Contribution to the Safety Assessment of Hair Straightening Products: Validation and Application of a Fast and Reliable Spectrophotometric Method for the Determination of Formaldehyde

Aim/background: Formaldehyde is a high reactive, flammable and low molecular weight substance, which has been classified as carcinogen (1B category) by different international entities; for this reason, European Commission have included it in the II annex (List of Substances Prohibited in Cosmetic Products) at the Cosmetic Ingredient Database (CosIng). Despite the risks associated with its use, formaldehyde has been detected as an active ingredient in hair straightening products due to its physicochemical properties, which allow it to modify the internal structure of the hair, achieving straight hair quickly, with good results, and at a very low cost. Methods: In this sense, the method applied in this research was based on the principle of spectrophotometric quantification, in which formaldehyde reacts with Nash’s reagent to form a colored compound called 3,5-diacetyl-1,4-dihydrolutidine (DDL). The method was validated by evaluating selectivity, linearity, LOD (Limit of Detection), LOQ (Limit of Quantification), accuracy, precision, and robustness, Results: It was possible to demonstrate that the method is selective, linear, precise, accurate, and robust within a concentration range of 0.019–2.5 ppm. Subsequently, the method was used to determine and quantify formaldehyde in 10 hair straightening products marketed in Medellín, Colombia, revealing the presence of formaldehyde in samples C and D, with 4.41 ± 0.41% and 4.99 ± 0.37%, respectively. Conclusion: While most of the evaluated products comply with the established regulations, continuous monitoring is necessary to ensure safety and prevent the commercialization of products that may pose a health risk.