And the 2025 Allergen of the Year is…toluene-2,5-diamine sulfate (PTDS).

Each year, the American Contact Dermatitis Society (ACDS) names an Allergen of the Year to raise awareness about common allergens.

Toluene-2,5-diamine sulfate (PTDS), also known as toluene diamine sulfate or 2,5-diaminotoluene sulfate, is an aromatic amine that can be used as a hair dye alternative for people who are allergic to paraphenylenediamine (PPD).  It is primarily found in hair dye and bleach-toner formulations, but PTDS may also be used in developing agents for color photography and to dye textiles and furs.

People may develop contact allergy to PTDS, and it is often omitted from standard patch test screening series, possibly leading to underdiagnosis. The North American Contact Dermatitis Group (NACDG) showed positivity in 1.6% and 1.7% of routinely patch-tested patients. Patch test results from the European extended baseline series showed PTD positivity of 1.4%, and a study of the Australian Baseline Series showed PTDS positivity of 12.9%, according to an article in Dermatitis. ( (Due to the similarity between PTDS and PTD, PTD data will be reported where PTDS-specific data are missing in this publication.)

Hairdressers  and people who dye their own hair are at increased risk of sensitization to hair dyes like PTDS, the study authors note.

“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,” the researchers urge.

Past ACDS Allergen of the Year Winners

2024- Sulfites

2023 – Lanolin

2022 – Aluminium

2021 – Acetophenone Azine

2020 – Isobornyl Acrylate

2019 – Parabens (Non) Allergen