Cytotoxic Evaluation of Hair Relaxers and Straighteners Actives in Human-Derived Skin Cells

Most hair relaxers or straighteners used for Afro-textured hair have a pH above 11. This pH, according to international occupational health and safety guidelines, is corrosive to the skin. Further, relaxers are associated with a 3-6 times increased risk of alopecia. The skin also gets into contact with these hair straightening products during application resulting in skin irritation. Using animals as test subjects to determine these products' potential risk and safety margin is standard practice; however, with the European Union (EU) ban on animal testing, the testing method needs to be replaced by newer in vitro laboratory technologies. This study used previously and newly established in vitro cell-based technologies and human-skin derived cells as laboratory models to evaluate relaxer-induced cytotoxicity. These technologies include endpoint (MTT, CCK-8 and flow cytometry) and real-time assays (real-time cell analysis (RTCA) and extracellular flux analysis). Cellular toxicity was evident following treatment of keratinocytes and fibroblasts with acceptable concentrations of thioglycolic acid (TGA, HSCH2COOH), ammonium bisulphite (NH4HSO3), lithium hydroxide (LiOH) and sodium hydroxide (NaOH) based relaxers. Real-time assays showed significant reductions (P<0.001) in cell index and ATP production following treatment of keratinocytes with as little as one-tenth of the acceptable concentrations. Increased apoptosis (47.9%, 58.0%, 76.7% and 80.3%) also occurred in cells after treatment with TGA, NH4HSO3, NaOH and LiOH, respectively. In conclusion, this study demonstrated that using skin cells in conjunction with advanced cell-based technologies could serve as alternatives to laboratory animals for accessing the toxicity margin of hair relaxers, straighteners and other cosmetics.