Methylisothiazolinone; 2-Methyl-4-Isothiazolin-3-One(in 22,951 products)Banned by Gov or classified as a carcinogen by IARC
Banned by Gov or classified as a carcinogen by IARC
1. Antimicrobial Preservative - Actively kills and inhibits the growth of unwanted microorganisms which may be harmful.
2. Preservative - Prevents and inhibits the growth of unwanted microorganisms which may be harmful
Methylisothiazolinone (short for MIT or MI; sometimes erroneously called methylisothiazoline), is a powerful synthetic biocide and preservative within the group of isothiazolinones. MIT is used in numerous personal care products such as cosmetics, lotions, moisturizers, sanitary wipes, shampoos, and sunscreens, and a wide range of industrial applications. 
An EU amendment effective from the 12th February 2017 onward and has stated that "no safe concentrations of Methylisothiazolinone for induction of contact allergy or elicitation have been adequately demonstrated." There is no safe concentration of MIT in leave-on products, so MIT has been banned in all leave-on products and wet wipes.
MIT will still be permitted in rinse-off products at a 0.01% concentration. In the interest of consumer safety, 1Source gives a rating of 10 for MIT due to the possibility that MIT might not be completely rinsed-off and the possibility that rinse-off products could be mistakenly treated as leave-on products. Nevertheless, methylisothiazolinone remains an approved ingredient in CosIng Annex 5. 1Source hypothesizes that either those two agencies rate ingredients independently of each other or that the CosIng Annex 5 has not yet been updated as of 2018. In the interest of consumer safety, 1Source gives a rating based on the agency with the harshest rating of the ingredient in question.
The Hong Kong Consumer Council has labeled methylisothiazolinone as a preservative which "might cause an allergic reaction in some people". As a preservative in shampoos, the Consumer Council has stated: "According to expert advice, applying shampoos with MIT or CMIT preservatives on open wound could lead to a higher chance of adverse discomfort reaction." In another Council report, it also warned that "wounds on ladies' private parts, could become irritated and uncomfortable, after using products containing MIT".
Positive patch test results from MIT has been steadily increasing over the years, from <2% in 2010 to >10% in 2013.  The sudden increase in MIT contact allergy remains unknown, though results from elsewhere in Europe show a prevalence "that is still below 10%".  In a separate study conducted on 10,000 individuals, there appears to be minimal correlation between MIT concentration and contact allergy rate: “MI [MIT] contact allergy varied during 2003–2009 between 0.5 and 1.9%. From 2003 until 2007 MI [MIT] was tested at 4 different concentrations. MI [MIT] tested at 475 ppm had a contact allergy rate of 1.0%. For MI [MIT] tested at 950, 1,000, and 1,500 ppm the figure was 0.5%.”  The MIT concentration (ppm) is independent of the contact allergy rate (%).
A study in Luxembourg also deduced that a 0.01% maximum concentration permitted in rinse-off products is still too high and that it should be further decreased to 0.0015%. "Current clinical data indicate that 100 ppm MI [MIT] in cosmetic products is not safe for the consumer...For rinse-off cosmetic products, a concentration of 15 ppm (0.0015%) MI [MIT] is considered safe for the consumer from the view of induction of contact allergy."  Though it agreed with EU findings that no safe concentrations for leave-on products have been found: "For leave-on cosmetic products (including 'wet wipes'), no safe concentrations of MI for induction of contact allergy or elicitation have been adequately demonstrated". 
1. PubChem: https://pubchem.ncbi.nlm.nih.gov/compound/39800
2. The rise in prevalence of contact allergy to methylisothiazolinone in the British Isles. (Contact Derm. 2014 Apr;70(4):238-40. doi: 10.1111/cod.12185.)
3. Methylisothiazolinone contact allergy is rising to alarming heights also in southern Sweden. (Acta Derm. Venereol. 2015 Jan;95(1):31-4. doi: 10.2340/00015555-1844.)
4. Opinion of the Scientific Committee on Consumer safety (SCCS) – Opinion on the safety of the use of Methylisothiazolinone (MI) (P94), in cosmetic products (sensitisation only). (Regul. Toxicol. Pharmacol., 76(), 211–212. doi:10.1016/j.yrtph.2016.01.001)
5. Methylisothiazolinone may induce cell death and inflammatory response through DNA damage in human liver epithelium cells. (Environ. Toxicol. 2018 Feb;33(2):156-166. doi: 10.1002/tox.22503. Epub 2017 Nov 7.)
1. EU Scientific Committee on Consumer Safety
2. Hong Kong Consumer Council 2017-2018 Annual Report
- Report: Improving Product Quality and Safety, p28-29
3. Hong Kong Consumer Council 2016-2017 Annual Report
- Report: Improving Product Quality and Safety, p26
4. EU CosIng Annex V, PRESERVATIVES ALLOWED IN COSMETIC PRODUCTS 
- Ref: V/39 and V/57
5. International Fragrance Association Transparency List 
Safety and Hazards (UN GHS):
1. Toxic if swallowed (H301)
2. Toxic if swallowed or in contact with skin (H301+H311)
3. Harmful if swallowed (H302)
4. Toxic in contact with skin (H311)
5. Causes severe skin burns and eye damage (H314)
6. May cause an allergic skin reaction (H317)
7. Causes serious eye damage (H318)
8. Fatal if inhaled (H330)
9. Toxic if inhaled (H331)
10. Harmful if inhaled (H332)
11. May cause respiratory irritation (H335)
12. Very toxic to aquatic life (H400)
13. Very toxic to aquatic life with long lasting effects (H410)
Potential Health Concerns For:
1. Dermatitis, Allergic Contact (PubMed ID:12081698)
2. Dermatitis, Contact (PubMed ID:25907379)