People have been asking this question for years.  There seems to
be little in the medical literature (if anything at all).  The 1997
Breastfeeding Answer Book says:

"No evidence exists that the nursing mother's use of hair care products,
such as hair dyes and permanents, has any effect on her breastfeeding
baby.  When a mother uses hair care products, some of the chemicals will
be absorbed through her skin.  If her scalp is healthy and intact, less
will be absorbed than if the skin on her scalp is scratched or abraded."
p.382

Hope this helps.
Sarah

Sarah Friend Barnett   LLLL, IBCLC
Bronx (New York City), NY  -  [log in to unmask]
" You are not obliged to finish the task,
 neither are you free to neglect it."       R. Tarfon