Hair dye molecules reach the brain through your skin
Anatomy
The semi-permeable barrier of your scalp allows synthetic pigments to enter the bloodstream and bind to proteins in the brain within minutes of application.
When you apply permanent hair color, the ammonia in the mixture swells each hair shaft by 20 percent to let the pigment in, but it also opens a direct gateway through your skin. Most modern dyes rely on PPD, a coal-tar derivative first synthesized in 1863. These tiny molecules are so small they diffuse through the skin ten times faster than larger industrial chemicals, eventually reaching the nervous system where they can bind to neural proteins.