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METHYLCHLOROISOTHIAZOLINONE.

Avoid CAS 26172-55-4 / PRESERVATIVE

Methylchloroisothiazolinone (MCI) is a broad-spectrum preservative typically used in combination with methylisothiazolinone (MI). While effective against microbial growth, its significant potential for skin sensitization and irritation necessitates strict concentration controls and limited application to ensure product safety.

Summary

preservative

Science

Mechanism of Action

This ingredient, an isothiazolinone, acts as a broad-spectrum antimicrobial agent. Its active sulfur moiety oxidizes thiol-containing residues within microorganisms, effectively eliminating aerobic and anaerobic bacteria, yeasts, and fungi, thereby preserving the integrity of cosmetic formulations.

Research

Clinical Evidence

High confidence
Effective range N/A
Optimal

The maximum recommended concentration for the MCI/MI mixture (3:1 ratio) is 0.0015% (15 ppm) for rinse-off products, and 0.00075% (7.5 ppm) for leave-on products (though banned in leave-on by SCCS).

Key findings

  1. 01 3.8% of patients exhibited a positive allergic reaction during patch testing, indicating significant sensitization potential.
  2. 02 1.6% of patients showed a positive allergic reaction during patch testing, suggesting a substantial allergenic risk even at lower concentrations.
  3. 03 Evaluated as non-sensitizing in a human repeated insult patch test when incorporated into a hand wash formulation.
  4. 04 Deemed essentially non-irritating in human patch tests, though allergic reactions were still noted at this level.
  5. 05 Demonstrated slight irritation in human patch tests.
  6. 06 Proven to be strongly irritating in human patch tests.

Transparency

Dusting Analysis

Not commonly dusted
Min. effective
0.0015% (15 ppm) for rinse-off products, 0.00075% (7.5 ppm) for leave-on products (as MCI/MI mixture 3:1 ratio).%
Red flag below
Concentrations at or above 0.01% (100 ppm) of the MCI/MI mixture have been linked to allergic reactions in human patch tests, with irritancy becoming notable at 0.02% (200 ppm) and severe at 0.04-0.08% (400-800 ppm).%

Methylchloroisothiazolinone is not a component that typically requires dusting during formulation. However, its low regulatory limits and documented potential for skin sensitization and irritation at concentrations exceeding 0.0015% in rinse-off products, or at any concentration in leave-on products due to the SCCS ban, highlight its significant allergenic profile and the need for strict adherence to safety thresholds.

The Formula

Formulation

Solubility
Water
Optimal pH ≤4.5
0 7 14

Stability

The compound maintains optimal stability under acidic conditions. Its rate of hydrolysis and degradation increases dramatically (by approximately 2000-fold) as the pH rises from 4.5 to 11. Stability can be enhanced through the formation of adducts with calcium or magnesium salts.

Synergies

  • Enhanced stability when formulated with calcium or magnesium salts.

Conflicts

  • Not permitted for standalone use; must be formulated in combination with methylisothiazolinone (MI).

Safety

Safety Profile

CIR Status
Safe with restrictions
Max tested
0.0015% (15 ppm) for rinse-off products and 0.00075% (7.5 ppm) for leave-on products (MCI/MI mixture 3:1 ratio), as per CIR.%
Sensitization risk High

The U.S. FDA identifies Methylchloroisothiazolinone as a standardized chemical allergen. The European Commission's Scientific Committee on Consumer Safety (SCCS) banned the MCI/MI mixture from all leave-on cosmetic products in 2014/2015 and restricted its use to a maximum of 0.0015% (15 ppm) in rinse-off products (3:1 MCI/MI ratio). Data are insufficient to determine its safety in products with potential for incidental inhalation.

Your Skin

Skin Compatibility

No Normal
No Dry
No Oily
No Sensitive
Irritancy High
Comedogenicity Unknown

Our Assessment

Verdict

Avoid

Given its high potential for skin sensitization and irritation, documented allergic reactions at low concentrations, regulatory bans in leave-on products, and status as an FDA-identified allergen, Methylchloroisothiazolinone presents an unacceptable risk for inclusion in precision skincare formulations.

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