Chemical Profile: Isothiazolinone Preservatives

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Why It Matters

Isothiazolinone preservatives are a common substitution for banned triclosan in antibacterial hand soaps, yet these substances come with their own set of risks. Isothiazolinone preservatives are established skin sensitizers and irritants and are linked to endocrine disruption and aquatic toxicity. In addition to hand soaps, they are found in various personal care products. While it may be tempting to fall for this ingredient substitution, isothiazolinone preservatives are no better than triclosan and other banned antimicrobials. At MADE SAFE, we advise consumers to be diligent in avoiding ingredients of this class.

What Is It?

Isothiazolinone preservatives, in addition to chloroxylenol, are a common substitution for triclosan in hand soaps for their antibacterial and antifungal properties.[1] [7] Two of the most commonly used ingredients of this class are methylisothiazolinone (MI) and methylchloroisothiazolinone (MCI), often in combination at a 3:1 ratio and referred to as methylisothiazolinone / methylchloroisothiazolinone (MCI/MI). MCI/MI is an effective antimicrobial at low concentrations.[5]

Chemical Profile Isothiazolinone Preservatives Infographic MADE SAFE Blog

MCI/MI is historically used as a combination in consumer products. In the early 2000s, methylisothiazolinone was permitted as a standalone preservative for industrial products, and in 2005, for cosmetics.[8] This regulatory change resulted in a 25-fold increase in methylisothiazolinone exposures from personal care products.[13]

Where It's Found

Isothiazolinone preservatives are found in cleaning products, shampoo, conditioner, body wash, and other common personal care and household products. Other isothiazolinone preservatives, benzisothiazolinone (BIT) and octylisothiazolinone (OIT), are less common in personal care products. BIT and OIT are used in industrial chemicals, paints, glues, leather, and cleaning products.[13]

The Health Concern

Both methylisothiazolinone and methylchloroisothiazolinone are known irritants, sensitizers, and causes of contact skin allergies.[3] [8]  Between 2009 and 2015, and following the permittance of standalone methylisothiazolinone in cosmetics, an epidemic of MCI/MI and/or MI-derived contact allergy emerged globally. In 2013, the American Contact Dermatitis Society named methylisothiazolinone the contact allergen of the year.[15]  As a result, MCI/MI was banned from leave-on products (but not rinse-off products) by the European Union as of 2016, followed by a ban of stand-alone methylisothiazolinone from leave-on products by 2017.[9] [10]   In that same year, methylisothiazolinone was also placed on the TEDX List of Potential Endocrine Disruptors.[16]

Isothiazolinone preservatives have been linked to impaired lung function following the use of humidifier disinfectants containing MCI/MI.[2] [6] [11]  Isothiazolinones are also suggested to be potentially linked to cytotoxicity, neurotoxicity, and carcinogenicity.[11] [14]  However, more research is necessary to support these claims.

Given their biocidal properties, isothiazolinone preservatives are considered to pose ecotoxicological harm once released to the environment.[12]  According to the European Union, MCI/MI is very toxic to aquatic life.[4]  Methylisothiazolinone has been shown to inhibit wound healing in aquatic animals.[17]  Methylisothiazolinone and octylisothiazolinone have been linked to developmental toxicity and endocrine disruption in zebrafish.[7]

Significant elimination of isothiazolinone preservatives has been shown following wastewater treatment, and biological degradation appears to be a relevant mode of their elimination in water.[12]  In higher concentrations, however, the antimicrobial properties of the preservatives can slow down their biodegradation.[12]  MADE SAFE does not permit isothiazolinone preservatives in certified products due to their associations with endocrine disruption in humans and aquatic animals, in addition to other potential adverse effects.

How to Avoid It

Read labels, avoid isothiazolinone derivatives, and also:

  • Be wary of products marketed as “antimicrobial,” “odor fighting,” “germ-killing,” or “antibacterial.”
  • Shop MADE SAFE Certified products
  • Wash hands with plain soap and water. It is one of the most important steps you can take to avoid spreading germs and getting sick. The CDC recommends scrubbing hands for at least 20 seconds (the amount of time it takes to sing Happy Birthday in your head).


