It is nearly impossible to identify all constituents of a shoe [1], as components may gradually change as shoe manufacturing, fashion, and technology evolve [5, 9]. Leather and shoe dyes were the most common shoe allergens in the early twentieth century [1]; however, chromium compounds, used for leather and non-leather synthetic uppers are still the most important shoe allergens throughout Europe and India [1, 4, 5, 10]. In the USA, Canada, Brazil, and Asia, rubber allergens have predominated shoe allergy since the 1950s [1]. Overall, the most common allergens to consider in patients with allergic contact dermatitis of the feet include constituents of rubber, leather adhesives, and less commonly, shoe linings and dyes .

The most important allergens include para-tertiary-butylphenol formaldehyde resin (PTBPF-R), mercaptobenzothiazoles (MBT), thiurams, potassium dichromate, colophony, and PPDA derivatives [1, 5, 7, 8]. Components containing rubber resins, including heel and toe counters and leather finish coats, may include MBT or thiurams [1]. One retrospective study found that positive patch test reactions to potassium dichromate, PTBPF-R, mercapto mix, and MBT were significantly associated with foot dermatitis, as opposed to dermatitis of other body areas [8]. Similarly, in another retrospective study PTBPF-R, potassium dichromate, and carba and thiuram mixes were identified as the most common positive patch test allergens identified in patients with primary foot dermatitis [2].

PTBPF-R has been used as an adhesive in rubber glues since the 1950s and was found to be the most common individual shoe allergen in a recent USA study [1, 2] Currently, it is found as a component of neoprene adhesives used for shoe linings. Glues containing this allergen are also found in other leather products including watch straps, handbags, building materials, and electrical products. PTBPF-R is commonly found in shoe lining and insole glues and, along with chromate, is an important allergen to consider in persons wearing orthopedic shoes and using prostheses [1].

Colophonium and modified colophonium are also found in heel and toe stiffeners; rubber latex and neoprene adhesives are used to glue insoles and linings in place [1, 6, 11]. Colophonium is a mixture of over 100 compounds derived from pine trees, and the resin acids contained are easily oxidized, becoming allergenic. Allergic patients should be counseled to wear unlined shoes or shoes with stitched linings; it is very difficult to find any lined shoe free from these tackifiers [11].

Leather is often tanned with trivalent chromium (Cr(III)) to maintain suppleness and durability. Hexavalent chromium (Cr(VI)) can occur as an impurity, is a potent sensitizer, and is the chromate used in patch testing [2, 10]. Contact allergy to chromium often results in particularly severe foot dermatitis. The incidence of chromium allergy was found to have significantly increased between 1995 and 2007 in Europe, most likely due to increasing leather shoe exposure, especially in women [4, 10]. In fact, in July 2007 the German Risk Assessment Institute (Bundesinstitut für Risikobewertung) recommended reducing the use of chromium salts in leather production as much as possible [10]. Beginning in 2015 the European Union will no longer allow leather shoes to be sold which contain more than 3 mg/kg chromium VI.

Other common shoe allergens include formaldehyde (used in leather tanning) and nickel, which is found in decoration and trim or buckles [1]. Metal salts like nickel and cobalt may be found in plastic footwear worn by medical personnel [1, 4, 12]. A recent report [12] suggested that cobalt may even be found in leather footwear and cause sensitization, and another study [4] reported that cobalt sensitization may often occur with chromium allergy. Allergies to dyes are much less common, with the exception of re-dyed leather or fabric shoes [1]. The most commonly used dyes are related to para-phenylenediamine and para-aminoazobenzene [1, 5].

Cases of occupational dermatitis from work in shoe production or shoe repair are uncommon; the major allergens were PTBPF-R and MBT [1].

In Europe several cases of severe ACD have been reportedly caused by dimethylfumarate present in antifungal packets in footwear or retail shoeboxes [1, 13]. Additionally, dimethylfumarate was responsible for a widespread epidemic of contact dermatitis caused by Chinese-imported furniture in Europe [13, 14]. Following this dimethylfumarate was banned in the European Union. Vesicular dermatitis of the soles bilaterally due to cinnamon powder used in odor-neutralizing agents has also occurred [15].