A Review of Aspects of Oxidative Hair Dye Chemistry with Special Reference to N-Nitrosamine Formation

Abstract

This review discusses a new aspect to the safety profile of oxidative hair dyes using data already in the public domain. These dyes contain secondary amines that are capable of forming potentially carcinogenic nitrosamine derivatives when exposed to atmospheric pollution. Numerous scientific articles confirm the existence of secondary amines in hair dyes (and their intermediates), the possibility of nitrosation by atmospheric NO_x of secondary amines to give the N-nitrosamines, and the significant safety risks on N-nitrosamines. It is believed that such nitrosamine derivatives should be investigated more fully in the interests of consumer safety.

Keywords: N-nitroso compounds (NOC); cancer; hair dye; nitrosamine; p-phenylenediamine (PPD); risk.

Scheme 1

The reaction of a secondary amine with dinitrogen trioxide (via nitrous acid) to form an N-Nitrosamine. R, R′ may be alkyl groups, aryl groups or carbon atoms in a ring structure.

Chart 1

Selected secondary amines present in the EU Cosmetics Directive for use in hair dyes.

Chart 1

Selected secondary amines present in the EU Cosmetics Directive for use in hair dyes.

Chart 2

The structures of four commonly used HC dyes.

Scheme 2

Formation of simple hair dyes from PPD and three different couplers.

Scheme 3

N-nitrosation of the secondary amine group in a hair dye trimer.

Scheme 4

A proposed mechanism for the reduction of a hair dye dimer/trimer and subsequent N-nitrosation.