At a glance
| Chemical family | Aromatic diamine — oxidative hair dye intermediate (2,5-diaminotoluene / p-toluenediamine) |
| CAS number | 95-70-5 |
| Classification | IARC evaluated (Monographs Vol. 16/27/Suppl 7) — inadequate evidence for carcinogenicity; not formally classified in a numbered group. EU Cosmetics Regulation Annex III restricts usage conditions. Potent contact sensitizer (Skin Sens. 1A) |
| Where you encounter it | Permanent oxidative hair dyes (one of the primary colour-developing intermediates); transfers from freshly dyed hair to pillowcases, towels, and bedding; salon environment |
| Sleep micro-environment relevance | Transfers from freshly dyed hair to pillowcases during sleep. Potent contact allergen — pillow contact can trigger facial and scalp dermatitis in sensitised individuals. The bedroom is where overnight exposure occurs |
Regulatory & certification status
| European Union | EU Cosmetics Regulation Annex III — PTD permitted in oxidative hair dye products under specified conditions (maximum concentration after mixing, mandatory warning labels including allergy patch test instruction). Not permitted for eyelash or eyebrow use. CLP Skin Sens. 1A, Acute Tox. 3. Regulatory |
| United States | FDA permits PTD in hair dyes. Hair dyes are exempt from colour additive pre-market approval under the FD&C Act (with a mandatory caution label). No concentration limit beyond the general safety standard. Regulatory |
| Canada | Health Canada Cosmetic Ingredient Hotlist restricts PTD — permitted in oxidative hair dyes only. Not for eyelash or eyebrow use. Regulatory |
| International | IARC evaluated (inadequate evidence; not classified). EU Annex III regulated. One of the most common hair dye allergens worldwide alongside PPD. Regulatory |
What it is
Toluene-2,5-diamine is an aromatic diamine used as a primary intermediate in oxidative (permanent) hair dyes. During the hair dyeing process, PTD reacts with hydrogen peroxide and a coupler molecule to form large pigment polymers that deposit within the hair cortex — this is the chemistry that makes permanent hair colour permanent (lasting through multiple washes). PTD is the methyl-substituted analogue of p-phenylenediamine (PPD), the most notorious hair dye allergen. PTD has similar sensitising potential to PPD, and the two cross-react — people allergic to PPD are often also allergic to PTD. IARC evaluated PTD in multiple monographs (Vol. 16, 27, and Supplement 7) and concluded that evidence for carcinogenicity was inadequate. The EU Cosmetics Regulation permits PTD under Annex III conditions (maximum concentration, mandatory warning labels, not for eyelash/eyebrow use).
Where it shows up in bedding
PTD enters the bedroom through a direct transfer pathway: freshly dyed hair releases dye residues onto pillowcases during the first few nights after colouring. This is particularly visible on light-coloured pillowcases — dark staining from hair dye transfer is a common complaint. For most people, this is a cosmetic nuisance. For the estimated 2-5% of the population with hair dye allergy (sensitisation to PTD, PPD, or both), overnight pillow contact with dye residues can trigger or maintain allergic contact dermatitis on the face, scalp margins, ears, and neck. Using a dark pillowcase for the first 2-3 nights after dyeing, and washing hair thoroughly after the dye process, reduces residue transfer.
Citations
- IARC (1987). Toluene-2,5-diamine. IARC Monographs Supplement 7. Source Peer-reviewed
- Thyssen, J.P. et al. (2009). The Epidemiology of Contact Allergy in the General Population — Prevalence and Main Findings. Contact Dermatitis, 61(5): 260-267. Source Peer-reviewed
- EU SCCS (2012). Opinion on Toluene-2,5-Diamine and Its Sulfate. SCCS/1479/12. Source Regulatory
Frequently asked questions
Can freshly dyed hair stain my pillowcase?
Yes. Freshly dyed hair releases dye residue — including unreacted PTD and its oxidation products — onto pillowcases for the first 2-3 nights after colouring. The staining is most visible on white or light-coloured pillowcases. Washing hair thoroughly after the dye process, using a dark pillowcase for the first few nights, and allowing dye to fully oxidise before bed all reduce transfer.
Can I get a rash from dye on my pillow?
If you are allergic to hair dye intermediates (PTD or PPD), overnight contact between your face and a pillowcase stained with dye residue can trigger allergic contact dermatitis — redness, itching, and eczema on the face, scalp margins, ears, and neck. This is a delayed-type (Type IV) reaction that appears 24-72 hours after contact. If you suspect hair dye allergy, a dermatologist can confirm with patch testing.
Related compounds
Embr is a sleep environment company researching and addressing the chemistry of the bedroom. Research and product development in progress.
Last reviewed 2026-07-08. If you find a factual error, contact us.