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

Summary

In the first conundrum, permanent hair dyeing uses aromatic amines such as p‐phenylenediamine (PPD), whose oxidation is pivotal to the dyeing process, but also generates potent allergens. Despite prolonged industry effort searching 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 sensitising 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 sensitisation each conundrum can be addressed from a new perspective. These principles indicate that where the exposed area of skin is small (<1cm²), 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 sensitisation is most likely to occur, indicates a strategy to reduce the chances of contact sensitisation in consumers from hair dyeing, and how we might mitigate the risk of active sensitisation from diagnostic patch testing.

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