Re: What are the negative effects of hair dye?

Jean-Marc:

     The answer to this question depends on about which negative effects 
you are interested.  I would classify them as constructed effects (Which 
would be social) and biological/toxicological effects.  I am guessing you 
are interested in the latter.

The social effects will vary, but these will primarily be dependent on 
geography (which country) and demographic.  Some examples

Dying hair might be seen as elitist in one country, and something only 
done by lower classes in another
Demographically, one group might look down on anothers hair dye.  An 
example of this would be adults (parents) making judgements and 
assumptions about a teenager that dyes their hair jet-black or perhaps 
pink.

Biologically, the effects of hair dye are primarily dictated by two things
1.) The chemical composition of the dye (vegetal vs petroleum based)
2.) The length/type of exposure

In general, there are two different kinds of hair dyes; oxidative and non-
oxidative.  The primary difference is whether the mordant (which 'Fixes' 
the dye to the hair is based on something like hydrogen peroxide or 
something like an acid or base.  Most commercial hair dyes use peroxide-
based mordants, many vegatal dyes, like Henna use an acid like lemon juice 
and sugar as the mordant.

Regardless of the origin of the dye many studies have failed to find any 
long-term change in the rates of cancer from modern vegetable or synthetic 
hair dyes.  Jacobs et al., and Burnett et al., both tested oxidative and 
non oxidative dyes applied in high-concentrations to mice and failed to 
detect any significant difference.

There are some known toxicological effects of dyes, though.  Darkening 
agents often contained lead acetate as a flux and sye stabilizer, and 
other dyes have had salts of lead.  In addition to these possible toxicity 
effects there is always the possibility of allergic reaction to the 
components of either natrual or synthetic dyes.  An example of this might 
be non-legume nut allergies with walnut-based dyes.  Some people have 
dermatitis reactions to the peroxides as well, which can result in hives, 
rash, hair loss and/or hair breakage. 

I hope this answers your questions.  Good luck with your science project

References:
Burnett C. et al., 1980 Journal Toxicology and Environmental Health; March 
6(2) pp247.
Jacobs MM., et al., 1984 Drug and Chemical Toxicity; 7(6) pp573