[1] Capkin, E., Ozcelep, T., Kayis, S., and Altinok, I. (2017). Antimicrobial agents, triclosan, chloroxylenol, methylisothiazolinone and borax, used in cleaning had genotoxic and histopathologic effects on rainbow trout. Chemosphere. 182: 720-729.

[2] Cho, H.J.; Park, D.U.; Yoon, J.; Lee, E.; Yang, S.I.; Kim, Y.H.; Lee, S.Y.; Hong, S.J. (2017). Effects of a mixture of chloromethylisothiazolinone and methylisothiazolinone on peripheral airway dysfunction in children. PLoS ONE. 12(4): e0176083.

[3] de Groot, A. C., & Herxheimer, A. (1989). Isothiazolinone preservative: Cause of a continuing epidemic of cosmetic dermatitis. Lancet, 11(1).

[4] European Chemicals Agency. Reaction mass of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-2H-isothiazol-3-one. REACH – Registration, Evaluation, Authorisation and Restriction of Chemicals Regulation Registered Substances Factsheets. Accessed September 19, 2022.

[5] Fewings, J. and Menne, T. (1999). An update of the risk assessment for methylchloroisothiazolinone/methylisithiazolinone (MCI/MI) with focus on rinse-off products. Contact Dermatitis. 41:1-13.

[6] Lee, E., Son, S.K., Yoon, J., Cho, H., Yang, S., Jung, S., Do, K., Cho, Y.A., Lee, S., Park, D., and Hong, S. (2017). Two Cases of Chloromethylisothiazolinone and Methylisothiazolinone-associated Toxic Lung Injury. J Korean Med Sci. 33(16): e119.

[7] Lee, S., Lee, J., Kho, Y., Ji, K. (2022). Effects of methylisothiazolinone and octylisothiazolinone on development and thyroid endocrine system in zebrafish larvae. Journal of Hazardous Materials. 425: 127994.

[8] Lundov, M.D., Krongaard, T., Menne, T.L., and Johansen, J.D. (2011). Methylisothiazolinone contact allergy: a review. British Journal of Dermatology. 165(6): 1178-1182.

[9] Official Journal of the European Union. (2014). Commission Regulation (EU) No 1003/2014 of 18 September 2014 amending Annex V to Regulation (EC) No 1223/2009 of the European Parliament and of the Council on cosmetic products. Accessed September 21, 2022.

[10] Official Journal of the European Union. (2016). Commission Regulation (EU) 2016/1198 of 22 July 2016 amending Annex V to Regulation (EC) No 1223/2009 of the European Parliament and of the Council on cosmetic products. Accessed September 21, 2022.

[11] Park, E. and Seong, E. (2020). Methylisothiazolinone induces apoptotic cell death via matrix metalloproteinase activation in human bronchial epithelial cells. Toxicology in Vitro. 62: 104661.

[12] Rafoth, A., Gabriel, S., Sacher, F., and Brauch, H. (2007). Analysis of isothiazolinones in environmental waters by gas chromatography – mass spectrometry. Journal of Chromatography A. 1164: 74-81.

[13] Reeder, M. and Atwater, A.R. (2019). Methylisothiazolinone and Isothiazolinone Allergy. Cutis. 104(2): 94-96.

[14] Silva, V., Silva, C., Soares, P., Garrido, E.M., Borges, F., and Garrido, J. (2020). Isothiazolinone Biocides: Chemistry, Biological, and Toxicity Profiles. Molecules. 25(4): 991.

[15] Sukakul, T., Kanchanapenkul, D., Bunyavaree, M., Limphoka, P., Kumpangsin, T., and Boonchai, W. (2018). Methylchloroisothiazolinone and/or methylisothiazolinone in cosmetic products – A market survey. Contact Dermatitis. 80:110-113.

[16] The Endocrine Disruptor Exchange, (TEDX). (2018). Search the TEDX list: Methylisothiazolinone. Accessed September 19, 2022.

[17] Van Huizen, A.V.; Tseng, A.; Beane, W.S. (2017). Methylisothiazolinone toxicity and inhibition of wound healing and regeneration in planaria. Aquatic Toxicology. 191:226-235.

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