2,4-Diaminophenoxyethanol HCl

What it is

2,4-Diaminophenoxyethanol HCl is an aromatic amine used in permanent hair colour as a developer component — technically an oxidative dye precursor. The suffix “HCl” denotes the hydrochloride form, a salt that makes the substance more soluble and more stable in storage. Its origin is synthetic. That is factual information, not an assessment.

On its own the substance does not colour anything. Only when the colour cream is mixed with the oxidising agent (usually hydrogen peroxide) is it oxidised and reacts inside the hair with so-called couplers to form larger, coloured molecules. These are created inside the hair shaft and are too large to wash out easily — which is why oxidative colour is permanent. After the development time and rinsing, the reactive starting material is largely consumed.

Why

• Why not “low risk”: oxidative dye precursors are the substance group that hair colour allergy warnings exist for in the first place. Aromatic amines of this kind can cause contact sensitisation — the immune system becoming primed to react on further contact. This classification applies to the substance group; substance-specific test data has not been evaluated here.
• Sensitisation is not the same as irritation. Irritation subsides once contact ends. Sensitisation, once acquired, normally persists and can trigger symptoms again on every further contact. That is precisely why this reads “increased attention” and not “low risk”.
• Exposure: the substance is not applied neat. It sits in the colour cream, is mixed with the oxidising agent immediately before use, acts for a limited time and is rinsed out. During the reaction the reactive starting material is largely consumed, so exposure to the unreacted form is limited to the application period.
• Separating hazard and risk: sensitisation potential is a property of the substance. Whether it becomes a problem in an individual case depends on pre-existing sensitisation, skin condition, contact intensity and frequency. For someone without a hair colour allergy and with intact scalp skin, a single professional application is a different situation from daily skin contact.
• Occupational use is the real difference: handling colour cream several times a day in a salon is an entirely different exposure situation from a client having their hair coloured a few times a year. Hand protection and working practice are the decisive factor here, not the substance property alone.
• Without a use level this remains, strictly speaking, a statement about a substance property and not a risk statement for a specific product. The concentration in the actual product is not available here.

When the assessment differs

• Known hair colour allergy: anyone who has already reacted to oxidative colour needs dermatological assessment. For this group the product is not suitable — regardless of how the product is classified overall.
• Broken, inflamed or freshly irritated scalp: a compromised skin barrier changes uptake and therefore the assessment. The manufacturer’s instructions on use after scalp irritation govern here.
• Occupational users: repeated contact over years rather than isolated applications. The focus here is occupational protection, not the product assessment.
• Use outside the intended purpose: oxidative colour for scalp hair is not intended for eyelashes and eyebrows. Such use is not covered by this classification.
• Children and adolescents: these products are intended for use on adults. Deviating uses have not been assessed here.

Open questions

• Regulatory status: no maximum level, annex reference or labelling requirement is quoted here at second hand. For a reliable statement the entry in the EU CosIng database must be checked, with its date.
• Substance-specific sensitisation data is not available here. The classification rests on knowledge about the group of oxidative dye precursors, not on verified single-substance data.
• Cross-reactivity with other aromatic amines has not been assessed.
• Concentration in a specific product: not available.
• Environmental data: not examined.

Related substances

Applies to: Dye precursors and couplers

• 2,7-Naphthalenediol — Coupler for oxidative hair colour
• 2-Methylresorcinol — Coupler for oxidative hair colour
• 4-Chlororesorcinol — Coupler for oxidative hair colour
• Resorcinol — Coupler for oxidative hair colour

Further substances from the same group for which a long-form assessment already exists.