Methylchloroisothiazolinone (MCI/Kathon CG) on your skin: a safety profile
High riskDermal sensitization is the defining health concern for MCI/Kathon CG. The compound is an extreme contact sensitizer — its EC3 in the LLNA of approximately 0.002% places it in the highest sensitization potency category. Sensitization requires two phases: an induction phase (first contact with the allergen in a susceptible individual) and an elicitation phase (subsequent contact triggering the allergic response). Once sensitized, affected individuals react to trace amounts of MCI/MI in cosmetics and household products (wet wipes, shampoos, rinse-off products where use has increased after leave-on ban). The cross-reactivity pattern between MCI and MI is complex — some sensitized individuals react to both, others to MI alone (which has become the dominant clinical concern following the shift to MI-only preservatives in cosmetics after the MCI/MI restriction). Occupational exposures in the paper and pulp industry, metalworking (cutting fluids), and water treatment (cooling towers) have caused occupational allergic contact dermatitis outbreaks. Wet wipes sensitization has emerged as a major clinical problem because wet wipes containing MI (typically at higher concentrations than cosmetics) are used on the face, perianal area, and for cleaning baby skin, enabling sensitization through particularly susceptible mucosal and perioral skin sites. Clinical presentation of MCI/MI allergic contact dermatitis ranges from localized facial eczema to widespread dermatitis affecting skin areas not directly in contact with the allergen.
What is methylchloroisothiazolinone (mci/kathon cg)?
The IUPAC name is 5-chloro-2-methyl-1,2-thiazol-3-one.
Also known as: 5-chloro-2-methyl-1,2-thiazol-3-one, 5-Chloro-2-methyl-4-isothiazolin-3-one, Methylchloroisothiazolinone, CMIT.
- IUPAC name
- 5-chloro-2-methyl-1,2-thiazol-3-one
- CAS number
- 26172-55-4
- Molecular formula
- C4H4ClNOS
- Molecular weight
- 149.6 g/mol
- SMILES
- CN1C(=O)C=C(S1)Cl
- PubChem CID
- 33344
Risk for people
High riskDermal sensitization is the defining health concern for MCI/Kathon CG. The compound is an extreme contact sensitizer — its EC3 in the LLNA of approximately 0.002% places it in the highest sensitization potency category. Sensitization requires two phases: an induction phase (first contact with the allergen in a susceptible individual) and an elicitation phase (subsequent contact triggering the allergic response). Once sensitized, affected individuals react to trace amounts of MCI/MI in cosmetics and household products (wet wipes, shampoos, rinse-off products where use has increased after leave-on ban). The cross-reactivity pattern between MCI and MI is complex — some sensitized individuals react to both, others to MI alone (which has become the dominant clinical concern following the shift to MI-only preservatives in cosmetics after the MCI/MI restriction). Occupational exposures in the paper and pulp industry, metalworking (cutting fluids), and water treatment (cooling towers) have caused occupational allergic contact dermatitis outbreaks. Wet wipes sensitization has emerged as a major clinical problem because wet wipes containing MI (typically at higher concentrations than cosmetics) are used on the face, perianal area, and for cleaning baby skin, enabling sensitization through particularly susceptible mucosal and perioral skin sites. Clinical presentation of MCI/MI allergic contact dermatitis ranges from localized facial eczema to widespread dermatitis affecting skin areas not directly in contact with the allergen.
Regulatory consensus
11 regulatory and scientific bodies have classified Methylchloroisothiazolinone (MCI/Kathon CG). The classifications differ — that's the data.
| Agency | Year | Classification | Notes |
|---|---|---|---|
| US EPA | 2000 | not classifiable as to human carcinogenicity (Group D) | |
| EFSA | 2014 | not evaluated for carcinogenicity; regulated as potent contact sensitizer under EU Cosmetics Regulation (EC) No 1223/2009; banned from leave-on cosmetics (2014 Commission Decision); restricted to 0.0015% in rinse-off cosmetics; SCCS 2014 opinion found no safe concentration can be established for use in leave-on products due to sensitization epidemic | |
| EPA CTX / Genetox | — | Genotoxicity: negative (single report) (Ames: None, 0 positive / 1 negative reports) | |
| EPA CTX / Genetox | — | Genotoxicity: negative (single report) (Ames: None, 0 positive / 1 negative reports) | |
| EPA CTX / Skin-Eye | — | Skin Sensitization: Sh (score: high) | |
| EPA CTX / Skin-Eye | — | Skin Sensitization: SkinSens1 (score: high) | |
| EPA CTX / Skin-Eye | — | Skin Irritation: Skin corrosion - category 1B (score: very high) | |
| EPA CTX / Skin-Eye | — | Skin Sensitization: Skin sensitisation - category 1 (score: high) | |
| EPA CTX / Skin-Eye | — | Eye Irritation: Category 8.3A (Category 1) (score: very high) | |
| EPA CTX / Skin-Eye | — | Skin Irritation: Category 8.2B (Category 1B) (score: very high) | |
| EPA CTX / Skin-Eye | — | Skin Sensitization: Category 6.5B (Category 1) (score: moderate) |
Regulators apply different standards of evidence — animal-data weighting, exposure-pattern assumptions, epidemiological power thresholds — which is why two scientific bodies can review the same data and reach different conclusions. The disagreement is the data.
