Chromium Management
APPROXIMATELY 85% of the leather currently produced in the world is chrome tanned.
This is due to the superior performance of chrome-tanned leathers, both due to the high stability of the collagen fibres firmly bound to the chromium, and to the advantages of the manufacturing process: relatively fast, convenient and easily reproducible, as well as the best known.
Chromium is a metallic element, naturally present in nature in the trivalent oxidation state, in the form of chromite.
In addition to being essential for some mammalian metabolic processes, it is used in processes and applications for many consumer goods: metal alloys, pigments for paints and dyes, cosmetics, packaging.
To date, most tanning chemical research, especially regarding fine leather (post tanning), has focused on chrome-tanned leathers; such in-depth knowledge allows for correct and efficient management in terms of process as well as recycling and recovery activities.
The vast majority of mined chromium is used in the high-quality stainless steel and chrome-plated goods industry.
In the tanning process, chromium is used in its oxidation state +3 or trivalent chromium, Cr(III), in the form of basic chromium sulphate to stabilise the leather.
Less than 2% of the chromium mined annually goes to the tanning industry.
To optimise its use, a chromium recycling process has been studied and created, applied in particular in the Tuscan district by Aquarno-Consorzio Recupero Cromo, which allows to reduce the consumption of virgin raw materials.
It has been found that of the quantity of chromium used for tanning, normally only 2/3 is fixed to the collagen of the leather, while the unreacted part remains in the waste bath, constituting a disposal problem.
Before being reused, the liquids and sludge resulting from the tanning processes must be subjected to appropriate purification treatments, regulated by law.
In addition to purification, the Aquarno plant is able to recover chromium from these processing residues.
This activity allows the reuse of chromium as a secondary raw material for new tanning processes and avoids the disposal of sludge in landfills which, deprived of chromium, can be used for other purposes.
Basic liquid chromium sulphate is obtained from the chromium recovery processes, the concentration of which is approximately 10%. The potential daily production of the plant is over 21,000 kg, reused in the tanning phase by the associated companies.
When addressing health risks related to the use of chromium in the tanning sector, it is important to distinguish the forms of oxidation that this element can assume and the related problems.
As explained above, chromium is used in its trivalent oxidation state Cr(III) for the tanning processes, in the form of chromium sulphate.
Chromium sulphate is managed in tanneries in according to Italian legislation on health and safety.
As far as the finished product is concerned, the effects of chromium (Cr(III)) on health are not dangerous: consider that pigments based on Cr(III) oxide are also widely used as a green colour for tattoos, without however giving rise to hypersensitivity reactions of the skin or of the body in general (also considering that tens of millions of individuals are involved in this case).
Often when discussing chromium in relation to leather goods, the oxidised form of chromium (Cr (VI)) is invoked by critics of the tanning industry; it is therefore necessary to provide clarity on the basis of existing scientific data.
Cr(VI), in itself, is suspected to be teratogenic and is classified as a 1A carcinogen (IARC) for almost all living organisms (including humans).
Its compounds are very toxic if ingested or inhaled, but these cases do not concern leather goods.
In case of direct contact with human skin, chromium may cause sensitivity, Allergic Contact Dermatitis (ACD).
The contact of human skin with chromium and its compounds occurs through costume jewellery, alloys, cement, leather, chemicals, anticorrosives, ceramics, wood, paints and varnishes, mordants and dyes, batteries and magnetic tapes, detergents and cleaning products, etc.
Leather is just one of the many materials that contain chromium.
Chrome-tanned leather represents over 85% of the leather produced in the world, yet since the first use of this tanning technique, no cases of widespread health problems related to leather produced in this way have ever been reported worldwide.
There are various technical and scientific explanations.
As highlighted in this study conducted by Dr Angelo Moretto, Director of the Hospital Occupational Medicine Clinic and Professor of Occupational Medicine at the University of Padua, a very low percentage of the world population is subject to risks of ACD sensitivity due to the presence of chromium in leather goods.
For an allergic skin reaction, it is the rate of release, rather than the amount of chromium in the object, that is relevant.
Low amounts of Cr(III) and Cr(VI) may be released from chrome-tanned leather.
Under ’normal’ leather conditions with pH values between pH 3.2 and 3.5 and extractable Cr(III) concentrations between 50 and 500 ppm (values generally considered standard for chrome-tanned leather) at a temperature below 100°C, the electrochemical equilibrium between Cr(VI) and Cr(III) is almost completely shifted towards the safe form of Cr(III); therefore, the risk of release of Cr(VI) is almost irrelevant, even in the presence of residual fractions of free chromium.
