Given the worldwide
popularity of hair dyeing, there is an urgent
need to understand the toxicities and risks associated with exposure
to chemicals found in hair dye formulations. Hair dyes are categorized
as oxidative and nonoxidative in terms of their chemical composition
and ingredients. For several decades, the expert panel’s Cosmetic
Ingredient Review (CIR) has assessed the safety of many of the chemicals
used in hair dyes; however, a comprehensive review of hair dye ingredients
and the risk of exposure to hair dyeing has not been documented. Herein,
we review the safety of the various chemicals in oxidative and nonoxidative
hair dyes, toxicities associated with hair dyeing, and the carcinogenic
risks related to hair dyeing. While many compounds are considered
safe for users at the concentrations in hair dyes, there are conflicting
data about a large number of hair dye formulations. The CIR expert
panel has ratified a number of coloring ingredients for hair dyes
and banned a series of chemicals as carcinogenic to animals and unsafe
for this application. The use of these chemicals as raw materials
for producing hair dyes may result in the synthesis of other contaminants
with potential toxicities and increased risk of carcinogenesis. It
is an open question whether personal or occupational hair dyeing increases
the risk of cancer; however, in specific subpopulations, a positive
association between hair dye use and cancer occurrence has been reported.
To address this question, a better understanding of the chemical and
mechanistic basis of the reported toxicities of hair dye mixtures
and individual hair dye ingredients is needed. It is anticipated that
in-depth chemical and systems toxicology studies harnessing modern
and emerging techniques can shed light on this public health concern
in the future.