4-Hydroxypropylamino-3-Nitrophenol

FunctionNitro hair dye (restricted-use)

What it is — 4-Hydroxypropylamino-3-Nitrophenol is a nitro-type direct hair dye — a pre-formed colourant that develops no chemistry on the hair and needs no peroxide.

What it does — It contributes a warm red to violet-brown tone and, like other nitro dyes, penetrates slightly further than a cationic basic dye, so its contribution to a shade lasts through more washes. In grey-coverage blends it is one of the components that keeps a brown from drifting flat or ashen as the lighter dyes wash out.

Typical use — Semi-permanent colour, colour-depositing conditioners and grey-toning shampoos. Korea's 식약처 lists it among ingredients not permitted in cosmetics generally, while allowing it in hair colour products at up to 2.6%; the EU restricts it to hair dye use on similar terms. The cap, not the ingredient itself, is what the regulation turns on.

Who it suits / good to know — There is no PPD, no resorcinol and no developer involved, so it avoids the sensitizers responsible for most hair-dye allergy. What you give up is one-wash coverage: tone builds over repeated use and fades gradually. Temporary staining of skin, nails and light fabrics during application is normal.

Safety — DHALIORA grades it mid risk: a restricted-use hair colourant permitted within a concentration limit, considerably gentler than oxidative dye chemistry but still a colourant in contact with the scalp. This reflects the ingredient in general and is not medical advice; patch-test if your scalp is easily irritated.