Hair dyes: a systematic review of pertinent in vivo human studies.

What is the safety of hair dyes in human use? To address this, the penetration of hair dyes into human bodies must be quantified. This article provides a critical overview to update work published in the area of in vivo hair dye penetration in humans. Studies are in humans, with one including rhesus monkey and one including in vitro human skin. MEDLINE, PubMed, Embase, Scopus, Web of Science, and Google Scholar databases were searched from 2004 to October 2019 for articles with experimental methodology on in vivo percutaneous absorption of hair dyes in humans and six studies were included. Findings indicate that percutaneous penetration occurs with hair dye use, with less absorbed in hairdressers. However, much remains unknown about the penetration of the over 200 commercially available hair dyes. A major void exists in data confirming or denying the completeness of excretion once absorbed.