The American Contact Dermatitis Society (ACDS) has named PTDS as the Allergen of the Year for 2025. This chemical compound, widely used in permanent hair dyes, often goes unnoticed in standard patch test screenings, leading to underdiagnosis of contact allergies. The prevalence of PTDS allergies is notably higher among individuals frequently exposed to hair dyes, such as hairdressers, though it can affect others as well.
The decision to highlight PTDS stems from its dual nature. On one hand, PTDS is an allergen that poses risks to those sensitive to it. On the other hand, it serves as a crucial alternative for individuals with PPD (para-phenylenediamine) contact allergies. Approximately half of the patients allergic to PPD may tolerate PTDS, making it a viable option for them.
PTDS and Its Place in Hair Dye Allergies
PTDS, or para-toluenediamine sulfate, is chemically similar to para-toluenediamine (PTD). This similarity has led researchers to rely on PTD data when PTDS data is insufficient. However, the unique chemical makeup of PTDS means individuals might react differently to each compound. “PTDS- and PTD-allergic patients are not likely to be able to use PPD as a dye alternative,” stated the authors of the ACDS report.
Despite its usefulness as an alternative for some, the chemical remains a concern. In studies involving hairdressers, PTDS positivity was recorded at 15.3% and 7.9%. By comparison, broader data from the North American Contact Dermatitis Group showed positivity rates of 1.6% and 1.7% from 2019-2022. These figures underscore the need for greater awareness and testing.
The Implications of Underdiagnosis
One challenge with PTDS is its exclusion from standard patch test screening series. This oversight can lead to underdiagnosis and prolonged exposure to an allergen. The ACDS aims to rectify this by advocating for the inclusion of PTDS in upcoming updates to the core screening series.
“Given the recent North American data as well as the importance of PTDS as an alternative hair dye option for some patients with PPD contact allergy, we advocate that PTDS be considered for inclusion in the next ACDS core screening series update,” stated the authors.
Furthermore, patients with PTDS allergic contact dermatitis (ACD) should be cautious about alternative hair dyes marketed as natural or organic, as these products might still contain PTDS or PPD. It remains crucial for individuals to review product ingredients meticulously.
Navigating Alternatives and Awareness
For those allergic to both PPD and PTDS, options are limited. Avoidance of hair dye or switching to henna-based products are recommended strategies. The ACDS hopes to raise awareness of PTDS’s dual role—not only as an allergen but also as a lifeline for some PPD-allergic individuals.
Botto, an expert in dermatology, elaborated on the importance of expanded allergy testing:
“But since PPD allergy is the most common one that we see, testing those patients who are PPD allergic and still want to dye their hair with expanded allergy testing to PTDS allows about half of them to use PTDS as an alternative.”
Moreover, understanding the manifestation of hair dye allergies is essential. “Hair dye allergy typically starts at the hairline, face and eyelids, and usually only presents on the scalp when severe,” explained Botto. An interesting aspect noted by researchers is that with PPD-related ACD, the scalp can sometimes remain unaffected due to the protective nature of sebum and hair.
“Interestingly, with PPD-related ACD, the scalp can be spared, likely due to sebum on the scalp and the hair itself,” remarked the authors.
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