Chrome (VI) - Toxicity and testing18 September 2001
As far as I am aware, nobody has ever proved that a positive chrome (VI) result means that chrome (VI) was actually in the leather'. There has been much work carried out into the determination of hexavalent chromium in leather, but as Dr Nick Cory, head of the Leather Research Laboratory, commented at the Y2K American Leather Chemists conference, the inference that there was chrome (VI) in the leather, and that it is not just an artefact of the determination method, is hard to substantiate. Chromate toxicity So, how does chromate affect the human body? It has been well documented that chromium in the oxidation state six is an established human carcinogen, associated with lung cancers. In a paper by O'Brien (1), the 1999 SLTC Procter Memorial lecturer, and Kortenkamp, they set out how chromate is believed to cause cancer and mutations by a direct modification of the DNA chain. They state: 'In vitro, on reduction, chromate can cause a range of DNA lesions including: * single strand breaks * formation of oxidised products, such as 8-hydroxyguanosine * causing the exclusion of bases to form apyridinic or apurinic sites (A-P sites) * causing crosslinks within the DNA or to proteins or peptides 'The relative importance of these different types of lesion is unclear, although some researchers think that the crosslinking mechanism is more important in lesion formation. However, oxidative damage such as strand break and A-P sites may be important.' They also point out that for the various mechanisms to occur, the only common factor is the need for a reducing agent. The mechanisms are thought to involve Fenton type chemistry and the effects of various reducing agents on the reaction mechanism has been followed using spin-trapping electron spin resonance. The paper also gives 69 references that explain in detail the various mechanistic routes. Whatever the mechanistic routes, the question still remains whether chromium (VI) is actually present in the leather for as Dr Cory said: 'You can have chrome sulfate powder and heat it all day long at as high a temperature as you like and as dry conditions as you like and you would not get any chromium (VI) produced. 'The point is that we need something to react with the trivalent chromium such as an unsaturated fatliquor and at a high pH.' Which is where the determination method, that requires an extract at pH7.7, could be producing false positive results. In a paper by Nygren and Wahlberg (2), on 'the specification of chromium in tanned leather gloves and relapse of chromium allergy from tanned leather samples', they state that the 'various forms of chromium have different biological effects and the determination of total chromium (both soluble and insoluble), which is often used in clinical studies, may not therefore be an adequate measure of the adverse exposure.' The aim of this pilot study was, therefore, to determine the total amount of chromium and the amount of sweat leachable chromium in different leather glove materials and also to apply samples of these materials in a patch test to chromium sensitive patients to see if they could provoke a reaction. The leathers used are set out in table 1. Testing From each pair of gloves, 15 discs were punched out. Ten discs from each pair of gloves were coded and packed in plastic bags and were used for patch testing on chromium sensitive patients. The remaining five discs from each pair of gloves were used for determination of the total chromium content and leachable chromium. The determinations were carried out as follows: Leachable hexavalent chromium was determined by leaching three discs (three of the five remaining discs) in synthetic sweat. The mean value of the three discs was used. Each disc was weighed and placed in a 20ml test tube and 5ml of synthetic sweat solution were added. Leaching was performed by ultrasound for 60 minutes. Hexavalent chromium was determined using spectrophotometry after diphenylcarbazide complexation. A 2ml aliquot was withdrawn from each sample and transferred to a 15ml calibrated flask. Complexing reagent (1ml). 0.5mol dm-3 sulfuric acid (5ml) and water to volume were added and the sample was left to stand for 10 minutes to allow formation of the complex. The absorbance at 540 nm was measured. Using 40 mm cuvettes, the hexavalent chromium was quantified using standards prepared in the synthetic sweat solution. The total chromium content was determined by acid digesting the leather which was then filtered and analysed for total chromium content atomic absorption. Spectroscopy Apart from being slightly naïve about tanning terminology, they state that 'the chromium tanned leathers tested contain about 3% (m/m) total chromium while vegetable tanned leathers contain less than 0.001%(m/m) of chromium. However, small amounts of chromium can be found in dyed vegetable tanned leather, probably emanating from the leather dye. Small amounts of hexavalent chromium were leached from both chromium tanned and vegetable tanned leather and no significant difference between the tanning processes was found (see table 1). They came to the conclusion that since chromium is leached from both chromium tanned and dyed vegetable tanned leather, the risk for relapse among chromium sensitised persons wearing chromium tanned protective gloves cannot be disregarded. This conclusion was reached despite the fact that the patch tests all proved negative even though these were carried out on eight men who tested positive to a control of potassium dichromate smeared on the skin in a petroleum jelly