The case for testing

In recent times tanners have seen a dramatic increase in the level of leather testing required to meet either legislation or pseudo legislation (ie customer requirements to demonstrate the absence of chemicals eg formaldehyde, where no legislative limit exists). The list of chemicals that tanners are required to demonstrate as absent include such materials as:

* Aromatic amines

* Chromium VI


As tanners we can have no reason to dispute the right of legislators to introduce restrictions or define limits on consumer products when there is a need to do so. In the case of the aromatic amine legislation for consumer products there is clearly a case for the carcinogenicity of the amines as dye intermediates but no demonstrative case that this applies to the leather once the dyes are incorporated into the leather. Nevertheless, tanners complied with this legislation (ie absorbed the extra costs of testing etc) and comply with the requirements even though they still believe that there is no demonstrative health risk from leathers containing the listed aromatic amines.

It is important that tanners produce leather commodities that will be safe and acceptable in the current market. There is no place for tanners hiding behind the concept ‘leather is perfectly safe – it always has been’ unless this is true! But the same argument holds true for the legislators and larger manufacturers consuming significant quantities of leather. When specifications and limits for the presence/absence of certain chemicals are being determined they should be set on the basis of sound science and not market hype. They should also always recognise that, scientifically ‘absence of’ should be determined in terms of the detection limit, and that there is no point in looking for substances that are not remotely likely to be present.

The formaldehyde case

Tanners are increasingly being asked to produce leathers that comply with different levels of formaldehyde. The limits requested can vary from 10 ppm to 300 ppm. BLC believes this degree of stringency to be totally unnecessary.

BLC suggested guidelines are that leathers should contain no more than 300 mg/kg of formaldehyde. With modern tanning techniques leathers typically contain no more than 400 mg/kg (often far less!) so achieving less than 300 mg/kg is commercially achievable and environmentally sound. As far as BLC is aware there have been no incidence of any person having any adverse reaction to contact with leather containing formaldehyde at these levels. The target of 300 mg/kg is at a level that should be easily attainable for industry with the use of modern processing techniques. It is also in accord with most guidelines where standards for formaldehyde could apply – leather usage would come under the category ‘no skin contact’. It is generally accepted that the ‘no skin contact’ category applies to leathergoods, outer garments, shoe and upholstery leathers. The term ‘skin contact’ is used for items such as shirts and towels in the textile sector.

The figures quoted should also be put into context with levels allowed within other industries. In fact, the level of formaldehyde allowed in cosmetics is 2000 mg/kg and formaldehyde is allowed as a preservative in oral hygiene products at 1000 mg/kg. Many products on the market contain in excess of 500 mg/kg. Also these products are in direct contact with the skin, often in moist environments eg hair care products and are not considered to be adversely affecting human health. If such products as cosmetics can operate at levels of greater than 500 mg/kg why are tanners being asked to produce leathers with less than 10 ppm for upholstery leathers? Surely this is an over-kill situation!

Formaldehyde test method

Currently the leather industry’s standard method for determining levels of formaldehyde is DIN 53315. This method consists of two parts, which can be interchanged depending on the apparatus that is available. The scope of the method indicates that the method determines the available formaldehyde as found under defined conditions of the extraction and test method. It is not a measure of free or available formaldehyde in use! In fact leathers showing amounts of formaldehyde under the test conditions of over 200 ppm do not show any signs of formaldehyde by smell. The nose is a very sensitive aldehyde detector, indicating that the method is not determining free or volatile formaldehyde.

The method comprises:

Part A: A colorimetric determination of the extractable formaldehyde. In this method, the leather is extracted at 40oC, after which the extract is treated with acetyl acetone. A yellow compound (3,5-diacetyl-1,4-dihydrolutin) is formed in the presence of formaldehyde which is quantified photometrically at 412nm. Within this method there is a check to determine the presence of other compounds that may result in a coloured compound when reacted with acetylacetone. This involves addition of Dimedone to the extract prior to addition of the acetylacetone. If the resulting solution has an absorbance in excess of 0.025 (for a 1cm cell) there is the possibility of a false/positive result being obtained. BLC continues to see a high level of false/positive results using this technique and now relies upon the Part B method for formaldehyde determination

Part B: A method based on HPLC. The formaldehyde is extracted at 40oC, after which the extract is reacted with 2,4-dinitrophenylhydrazine (DNPH). The resulting extract is then separated using HPLC, with UV detection at 350nm.


Tanners will wish to continue to supply leather of commercially saleable quality that is safe for the consumer. BLC strongly supports this objective. However, tanners should not be subjected to meeting requirements for leather where increasingly stringent limits add cost to production for no added benefit. The case put forward in this article indicates that the stringent limits for formaldehyde of say 10 or even 75 mg/kg are restrictive and offer no increased security for the consumer ie they are strict limits based upon hype and not science.

A recent issue has come to our attention where limits for tri-butyl-tin (TBT) in leather are being set at the ppb (parts per billion) level. TBT is a chemical that is used in the marine industry to prevent fouling on ships, and has nothing to do with leather manufacture or the chemicals tanners use. Surely this is another case of over-kill and inappropriate application of specifications which add cost to leather manufacture but no added value to the consumer.