Quantification of the exposure of consumers and hairdressers to formaldehyde and oxalic acid after application of hair straightening products containing glyoxylic acid.

Glyoxylic acid (GA), the active ingredient of heat-activated hair straightening products (GHSPs) has been discussed in the context of acute kidney injury (AKI) via oxalic acid (OA) crystal formation and of heat-decomposition resulting in release of formaldehyde. Assessment of dermal absorption following realistic GHSP treatments with two marketed products resulted in maximally 7.73 μg/cm2 of bioavailable GA/application. Considering the scalp area of 580 cm2, the systemic GA exposure of consumers was 4.48 mg, equivalent to 1.1 mg OA taking into account that 25% GA is dermally metabolized to OA. Assessment of GA exposure to stylists' hands indicated systemic OA levels in the range of 0.026 mg/day (assuming 3 treatments in one day). Compared to typical dietary and endogenous sources of OA of 22-45 mg/day, GHSP-dependent OA exposures of consumers and stylists were 20-41-fold and > 860-fold lower, respectively. PBK prediction of GHSP-dependent renal OA excretion of consumers was <1.27 mg/day compared with normal excretion of ∼25 mg/day and the 40 mg/day threshold associated with AKI. Predicted kidney OA concentrations during GHSP exposure further indicated that OA crystal formation is unlikely to occur. Assessment of the release of the GA heat-decomposition byproduct formaldehyde in the ambient air during GHSP treatment was <25 μg/m3 and below regulatory exposure limits e.g., 100 μg/m3 (WHO indoor air quality limit). In conclusion, quantification of GA-dependent formaldehyde and OA exposure indicates no health concerns for consumers and stylists under the described realistic GHSP treatment conditions.