Addressing the conundrums of p‐phenylenediamine hair dye allergy by applying Friedmann's principles of contact sensitization

In the first conundrum, permanent hair dyeing involves the use of aromatic amines such as p‐phenylenediamine (PPD), whose oxidation is pivotal to the dyeing process, but also generates potent allergens. Despite prolonged efforts by industry to search for safer alternatives, hair dyeing is still reliant on this type of aromatic amine. In the second conundrum, patch testing with 1% PPD remains the most useful screen for hair dye contact allergy. However, there is a very small but real risk of actively sensitizing the patient. Lowering the PPD concentration below 1% significantly reduces test sensitivity and diagnostic utility. Here, we argue that by applying Friedmann's principles of contact sensitization each conundrum can be addressed from a new perspective. These principles indicate that, when the exposed area of skin is small (<1 cm²), induction of contact allergy is sharply reduced, whereas elicitation of allergy is unaffected. Careful reflection on this principle suggests that we can predict where hair dye sensitization is most likely to occur, indicates a strategy to reduce the chance of contact sensitization occurring in consumers as a result of hair dyeing, and how we might mitigate the risk of active sensitization resulting from diagnostic patch testing.     

Active sensitization and occupational allergic contact dermatitis caused by para-tertiary-butylcatechol

Para-tertiary-butylcatechol (PTBC) is a rare allergen which is used in the rubber, paint and petroleum industries. We present 9 patients who were sensitized to PTBC and examined at the Finnish Institute of Occupational Health (FIOH) between 1974 and 1995. 3 of the patients had been exposed to PTBC in their work. 2 of them also had allergic reactions to para-tertiary-butylphenol (PTBP)-formaldehyde resin and to PTBP 5 of the patients became sensitized to PTBC from patch testing. PTBC was found to be one of the most common causes of active sensitization in our clinic. Accordingly, at the FIOH, the patch test concentration of PTBC was lowered to 0.25% and this lower concentration is recommended for general use.     

Occupational contact allergy to formaldehyde and formaldehyde releasers

Background: Formaldehyde allergy is common and usually derives from formaldehyde‐releasing biocides in cosmetic and other products. Objectives: To analyse patterns of patch test reactions to formaldehyde and formaldehyde‐releasing compounds and the sources of sensitization. Patients/Methods: At the Finnish Institute of Occupational Health, we screened the patch test files for allergic reactions to formaldehyde and 12 formaldehyde‐releasing compounds. All patients with contact allergy to any of the substances were included, and their records were reviewed. Results: Between January 2001 and May 2007, we had patch tested 81 patients with formaldehyde allergy and 18 with independent allergy to some formaldehyde releaser. Of the formaldehyde allergies, 60 were new sensitizations, 25 of which were considered to be occupational. The most common source of occupational sensitization was metalworking fluids followed by creams and related products. Exposure to formaldehyde‐releasing preservatives in liquid soaps and other rinse‐off products was common in both occupational and non‐occupational cases. Reactions to formaldehyde‐releasing compounds were seen in 79% of the formaldehyde‐allergic patients. Conclusions: Occupational formaldehyde allergy was common and occurred in metalworkers, hairdressers, masseurs, and workers using protective creams, detergents, and liquid soaps. When compared with studies on general dermatological patients, contact allergy to formaldehyde releasers without formaldehyde allergy was rare.     

Frequency of sensitization to methyl aminolaevulinate after photodynamic therapy

Background:  Allergic contact dermatitis to methyl aminolaevulinate (Metvix™) after topical application in photodynamic therapy (PDT) has previously been described in case reports.
Objective:  To compare the frequency of sensitization to Metvix^® cream in a group of patients previously treated at least five times with Metvix-PDT with the frequency observed in an unexposed control group.
Methods:  Twenty patients treated five times or more with Metvix-PDT and 60 controls with no prior exposure to Metvix^® were patch tested with Metvix^® cream and Metvix^® placebo cream. Subsequently, the patients were interviewed to determine the relevance of a positive patch test reaction to Metvix^®.
Results:  Of 20 patients treated with Metvix-PDT, 7 were sensitized to Metvix^® cream, giving a sensation risk of 35%. In the control group, 1 of 60 became sensitized after a single exposure to Metvix^® cream (1.7%). There was no reaction to the placebo cream. The positive patch tests to Metvix^® were considered relevant in four of seven patients (57%).
Conclusions:  This study demonstrates a considerable risk of sensitization after Metvix-PDT. We suggest that the patients are interviewed to detect late or persistent local reactions after PDT. These reactions are often considered to be local infections but may represent allergic contact dermatitis, and therefore, patients should be offered patch testing with Metvix^® cream.