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6-HYDROXYINDOLE.

Questionable CAS 2380-86-1 / HAIR DYEING

6-Hydroxyindole is a bicyclic arylamine compound that functions as a coupler in oxidative hair coloring systems. When combined with hydrogen peroxide and primary intermediates, it undergoes coupling reactions to create permanent color pigments within the hair shaft. This ingredient is specifically designed for hair applications rather than general skincare use.

hair dyeing

Science

Acts as an oxidative hair dye precursor that requires hydrogen peroxide activation to form large, colored pigment molecules through coupling reactions with primary intermediates in alkaline conditions


Research

High confidence
Effective range 0.5–0.5%
Optimal

0.5%

Key findings

  1. 01 Demonstrated safe use in oxidative hair dye formulations with no health risks beyond sensitization potential
  2. 02 Showed slight to moderate skin irritation in 2-week guinea pig studies, though no ocular irritation occurred

Transparency

Not commonly dusted
Min. effective
0.5%
Red flag below
0.6%

Not commonly used in skincare; primarily restricted to hair dye applications with strict concentration limits due to sensitization concerns


The Formula

Solubility
Both
Optimal pH 4.5 – 10
0 7 14

Stability

Highly unstable when exposed to atmospheric oxygen, forming unwanted colored polymers. Requires antioxidants like sodium sulfite and nitrogen atmosphere storage. More stable in acidic conditions below pH 5, though typically used in alkaline hair systems.

Synergies

  • hydrogen peroxide
  • primary intermediates
  • sodium sulfite

Conflicts

  • atmospheric oxygen
  • strong oxidizing agents
  • light exposure

Safety

CIR Status
Safe as used
Max tested
0.5%
Sensitization risk High

Approved specifically for oxidative hair dye use only, with notable sensitization potential requiring careful handling


Your Skin

No Normal
No Dry
No Oily
No Sensitive
Irritancy Moderate
Comedogenicity Unknown

Our Assessment

Questionable

Specialized hair dye ingredient with high sensitization risk and no established skincare benefits, making it unsuitable for precision skincare applications.


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