Methylchloroisothiazolinone and Methylisothiazolinone 10 Years After Regulation: A Case Story of Liquid Soaps and Shampoos

Background: Contact allergy to methylchloroisothiazolinone (MCI) and methylisothiazolinone (MI) remains common in the EU despite regulation. Objectives: To assess the extent of use of MCI/MI and MI in cosmetic products on the Danish market, identify product categories involved and evaluate compliance of rinse-off products with the 15 ppm limit. Materials and Methods: Products listing MCI/MI or MI were identified through Kemiluppen, a Danish database of marketed cosmetics. Thirty products (16 shampoos and 14 liquid soaps) were examined using liquid chromatography–tandem mass spectrometry: 25 labelled as containing MCI/MI or MI, 5 unlabelled. Results: In total, 257 cosmetic products containing MCI/MI or MI were identified, primarily shampoos (127), conditioners (98) and liquid soaps (8). Chemical analysis showed that 2 of 25 labelled products (8%) exceeded the 15 ppm limit, and 4 products did not meet the required 3:1 MCI/MI ratio. Conclusion: MCI/MI is used infrequently in cosmetic products. All, except two products complied with the limit in the regulation. Given the continued occurrence of contact allergy, this indicates that the limit (15 ppm) may be too high to be sufficiently protective. Further investigation into MCI/MI use in cosmetic products is warranted, alongside more proactive risk assessment and management strategies.