Where you encounter methylchloroisothiazolinone (mci/kathon cg)
- Industrial Facilities — Manufacturing plants, Chemical storage areas, Waste treatment sites
- Occupational Environments — Factories, Warehouses, Transportation vehicles
- Personal Care — shampoo, conditioner, lotion, cosmetics, sunscreen
Safer alternatives
Lower-risk approaches that achieve a similar outcome to Methylchloroisothiazolinone (MCI/Kathon CG):
-
Physical/mechanical pest control (IPM)
Trade-offs: More labor-intensive. May not be sufficient for severe infestations.Relative cost: 1.2-2×
-
Phenoxyethanol
Relative cost: 1.2-2×
-
Benzisothiazolinone (BIT) at lower concentration
Relative cost: 1.2-2×
Frequently asked questions
Is methylchloroisothiazolinone (mci/kathon cg) safe for you?
Dermal sensitization is the defining health concern for MCI/Kathon CG. The compound is an extreme contact sensitizer — its EC3 in the LLNA of approximately 0.002% places it in the highest sensitization potency category. Sensitization requires two phases: an induction phase (first contact with the allergen in a susceptible individual) and an elicitation phase (subsequent contact triggering the allergic response). Once sensitized, affected individuals react to trace amounts of MCI/MI in cosmetics and household products (wet wipes, shampoos, rinse-off products where use has increased after leave-on ban). The cross-reactivity pattern between MCI and MI is complex — some sensitized individuals react to both, others to MI alone (which has become the dominant clinical concern following the shift to MI-only preservatives in cosmetics after the MCI/MI restriction). Occupational exposures in the paper and pulp industry, metalworking (cutting fluids), and water treatment (cooling towers) have caused occupational allergic contact dermatitis outbreaks. Wet wipes sensitization has emerged as a major clinical problem because wet wipes containing MI (typically at higher concentrations than cosmetics) are used on the face, perianal area, and for cleaning baby skin, enabling sensitization through particularly susceptible mucosal and perioral skin sites. Clinical presentation of MCI/MI allergic contact dermatitis ranges from localized facial eczema to widespread dermatitis affecting skin areas not directly in contact with the allergen.
What products contain methylchloroisothiazolinone (mci/kathon cg)?
Methylchloroisothiazolinone (MCI/Kathon CG) appears in: Manufacturing plants (Industrial facilities); Chemical storage areas (Industrial facilities); Factories (Occupational environments); Warehouses (Occupational environments); shampoo (Personal care).
Why do regulators disagree about methylchloroisothiazolinone (mci/kathon cg)?
Methylchloroisothiazolinone (MCI/Kathon CG) has been classified by 11 agencies including US EPA, EFSA, EPA CTX / Genetox, EPA CTX / Genetox, EPA CTX / Skin-Eye, with differing conclusions. Regulators apply different standards of evidence (animal data weighting, exposure-pattern assumptions, epidemiological power thresholds), which is why two scientific bodies can review the same data and reach different conclusions. See the regulatory consensus table on this page for the full picture.
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Open in body View raw API dataSources (2)
- US EPA Methylchloroisothiazolinone: Group D Not Classifiable; FIFRA Registered Antimicrobial Pesticide; Extreme Skin Sensitizer EC3 ~0.002%; Kathon CG Industrial Biocide; Aquatic LC50 0.1–1 mg/L; Algal EC50 0.02–0.05 mg/L (2000) — regulatory
- EFSA/SCCS MCI/Kathon CG: EU Leave-On Cosmetics Ban 2015; Rinse-Off Maximum 0.0015%; Contact Sensitization Epidemic Europe 1980s–2010s; LLNA Extreme Sensitizer; Sensitization Rate Reversal Post-Restriction; Wet Wipes MI Cross-Reaction (2014) — regulatory
Reference data, not professional advice. Aggregates publicly available regulatory and scientific data; not a substitute for veterinary, medical, legal, or regulatory advice. Why we built ALETHEIA →