Sodium Bromothymol Blue (C186) (C27H27Br2NaO5S, CAS number: 34722-90-2) is a weak acid. The degree of ionisation depends on the pKa and the pH of the solution resulting in two possible forms and different colours. The cyclic nonionised form is mostly present at pH < pKa (7.1). At pH 7.1, 50% of each form is present in solution. As the pH increases, the open ionised form is mostly present, at pH > pKa. Sodium Bromothymol Blue is used as cosmetic ingredient in hair dyeing, non-oxidative hair colouring products with a final on-head concentration of up to 0.5%.
Following the request from the EU Commission for a scientific opinion on the hair dye Sodium Bromothymol Blue (C186), on October 2022 the Scientific Committee on Consumer Safety (SCCS) issued a preliminary opinion which public consultation period ends on 23rd December 2022.
In light of the data provided, when used in non-oxidative hair colouring products up to a maximum on-head concentration of 0.5%, the SCCS is of the opinion that the complete safety of Sodium Bromothymol Blue could not be assessed because:
- A Toxicological Threshold of Concern (TTC) approach was used to assess the safety of three impurities present in Sodium Bromothymol Blue. However, the SCCS estimated the Systemic Exposure Dose (SED) to exceed the TTC threshold for Cramer class III for non-genotoxic substances. At the same time, the use of TTC on its own to justify the safety of substances that are regulated under the EU Cosmetic Regulation is not sufficient to waive the information requirements on essential toxicological endpoints.
Regarding the scientific concerns of the use of Sodium Bromothymol Blue in cosmetic products, the SCCS concluded that:
- While the use of TTC is acceptable to justify the safety of impurities and cosmetic ingredients that are added to a final product at low concentrations, it is not acceptable for the substances that are regulated under the EU Cosmetic Regulation. For this, data from New Approach Methodologies (NAMs) that are scientifically-accepted for the purpose, and/or other acceptable in vivo data on systemic toxicity, are required in an overall weight of evidence for safety.
Even though the Applicant has provided data on two batches of Sodium Bromothymol Blue, the SCCS has concerns about the impurities identified in one of the batches (Guanghua batch), as the levels of these impurities exceed the TTC threshold both individually and collectively. Therefore, the SCCS considers that the presence of these impurities is not safe, and a batch of Sodium Bromothymol Blue matching the impurity profile of Guanghua batch cannot be recommended for use in marketed products.
1. Scientific Committee on Consumer Safety (SCCS) – Opinion on Sodium Bromothymol Blue (C186) (CAS No. 34722-90-2), preliminary version of 24-25 October 2022, SCCS/1645/22