Well sure, but the scalp underneath the hair that's gotta have some skin properties - sure it's thicker than the rest of the skin and has some protective proper

Well sure, but the scalp underneath the hair that's gotta have some skin properties - sure it's thicker than the rest of the skin and has some protective properties against some forms of environmental damage but the dye does go in there and years of use should cause _some_ thing - not sure there's any research around what exactly.