carrier. There must also be queries about the use of synthetic sweat, in this case a mixture of sodium chloride, lactic acid and ammonia to adjust the pH to pH6.5, which has been the subject of hot debate at many IULTCS physical testing committee meetings, and at the pH values of the solutions which were set at pH7.7 I suppose it could also be questioned as to whether ultrasound is a suitable vehicle to replicate sweating next to the skin. The paper highlights the problems the leather industry faces in convincing consumers that chromium tanned leather is safe and highlights the problems the industry faces with the over-testing of samples in an effort to find a problem, where one either does not exist or where the risks are nominal. Sources Unido (3) carried out some research for pollution control in the tanning industry in south-east Asia, and investigated the sources of hexavalent chromium in leather and its products. They said: 'The oxidation of chromium (III) into chromium (VI) normally occurs in the presence of strongly oxidising agents in the acid environment, but can also take place in the presence of mild oxidising agents at high pH. 'In leather processing, neutralisation is a stage when such conditions could be created.' However, when they compared the neutralisation of wet-blue for upholstery crust, clothing and water resistant shoe-upper leathers, using a standard bicarbonate/formate recipe, they found no correlation between the chromate reduction potential of the float and the Cr (VI) content of the leather produced. Nor could a relationship be established between the pH and the hexavalent chromium content. Only, after extreme heating, eg 80ºC for at least 24 hours, was Cr (VI) detected. It is worrying to note that these conditions, while not generally realistic of conditions of usage for leather, are similar to those required by car manufacturers in their tests for automotive leathers. Surely, this is another case where there is a requirement for unrealistic testing conditions to 'find' something in the leather that would not be there under standard operating conditions. The use of wetting-back agents such as ammonia, sodium bicarbonate, cationic auxiliaries or reducing auxiliaries were also investigated for dyeing various leathers. It was found that generally, the values for Cr (VI) were below 'detectable limits'. This in itself is an interesting statement. Why 'below detectable limits?' Why not use the phrase 'not present?' If, as one manufacturer has begun to demand, the tanner has been asked to determine whether tri-butyl-tin (4) compounds are present in the leather, and none are found - is that because it is 'not detectable' or because it is 'not present'. I assume that most chemists would say the latter, but because there is a culture of belief that Cr (VI) exists in leather, the statement when none is found becomes it is 'not detectable'. This suggests the Cr (VI) is present, but that we have not yet got a method sensitive enough to detect; a misleading signal to the consumer, when we could justifiably say, as with the tin compounds, 'this chrome species does not exist in your leather.' The Unido report states that: 'In general it was found that the Cr (VI) content of air-dried leathers' was unaffected by the type of wetting-back employed, unless the leathers were heated to above 80ºC for as least 24 hours. They recommend 'the use of a reducing auxiliary prior to dyeing, instead of ammonia or other stronger alkaline reagents.' Dr Cory, in his discussion of a paper concerning 'chromium (VI) traces in leather', also states: 'I put it to you that when one heats it [the undyed Cr leather], we get oxidation of fatliquors to produce hydrogen peroxides and, in fact, what happens under the alkaline conditions of the test, is that the alkalinity is sufficient for the hydroperoxides to oxidise the chrome (III) to chrome (VI). 'The point is that we need something to react with the chrome (III), such as an unsaturated fatliquor, and a high pH. The high pH is not in the leather, but in the extraction.' The paper (5) referred to by Dr Cory, also found that when the humidity was high, the chrome (VI) levels detected fell and vice versa, (see table 2). Dr Cory commented: 'There is one thing that complicates these results. When you have higher moisture content in the leather, that moisture is inhibiting the access of oxygen and, therefore, is inhibiting the oxidation of the fatliquor. 'When you have dry conditions, you have enhanced fatliquor oxidation. When you carry out the analysis, we have more hydroperoxides present to oxidise the leachable chromium (III).' So, fatliquors, and the influence of relative humidity on them, play an important part in the levels of chromium (VI) detected in the analysis. This is in agreement with the Unido report, which states that natural or synthetic fatliquors without unsaturated fatty acids 'do not lead to chromate formation.' Conclusion Although there has been much work carried out by the leather industry and others, the need to establish whether chromium (VI) species are present in the original leather and not just a result of the method is still paramount. As Rodney Hammond, from Seton Leather said: 'You are talking about a very important subject for the leather industry and for our customers. If there is a chance that the finding of chromium (VI) is not due to the extraction, but is actually in the leather, we need to know that as an industry, so that we can make changes to not minimise, but to eliminate the possibility of chromium (VI) being present after the manufacturing process.'