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Methylisothiazolinone

Also known as: MI, methylisothiazolinone, isothiazolinone preservative, MCI/MI, Kathon CG, Methylisothiazolinone

Overview

Methylisothiazolinone (MI) is a synthetic biocide and preservative, often used in combination with methylchloroisothiazolinone (MCI) as MCI/MI. It is not a naturally derived substance nor a dietary supplement ingredient. Instead, MI is widely employed in cosmetics, personal care products (like shampoos and lotions), household cleaning products, and various industrial applications to prevent microbial growth. Its primary function is to inhibit bacterial and fungal contamination in water-containing products, thereby maintaining product integrity and extending shelf life. MI is effective at very low concentrations and is valued for its broad-spectrum antimicrobial activity. The safety and allergenic potential of MI have been extensively studied, particularly in dermatology and toxicology, with numerous human studies, systematic reviews, and safety assessments by expert panels confirming its role as a significant contact allergen.

Benefits

Methylisothiazolinone (MI) does not offer any direct health benefits as a supplement ingredient. Its sole 'benefit' lies in its function as a preservative. As an antimicrobial agent, MI effectively prevents the growth of bacteria, fungi, and other microorganisms in water-containing products. This action helps to maintain the integrity, safety, and shelf-life of cosmetics, personal care items, and household products by preventing spoilage and contamination. While this is beneficial for product preservation, it does not translate into a health benefit for the user, and its primary interaction with the human body is as a potential allergen.

How it works

Methylisothiazolinone (MI) functions as an antimicrobial agent by disrupting the cellular integrity of microorganisms. Its primary mechanism involves targeting and inhibiting essential enzyme systems within microbial cells, which are crucial for their survival and reproduction. Additionally, MI is known to damage microbial cell membranes, leading to leakage of intracellular components and ultimately, cell death. While highly effective against bacteria and fungi, in humans, MI primarily acts as a contact allergen. It can trigger an immune response upon skin contact, leading to allergic contact dermatitis. MI is poorly absorbed through intact skin, but its penetration increases through damaged skin, which can heighten the risk of allergic reactions.

Side effects

Methylisothiazolinone (MI) is primarily associated with allergic contact dermatitis, which is its most common side effect, affecting a significant percentage of exposed individuals. Symptoms include redness, itching, swelling, and rash at the site of contact. The prevalence of MI sensitization has increased globally, leading to restrictions on its use in leave-on cosmetic products in many regions. Less commonly, MI can cause irritant contact dermatitis, which is a non-allergic skin reaction. Severe allergic reactions are rare but possible. There are no significant drug interactions reported for MI. Contraindications include individuals with a known allergy or sensitivity to MI or its combination with methylchloroisothiazolinone (MCI/MI). Increased caution is advised for individuals with sensitive skin or pre-existing dermatological conditions, as they may be more susceptible to adverse reactions. Due to its high sensitization potential, MI's use in products that remain on the skin has been largely phased out or heavily restricted.

Dosage

Methylisothiazolinone (MI) is not a supplement and therefore has no recommended dosage for ingestion or therapeutic use. It is used as a preservative in various products at very low concentrations. Typically, MI is found in rinse-off products (e.g., shampoos, body washes) at concentrations ranging from 0.0015% to 0.01%. For leave-on products (e.g., lotions, creams), its use has been significantly restricted or banned in many regions due to its high potential for causing allergic contact dermatitis, even at very low concentrations. The aim of these low concentrations is to provide effective antimicrobial preservation without exceeding safety thresholds for skin exposure, although its allergenic potential remains a concern.

FAQs

Is MI safe in cosmetics?

MI is considered safe at very low concentrations in rinse-off products. However, due to its high potential for causing allergic contact dermatitis, its use in leave-on cosmetic products has been restricted or banned in many regions.

Can MI cause allergies?

Yes, MI is a well-documented and potent allergen. It is a common cause of allergic contact dermatitis, with an increasing prevalence of sensitization observed globally.

How can I avoid an MI allergy?

If you are sensitive or allergic to MI, you should carefully check product labels and avoid products containing 'Methylisothiazolinone' or 'MCI/MI'. Patch testing can confirm sensitivity.

Research Sources

https://journals.sagepub.com/doi/10.1177/10915818211016382 – This amended safety assessment reviewed extensive patch test data, showing a significant increase in MCI/MI hypersensitivity from 0.5% to 6.0% in European patients between 1993 and 2014. The study confirmed the high allergenic potential of MI and recommended cautious use in cosmetics due to rising sensitization rates.
https://www.cir-safety.org/sites/default/files/mcimi122019FAR.pdf – This systematic review and meta-analysis concluded that MI is a common cause of allergic contact dermatitis worldwide. It highlighted insufficient genotoxicity data for safe use in leave-on products, leading to regulatory restrictions, and noted no evidence of carcinogenicity but significant sensitization risks.
https://journals.sagepub.com/doi/pdf/10.1177/10915818211016382 – This source, identical to the first, further supports the findings of increased hypersensitivity to MCI/MI. It emphasizes the need for regulatory bodies to consider the rising prevalence of contact allergy when setting safety standards for cosmetic ingredients.
https://www.cir-safety.org/sites/default/files/MI-MCI.pdf – This document from the Cosmetic Ingredient Review (CIR) Expert Panel provides a comprehensive safety assessment of Methylchloroisothiazolinone and Methylisothiazolinone. It details the scientific basis for their safety conclusions, including data on absorption, metabolism, and toxicity, ultimately supporting the need for concentration limits due to sensitization concerns.