CI 42090
Reviewed by Emma · Last updated 14 Aug 2026
A synthetic blue colourant used to add colour to skincare formulations
Best For Skin Types
All
Usage Frequency
N/A
What Is CI 42090?
CI 42090, also known as Acid Blue 9 or Allura Blue AC, is a synthetic azo dye approved for use in cosmetics. It belongs to a class of colour additives that are chemically synthesised rather than derived from natural sources. In skincare products, it serves purely as a cosmetic colorant—it does not provide therapeutic benefits to the skin itself.
This ingredient is regulated by cosmetic authorities including the EU and UK, which maintain strict purity and safety standards for approved colorants. It appears in approximately 6% of skincare formulations, where it is used to give products their desired visual appearance.
How It Works
CI 42090 is a colour molecule that absorbs and reflects light in the blue spectrum. It remains on or near the skin surface and does not penetrate significantly into deeper layers under normal use. The ingredient itself does not interact with skin biology or exert pharmacological effects.
Clinical Evidence & References
Research on synthetic colorants in cosmetics has focused primarily on safety and allergenicity rather than skin benefits. Adverse reactions to approved cosmetic colorants are considered uncommon, with documented reactions—when they occur—typically being mild and involving the skin. Most studies indicate a low prevalence of adverse reactions in the general population, though higher rates have been observed in atopic individuals.
There is no evidence that CI 42090 provides direct skincare benefits beyond its colour function. Its safety profile in cosmetics is well-established through regulatory approval processes.
How to Use
CI 42090 is used as needed by formulators to achieve desired product colour. As a consumer, no specific usage instructions are required. The ingredient works in the background of your formulation and does not require adjustment based on application frequency or skin type.
Safety & Precautions
Frequently Asked Questions
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