therefore, creating heavily polluting industrial areas .
The United States Department of Labor (OSHA - Occupational Safety & Health Administration) states on its homepage the following information for workers dealing with Cr VI:
All hexavalent chromium compounds are considered carcinogenic to workers. The risk of developing lung, nasal, and sinus cancer increases with the amount of hexavalent chromium inhaled and the length of time the worker is exposed. Certain hexavalent chromium compounds produced lung cancer in animals that had the compounds placed directly in their lungs.
Direct eye contact with chromic acid or chromate dusts can cause permanent eye damage. Avoid eye contact with dusts, fumes, smoke, liquids, mists, and aerosols containing hexavalent chromium.
Hexavalent chromium can irritate the nose, throat, and lungs. Repeated or prolonged exposure can damage the mucous membranes of the nasal passages and result in ulcers. Some employees become allergic to hexavalent chromium so that inhaling the chromate compounds can cause asthma symptoms such as wheezing and shortness of breath.
Prolonged skin contact can result in dermatitis and skin ulcers. Some workers develop an allergic sensitization to chromium. In sensitized workers, contact with even small amounts can cause a serious skin rash.
Tannery workers are exposed to different chemicals and not ‘only’ chromium. Therefore, negative health effects on tannery workers may even be more diverse than described above. More information on health effects can be found at the homepage of the International Labour Organization of the United Nations:
Current legislation in the European Union
In the European Union, a common threshold of 3 mg/kg (0.0003 % by weight) of chromium VI is applied for all leather articles and articles containing leather, from 1 May 2015. Chromium VI will be regulated by REACH (Regulation on Registration, Evaluation, Authorisation and Restriction of Chemicals, came into force on 1 June 2007). This will not apply for products that were placed on the market before 1 May 2015. Furthermore, there will be a 12-month period after the entry into force of this Regulation for the stakeholders concerned to take measures to comply with this Regulation, including addressing articles already in the supply chain, and held in stock.
Commission Regulation (EU) No. 301/2014 of 25 March 2014 amending Annex XVII to Regulation (EC) No. 1907/2006 of the European Parliament and of the Council on the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) as regards chromium VI compounds text with EEA relevance. Available in all EU languages at: http://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32014R0301
This regulation is a very important step by the European Union to protect its customers. But this regulation does not mean no chromium is being used in leather production and, therefore, means that not all people in the supply chain are protected from harmful hexavalent chromium.
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