Latex protein allergies in footwear

Reviewing the issues surrounding the levels of proteins in natural latex rubber products.

by John Hubbard

Recent concerns have been raised regarding the increase in the incidence of reported allergies to soluble latex proteins in gloves and other dipped latex products. It has been claimed that up to 1,200 workers develop latex protein allergies every year due to wearing latex gloves. Latex may also be used in the production of footwear, so interested parties within the supply chain – from shoemakers to retailers – need to be aware of health problems that could be caused to prospective customers.

Allergic reactions to rubber are categorised as either ‘Type I’ or ‘Type IV’. A Type I allergic reaction may affect the bronchopulmonary tissues (asthma), eyes (conjunctivitis), gastrointestinal tract (gastroenteritis), nasopharynx (rhinorrhea or rhinitis) and skin (urticaria and eczema). In some cases, it can lead to anaphylaxis, which can be considered life threatening. The type of reaction observed is often dependent on the length and duration of use, combined with the sensitivity of the individual, but normally occurs rapidly on contact with the allergen. The cause of these allergic reactions is considered to be due to certain allergens found within the residual extractable fraction of some latex products.

Type IV allergies are those that are typically characterised by a dermatological reaction to the wearer’s skin and are linked with the vulcanising chemicals used in the manufacture of vulcanised rubber. These include mercaptobenzothiazole and the thiuram group of chemicals. Initial reactions to Type IV allergens are normally slower than those to Type I allergens. Type IV rubber allergies are probably the third most likely cause of an allergic reaction to footwear materials – the first being to chromium in leather and the second a reaction to para tertiary butyl phenol formaldehyde (PTBF) resins in adhesives.

Latex in footwear

In footwear, latex is usually present as either an adhesive or as a soling material. Therefore, the risk of a Type I allergic reaction is limited, because the products are not produced using the dipping process (which results in the presence of the allergen proteins). For very sensitive customers, avoidance may be recommended. Sufferers are, therefore, advised to choose soles that are clearly not rubber-based, such as leather, polyurethane (PU) or polyvinyl chloride (PVC), and to avoid adhesives by selecting unlined footwear with a loose insock.

From a supplier’s point of view, it is possible to determine the protein level in a natural latex product, so indicating the severity of risk to latex protein sufferers from contact with that article. It must be taken into consideration that most of the reported reactions to natural latex are associated with medical gloves and surgical devices, because of the dipped process of production and the intimate contact involved in their use.

There are a number of methods available for the measurement of these protein levels. The modified Lowry Assay is the most preferred, as it is sensitive, relatively reproducible and easy to carry out. This method is required by EN 455 part 3:2006 – ‘Medical gloves for single use’.

This involves the extraction of all soluble proteins present using a buffer solution followed by precipitation, and then quantification using a calibration curve constructed from known standards and ultraviolet (UV) spectroscopy at 750nm. The lower quantification limit of this method is approximately 10μg of protein per gram of glove material analysed.

The likelihood of a reaction to latex proteins can be reduced by modifying the production process in order to lower protein levels in the final product to a minimum. In addition, information can be provided to the user on the levels of soluble protein content present, thus allowing for a wider choice of products suitable for an individual.

There are no firm legal requirements for a maximum level of soluble protein, but glove manufacturers are required to provide information on the levels detected under test conditions. Guideline recommendations provided by ASTM and other bodies are currently intended for healthcare professionals, but it is intended to include other end uses at a later date. A maximum level of 200μg/g has been approved by The ASTM.

Furthermore, research suggests that levels of up to 50μg/g of material would not be considered to cause problems of allergic reactions – even in those already sensitive to latex proteins – except for the very small percentage of the population who are considered as being hypersensitive to these substances.

SATRA can carry out testing of all latex products using the modified Lowry method to both the ASTM method and EN 455 part 3, and provide information on current legislative requirements and guideline recommendations.

Publishing Data

This article was originally published on page 12 of the July/August 2014 issue of SATRA Bulletin.

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