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Methylchloroisothiazolinone

MCI

Worth knowingPreservativepreservative

EU-restricted, a common contact allergen, or subject to live debate.

Also written as: MCI · CMIT · 5-Chloro-2-methyl-4-isothiazolin-3-one · Kathon CG (as MCI/MI 3:1)

What it is

Methylchloroisothiazolinone is not permitted on its own in EU cosmetics; it is allowed only as the fixed 3:1 mixture with methylisothiazolinone, which is why the two names almost always appear together on a label. The blend has been recognised as a significant contact allergen since the 1980s. Since 2015 it has been confined to rinse-off products only.

What it does well

  • Very broad-spectrum preservation at parts-per-million levels

    The MCI/MI blend is one of the most powerful cosmetic preservatives available, which is why it survived in rinse-off use. Again, this is a technical property rather than a consumer benefit.

    Strong evidence

Worth knowing

  • A long-established, clinically important contact allergen

    MCI/MI positivity in European patch-test clinics peaked at 5.4% to 7.6% around 2013-2014 before the restrictions took effect. Sensitisation is usually lifelong.

    Matters most toAnyone with persistent hand or facial eczema; anyone already sensitised to MI, since cross-reaction is common.

  • Cannot be combined with MI on its own in the same product

    EU rules do not permit the MCI/MI mixture and standalone MI to be used together in one product, to prevent stacking of isothiazolinone exposure.

    Matters most toNobody directly; it is a compliance rule for manufacturers.

Where the EU stands

Concentration-restricted

Permitted only as a 3:1 mixture of methylchloroisothiazolinone and methylisothiazolinone, at a maximum of 0.0015% and in rinse-off products only. The rinse-off-only restriction has applied since 16 July 2015 (Regulation (EU) No 1003/2014).

Regulation (EC) No 1223/2009, Annex V entry 39, as amended by Commission Regulation (EU) No 1003/2014 · source ↗

Sources (2)

Every link goes to the original: a journal, a PubMed record or an EU document.

  1. [1]
    Trends in the Prevalence of Methylchloroisothiazolinone/Methylisothiazolinone Contact Allergy in North America and Europe

    JAMA Dermatology · 2023 · doi:10.1001/jamadermatol.2022.5991

    MCI/MI positivity peaked at 7.6% (ESSCA) and 5.4% (IVDK) in Europe in 2013-2014 and then declined, while North American rates rose to 10.8%.

  2. [2]
    Contact dermatitis associated with preservatives: Retrospective analysis of North American Contact Dermatitis Group data, 1994 through 2016

    Journal of the American Academy of Dermatology · 2021 · doi:10.1016/j.jaad.2020.07.059

    Isothiazolinones were the leading preservative allergens in a series of 50,799 patients, with 22.3% reacting to at least one preservative overall.