The "cayenne" pepper punctate purpura reaction seen with cobalt (Figure 1A) is described in the literature (Fischer & Rystedt, 1985). This is because of the cobalt ions causing an irritation reaction down the eccrine ostia. Patients with atopic dermatitis are more prone to irritant reactions (Thyssen, 2013), which can be overread on patch testing. One notable allergen, which has a specific irritant look on atopic skin seen during patch testing, is paraphenylenediamine (PPD). The vellus hairs within the patch site become dyed black, and there is a folliculocentric papular reaction that may or may not have erythema but notably has no confluent induration (Figure 1B).

FIGURE 1. (A) Cayenne pepper punctate purpura reaction seen with cobalt. (B) Dyed black hairs and folliculocentric papular reaction seen with paraphenylenediamine.

PPD is one of the five substances deemed by the Food and Drug Administration as a "strong sensitizer." The other four strong sensitizers are powdered orris root, epoxy resins, formaldehyde, and oil of bergamot (United States Consumer Product Safety Commission, 2013). A careful history can easily elicit whether there has been exposure to PPD, for example, hair dyeing practices and black henna tattoo applications. For this reason, especially in children under 12 years old (Worm et al., 2007) or those who meet the Hanifin and Rajka's criteria for atopic dermatitis (Hanna, Moennich, & Jacob, 2009), PPD should only be tested when history suggests it may be relevant.

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