The "Safer" Hair Dye Ingredient That's Now the 2025 Contact Allergen of the Year: Toluene-2,5-diamine sulfate (PTDS)
A chemical often marketed as a gentler alternative in PPD-free hair dyes has a new spotlight: toluene-2,5-diamine sulfate (PTDS) has been named the Allergen of the Year for 2025 by the American Contact Dermatitis Society (ACDS).
PTDS, also called toluene diamine sulfate or 2,5-diaminotoluene sulfate, is an aromatic amine commonly used in:
- permanent and semi-permanent hair dyes
- permanent and semi-permanent hair dyes
- bleach-toners
- other coloring products
It's frequently found in "PPD-free" formulations as a substitute for paraphenylenediamine (PPD), the classic culprit behind many hair dye allergies. While it helps some people with PPD allergy avoid reactions, PTDS itself is a potent contact allergen—often underdiagnosed because it's not routinely included in standard patch test series!
It's frequently found in "PPD-free" formulations as a substitute for paraphenylenediamine (PPD), the classic culprit behind many hair dye allergies. While it helps some people with PPD allergy avoid reactions, PTDS itself is a potent contact allergen—often underdiagnosed because it's not routinely included in standard patch test series!
Patch test positivity rates show its relevance: around 1.4–1.7% in North American and European data, with one Australian study reporting up to 12.9%.
Hairdressers and frequent home hair dyers face the highest risk of sensitization. Reactions typically cause allergic contact dermatitis, manifesting as an itchy, inflamed scalp soon after dyeing. Cross-reactivity with PPD is common, and about half of those allergic to PPD may also react to PTDS.
Hairdressers and frequent home hair dyers face the highest risk of sensitization. Reactions typically cause allergic contact dermatitis, manifesting as an itchy, inflamed scalp soon after dyeing. Cross-reactivity with PPD is common, and about half of those allergic to PPD may also react to PTDS.
The ACDS chose PTDS to highlight its dual nature: a helpful option for some, but a growing source of allergic reactions that deserves more awareness and testing.
Many patients with itchy scalps after dyeing are reacting to PPD, and switching to PTDS-containing "PPD-free" brands may only transfer the problem.
For better tolerance in PPD-allergic cases, patients may try products with modified alternatives like Me-PPD (found in Wella Koleston Perfect).
Many patients with itchy scalps after dyeing are reacting to PPD, and switching to PTDS-containing "PPD-free" brands may only transfer the problem.
For better tolerance in PPD-allergic cases, patients may try products with modified alternatives like Me-PPD (found in Wella Koleston Perfect).
The takeaway? If hair dye causes reactions, don't assume "PPD-free" is automatically safe - patch testing for both PPD and PTDS may be the key, and clinicians may push for PTDS inclusion in future standard screening panels.
This designation serves as a reminder that even "alternatives" can carry hidden risks in the quest for the perfect color.
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This designation serves as a reminder that even "alternatives" can carry hidden risks in the quest for the perfect color.