PTDS allergen of the year

THE 2025 ALLERGEN OF THE YEAR IS TO DYE FOR

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By Christen Mowad, MD, FAAD
Oct. 29, 2025
Vol. 7, No. 43

The "Allergen of the Year" announcement is always much anticipated in the world of contact dermatitis and patch testing. This year the honor has gone to toluene-2,5-diamine sulfate (PTDS). (1) PTDS is an aromatic amide that is used as a hair dye, textile dye, and in color photography. It was named allergen of the year of 2025 to not only highlight its potential as an allergen but also its potential as an alternative hair dye chemical for those who are allergic to paraphenylenediamine (PPD). PTDS is used in many permanent hair dyes and is also known as 2,5-diaminotoluene sulfate, p-toluenediamine sulfate, toluene-2,5-diamine hemisulfate, 2,5-toluenediamine sulfate, and paratoluenediamine sulfate. As with so many other allergens, this chemical is not included in the FDA-approved standard screening series. It is also not in many standard expanded screening series and therefore can be missed as a cause of allergy — another reason for highlighting this allergen. 

For those that are allergic to PPD, testing to PTDS is useful as it can function as an alternative to PPD if the patient is negative upon testing to PTDS. Relevance for those allergic to PTDS is over 70% in many studies and this is especially true for hairdressers given its narrow range of use. Clinically, PTDS ACD presents as an eczematous reaction in places where hair dye comes in contact with the skin — most commonly the hands, hairline, and periorbital areas. This reaction is typically associated with pruritus and can have vesiculobullous findings depending on the severity of the reaction. 

The image demonstrates an allergic contact dermatitis that developed within 48 hours after applying black hair dye containing paraphenylenediamine. JAAD Case Reports 2024; 43: 7-8.

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Occupational contact dermatitis (OCD) to this chemical is most often seen with hairdressers who are 1.7-2.5 times greater than the general population to have hand dermatitis. Cross reactions between PTDS and PPD can occur. PTDS allergic patients are often PPD allergic. Several studies have demonstrated that 80-100% of patients with PTDS allergy are also allergic to PPD. The reverse does not hold as studies show that 30-50% of PPD allergic patients are also allergic to PTDS, indicating that PTDS may be an alternative hair dye for those allergic to PPD. However, patch testing should be performed first to confirm if PTDS is tolerated. 

The allergen of the year is a tool for those firmly entrenched in the patch testing world to highlight allergens of particular significance. PTDS has been given the honor this year to showcase its potential as an alternative hair dye chemical to those who are PPD allergic and to highlight its potential as an allergen as well that must be considered given it is not routinely tested in standard and even expanded series. 

Point to Remember: Toluene-2,5-diamine sulfate (PTDS) may be an allergen but may also be used as a substitute in some patients allergic to the hair dye paraphenylene diamine. 

Our expert's viewpoint

Bruce A. Brod, MD, MHCI, FAAD
Associate Dean of Continuing Medical Education
Clinical Professor of Dermatology
University of Pennsylvania Perelman School of Medicine

Color me Cautious

The 2025 Allergen of the Year, toluene-2,5-diamine sulfate (PTDS), is a fitting choice not just for its allergenic potential, but for its dual identity as a PPD alternative and a frequent co-sensitizer. (1) Structurally similar to para-phenylenediamine (PPD), PTDS is often found in oxidative hair dyes alongside other aromatic amines. In my patch test clinic when testing suspect dye allergy patients, I routinely include m-aminophenol, p-aminophenol, and other aromatic dyes, as well as both PPD and PTDS for patients with scalp, neck, or facial dermatitis linked to hair dye use. 

The FDA requires warning labels on consumer products for coal-tar-based dyes like PPD and PTDS, advising consumers to always perform a patch test 48 hours before each use of hair dye even if they've used the product before as reactions can develop over time. (2)

In addition to PPD and PTDS, hair dyes often contain other potential allergens, including fragrance components and preservatives like methylisothiazolinone, which can contribute to allergic contact dermatitis and should not be overlooked in patch testing.

The good news is that the market for natural hair dyes is expanding. Products based on pure henna (Lawsonia inermis), indigo, cassia, and herbal blends are increasingly available and often marketed as safer alternatives. These dyes are generally free of oxidative agents and aromatic amines, making them less likely to cause allergic contact dermatitis.

However, natural doesn't always mean safe. A growing concern is the adulteration of hair dyes, especially in small ethnic markets. A recent study analyzed 51 hair dye products marketed as "PPD-free" and found that nearly 10% contained undeclared PPD, with some at concentrations high enough to trigger allergic reactions in sensitized individuals even though the ingredient was not listed on the label. (3)

The bottom line is most oxidative dyes are complex chemical mixtures. PTDS may be a substitute for some, but it's no panacea. Comprehensive patch testing, patient education, and awareness of both synthetic and natural allergen risks are essential. As we continue to explore safer alternatives, we must also advocate for clearer labeling, regulatory oversight, and expanded allergen screening to better protect our patients.

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References

1. Atwater AR, Botto N. Toluene-2,5-Diamine Sulfate: Allergen of the Year 2025. Dermatitis. 2025;36(1):3-11. doi:10.1089/derm.2024.0384. 

2. U.S. Food and Drug Administration. Hair Dye and Hair Dye Safety. Updated 2023. Available at: https://www.fda.gov. 

3. Needle CD, Milam EC, Korman A, et al. Contact Allergens in "PPD-Free" Hair Dyes. Dermatitis. 2025;00(0):1–7. doi:10.1089/derm.2024.05423. 

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