Furthermore, as leather is made up of over 95% of its weight of collagen proteins, it is naturally equipped with a self-reducing mechanism that prevents the oxidation of chromium III to chromium VI.
Only in rare cases, in inadequately preserved leather and subjected to extreme conditions (high temperatures, UV radiation, pH variations), the unreacted and residual Cr(III) in the product can oxidize to Cr(VI), i.e., to hexavalent chromium.
In the event of chromium release, Cr(III) is released in larger quantities, and is a much less potent sensitiser than Cr(VI) since it has lower skin permeability and consequently, less frequently causes allergic dermatitis (source ISSN ISTISAN Report 9/12).
In general, it can be summarised that Cr(III):
The concentration of Cr(VI) eventually released, however, is not sufficient to cause a skin reaction for a series of reasons.
First of all, as established by REACH Regulation 1907/2006, Annex XVII, Entry 47, to date the presence limits of Chromium VI permitted in leather is 3 ppm (or mg/Kg), a limit beyond which the leather cannot be marketed.
As also demonstrated in the study submitted to support comments on the proposal to restrict skin sensitisers, downloadable on page 95 of the following link on the ECHA website, the current limits imposed by REACH, in addition to being more than sufficiently cautious, are also based on incorrect assumptions. As highlighted, the risk of allergic dermatitis due to direct contact of chromium VI with the skin would have a safety threshold of up to 25 ppm exposures (eight times higher than the legal limit!).
Chromium is not the only substance that can be attributed to the cause of ACD
ACD is often attributed, as a ’default’, to the presence of chromium in leather goods, without considering that tanned leather is the result of the combination of several substances and in particular, if the quality of the leather is very low, and this is not the case for leather produced in Italy, there may be other substances that can cause allergic/irritating skin reactions.
However, the Italian leather industry did not stop at these conclusions and, in order to pursue its commitment to consumer prevention and worker health, it learned to effectively manage and control the potential risk arising from the oxidation of Cr(III) to Cr(VI) as early as the 1990s.
In fact, consumers use leather garments in the most diverse conditions in terms of hot and cold temperatures, relative humidity, contact with the skin, sweat and water immersion.
Consequently, the general concern regards the possible oxidation of free (not collagen-bound) Cr(III) released from the leather (200-500 mg/kg and above) at more than 3 ppm Cr(VI).
Consumer safety is guaranteed by the adoption of good manufacturing practices (also indicated by ECHA) applied in the production of leather and manufactured goods.
Further checks along the supply chain verify the quality of the leather and the non-detectable nature of chromium (VI) in leather goods.
Every year, tanneries carry out thousands of control tests, both on finished products and after ageing treatments.
Only in very rare cases, mostly due to inadequate quality of the semi-finished product imported from outside Europe, are positive cases of Cr(VI) found on Italian leather, always and in any case at very low values, around 3 mg/kg.
To support the supply chain, UNIC has always been involved in projects to standardise prevention practices and in research into the risks due to the formation of chromium VI.
Guidelines with Federchimica CrVI
Numerous projects have been carried out by the sector on this topic: one of these saw the collaboration between UNIC and Federchimica for drafting the ’Guidelines for avoiding the formation of chromium VI,’ which describe the good manufacturing practices implemented by the Italian leather supply chain to avoid the onset of conditions in the leather that could favour the formation of hexavalent chromium: over time, their application has minimised the risk of chromium oxidation.
UNIC research with Poteco, Milan Polytechnic Institute and SSIP on Fattening Agents and Antioxidants/Reducing Agents
To further investigate the mechanisms that induce the formation of Cr(VI), multi-stage research was undertaken which focused on studying the oxidation reaction from Cr(III) to Cr(VI), its causes and possible remedies.
The first phase of the research, carried out in collaboration with Poteco and the Milan Polytechnic Institute, identified antioxidant products capable of preventing or attenuating the oxidation process, even in conditions of thermal-UV stress brought to extreme conditions.
Following preliminary tests on chromium salts and standard leather powder carried out by the Milan Polytechnic Institute, further experiments on leather at laboratory scale are currently underway by SSIP-Stazione Sperimentale per L’Industria delle Pelli, which will be followed by tests in the tannery on whole leathers.
UNIC Participation as a CEN Member for the Development of Chemical and Physical Methods for Analysing Chromium IV in Leather
As regards the method for the determination of Cr(VI), UNIC has collaborated over the years with the CEN-WG1 working group, chemical methods for leather, to eliminate method interferences due to the presence of dyes.
Subsequently, it collaborated with universities (Como and Northampton) and the Milan Polytechnic Institute to study a method for determining Cr(VI) in dust in the tanning work environment (shaving), as an alternative to the official NIOSJH method which caused the formation of false-positive results.