Monday, October 15, 2007
Itchy Feet: Shoe allergens
The manufacture of modern shoe is complex and few people are aware of all the chemicals that have been used to prepare the shoe’s parts. Shoes are constructed with various materials glued together with different adhesives, all these steps involve chemicals that cause sensitization and footwear allergy. The common rubber allergens are phenolic resins, thioureas, carbamates and additives. Diagnosis is frequently hampered by presenting patterns which are obscured as shoe allergy can mimic other dermatoses of the feet e.g. atopic dermatitis, or chronic itchy inflammation of the skin that is common in people who have hay fever or asthma. Contact dermatitis may appear in acute, subacute, intermittent or chronic forms and many of the constituents of shoes, such as rubber, adhesives, chemicals used in tanning leather, dyes, biocides and trim can all be the source of skin irritation. The hot humid environment within the shoes gives an ideal environment for contact dermatitis. Sometimes metal buckles or shoelace grommets made from nickel may also cause contact allergy. Research has shown rubber chemicals are the most common culprit in shoe-related allergies and subsequent skin irritations usually are confined to a specific area with clearly defined boundaries. The allergy can cause red and swollen skin that may blister. There are two types of contact dermatitis. In the irritant type, exposure to substances such as soaps, detergents or metals may irritate the skin. In the allergic type, exposure to a substance is the cause, but the initial exposure or even numerous subsequent exposures will not cause an allergic reaction. Allergic contact dermatitis of the foot develops over time, as the skin of the foot is repeatedly exposed to an allergen (the substance that causes allergic reaction). In some cases this may take years. The pattern of shoe dermatitis usually corresponds to the location of the offending substance in the shoe. Shoe dermatitis is usually symmetrical (same pattern on both feet) but may also be patchy and or unilateral (one foot). Allergies on the top of the foot may be caused by allergens in glues or chromates and vegetable tannins in leather, or synthetic materials like polyurethane or neoprene foam. Leather traditionally is chrome-tanned, exposing the wearer to potassium dichromate. Tanning is followed by oiling, dyeing and finishing which give an attractive but tough outer coat to the leather. Shoe counters and toe boxes which give shape and support to shoe contain a number of allergens like adhesives and biocides. Dermatitis on the sole of the foot may be due to rubber or other materials in the insole, lining or glue which holds these two layers in place. Adhesives like hot melts, urethane, neoprene and natural rubber (latex) all cause allergies and various substances like p-tert-butylphenol-formaldehyde resin (PTBP-FR), colophony and epoxy resins impart allergenicity to these adhesives. Sport shoes are frequently implicated in shoe allergy as are work shoes and footwear that keep the feet warm. In the US reports of eczema caused by contact with insoles in Nike shoes have been on the increase. Wearing socks may temporarily reduce the risk but often it is necessary to patch testing for allergens. This involves cutting up the shoes into small pieces and testing them against the person’s skin for 24 hours. The tape strips are removed and the skin inspected for reaction, such as a small red spot that appears at the patch site. Once identified the cure often involves a complete change of footwear. Depending on the cause, will determined whether all leather shoes or synthetic or hemp or vegetable–tanned leather footwear is recommended. In cases where dermatitis was confined to the soles of the feet, replacing the insoles with felt or cork might suffice. Sandals, on the other hand, are less often associated with shoe allergy. Their open design allows feet to "breathe," and with no foot cover there is little material to irritate the feet.