Cosmetic products for skin lightening are becoming increasingly popular worldwide. Their use is most widespread in Asian countries, where fair skin is considered the ideal of beauty. In other countries, they are mainly used for the localized lightening of pigment spots. These products typically contain one or more active ingredients that lighten the skin or prevent the formation of darker pigmentation through specific mechanisms.
As skin lightening cosmetic products are increasingly found on our market, the main goal of our study was to evaluate these products regarding the content of active ingredients. The ingredients we selected were: ascorbic acid and its derivatives, α-arbutin, azelaic acid, ferulic acid, kojic acid, α-lipoic acid, salicylic acid, and niacinamide, which are most commonly used active ingredients in skin lightening products. For this purpose, we developed and adequately evaluated four analytical methods based on high-performance liquid chromatography. After market review, we selected a representative sample of 25 products containing one or more active ingredients for skin lightening. We selected products of various forms (creams and serums), price ranges, and included products with declared active ingredient content. We evaluated the content of these ingredients in the products and checked the accuracy of their labeling. Most of the tested products contained the active ingredients declared on the packaging, but we also discovered discrepancies in 5 out of 25 products. For products with a declared active ingredient content, a difference between the declared and the found content was found in almost all products. The determined values ranged from 0.2 % to 170 % of the declared amount; however, for most products the content found was close to the amount declared on the packaging (100 ± 20 %). We compared the results with the prices of cosmetic products and found no direct correlation between price and product quality. The selected products were exposed to elevated temperatures (40 °C) for four weeks and also tested after four weeks at room temperature in their original packaging. We found that the stability depends not only on the active ingredients but also on other ingredients of cosmetic products, confirming the temperature dependence. The greatest decrease in content was observed for ascorbic acid and its derivatives. We also checked the long-term stability of products that had been opened four years ago and stored at room temperature in their original packaging. These products mostly contained ascorbic acid. In these products, we found significant deviations from the stated concentration.
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