Areas of application include adhesives, textile processing, as a preservative in some paints, coating products and also in some cosmetics. Formaldehyde is also used in the production of some polymers. When combined with phenol, urea or melamine, formaldehyde produces a hard thermoset resin. It is also used during embalming processes to preserve corpses.
Potential exposure routes for formaldehyde are as wide ranging as smog, cigarettes and tobacco smoke and some household sources such as fibreglass, carpets, foam in cushions, permanent press fabrics, paper products, household cleaners, shampoos, bubble bath, medicines and disinfectants.
Why is it restricted?
The use of formaldehyde in many construction materials means that it is a common indoor air pollutant. At concentrations above 0.1ppm in air, inhaled formaldehyde can irritate the eyes and mucous membranes, potentially resulting in watery eyes, headache, a burning sensation in the throat, and difficulty breathing. Formaldehyde is also classified as a carcinogen. 
How is it relevant to leather?
Historically, formaldehyde has been used as a tanning agent due to its ability to crosslink proteins such as collagen. In addition, it has been used (again historically) to crosslink protein finishes such as casein. 
Formaldehyde or materials that are formaldehyde release agents may, however, be used in some areas of tanning, specifically in the production of wet-white, to allow stabilisation prior to splitting or shaving and in the production of syntans.
In modern processing, formaldehyde can be used in the manufacture of certain polymeric-based synthetic tanning agents. Its presence in leather may be as a result of the condensation residue from some synthetic tanning agents.
Theoretically, formaldehyde should be fixed to the collagen during processing (considering that formalin is used as a preservative because of its reactivity with protein). However, some of the reactions used in the preparation of syntans are reversible (eg production of melamine formaldehyde resins). This means that, under certain conditions, it is possible that formaldehyde is liberated. Also oxidisation of oils/fats can result in formaldehyde formation.
Practical advice on avoiding formaldehyde formation (as determined by the current industry test methods) includes:

  • Careful management of formaldehyde releasing compounds, for example:

–  Aldehyde tanning agents
–  Formaldehyde resin retannages
–  Preservatives (formaldehyde can sometimes be used as a preservative in processing chemicals)

  • The use of oxidisable oils should be carefully controlled as these can oxidise on exposure to air, moisture and heat, resulting in possible formaldehyde formation.
  • Adding reducing agents to the float (such as sodium metabisulfite) may help avoid formaldehyde liberation.

What is the legislation?
Restricted substance legislation is highly variable depending upon the final application of the leather in the product and the target user. Also there are considerable variations in legislation depending on the country where the leather or product is manufactured or sold. It is one of the key examples of how variable the restrictions can be.
Within Europe there is no general legislation that limits the presence of formaldehyde in leather.  There are some individual countries that have restrictions on its presence in consumer products and formaldehyde is restricted within the recently published EN71-9 Toy standard. Various Eco-labels require that levels of formaldehyde are determined and the automotive industry tends to have quite strict limits on formaldehyde release from car interior materials. Table 1 lists some of the limits currently in place for formaldehyde. In addition to these restrictions on the extractable levels of formaldehyde present in leather, there are also some Eco-labels (Blue Angel for example) that have restrictions on the volatile formaldehyde released from leather and products.
BLC guidelines state that leathers should contain no more than 200ppm of formaldehyde for articles in general use. If the item is in direct skin contact this should be 75ppm, and 20ppm for items used by babies (<36 months). Typically, with modern tanning techniques, leathers contain 400ppm or less.
As far as BLC is aware there have been no incidences of any person having an adverse reaction to contact with leather containing formaldehyde at these levels. The target of 200ppm is at a level that should be easily attainable for industry with the use of modern process techniques. It is also in accord with most standards where there is no direct and prolonged skin contact. 
The figures quoted should also be put into context with levels allowed within other industries. In fact, the level of formaldehyde allowed in cosmetics (Directive 76/768/EEC) is 2000ppm and formaldehyde is allowed as a preservative in oral hygiene products at 1000ppm.
Methods of analysis
There are three separate methods used in the leather industry for the analysis of formaldehyde.  Their application depends on the final use of the product along with the technical level of the laboratory carrying out the analysis. The most commonly used methods for analysis are described in CEN ISO TS 17226. This has two parts: a colorimetric method and an HPLC method.
Colorimetric method: This involves a colorimetric determination of the extractable formaldehyde. The leather is extracted at 40°C, 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.
HPLC method: This is based on determining the extractable formaldehyde using high performance liquid chromatography (HPLC). The formaldehyde is extracted at 40°C, after which the extract is reacted with 2,4-dinitrophenylhydrazine (DNPH). The resulting extract is then separated using HPLC, with UV detection at 350nm.
The CEN and ISO committees responsible for these methods are currently in the process of converting them to become full standards. When complete they will be referenced as EN ISO 17226-1 and EN ISO 17226-2. Part 1 is the HPLC method and will include a statement to the effect that this is the more reliable technique to be used in any case of dispute. This is due to the potential for interference and false positive results that may occur with a colorimetric analysis.
Also used in the leather industry but to a lesser extent is the Japanese method JIS L 1041 – 1983 (revised 1994). This is similar to the colorimetric method listed in CEN ISO TS 17226, with some modifications to the extraction procedure.
In addition, the automotive industry uses a separate analysis based on a headspace extraction technique. Within this method the sample is suspended in a sealed container over a defined amount of water. After heating, any formaldehyde liberated by the leather should be dissolved in the water. This resulting solution is then analysed colorimetrically. Acceptable limits as analysed by this method are typically 10ppm.
Conclusions
Testing of formaldehyde in consumer products is not straightforward. Even within a single material, such as leather, there are several testing methods that can be applied. Different materials (such as wood, cosmetics and textiles) will also have their own test methods that are appropriate. In addition to the complexity of the testing, there is also the variability in legislation and regulations that affect the products. BLC is able to offer support and advice to customers who have concerns about the presence of formaldehyde in their products.