I don’t know many people who can actually say either of these names. There are a number of acronyms used, but my favourite coping strategy is used by the owner of a small personal care company who simply refers to the combination as ‘methy whatsits’. Now I’ve put that out there I hope you’ll join me in trying to make that the standard term for this preservative combination. I say combination because these two preservatives always used to be used together. Recently it has been possible to use just the Methyl-isothiazolinone. I’ll come to the motivation behind that a bit later.
They are very good preservatives. The combination works extremely well at astonishingly low levels. Around 15ppm is just about the maximum that is ever used and it often does well at much lower levels.
Both the components are highly water soluble, and have hardly any potential to penetrate the skin or to build up in the body if they manage to do so. They are also readily biodegradable. The combination of very low use levels, high biodegradability and zero risk of accumulation in fatty tissues make them a very environmentally friendly option. If you have read Silent Spring, or my review of it, you’ll be well aware that accumulation in tissues can have unpredictable and undesirable consequences. No such worries with the methy whatsits.Methylchloroisothiazolinone
They have also been extensively tested for potential carcinogenicity, and have passed with flying colours.
Unfortunately they have an Achilles heal. At high concentrations they are quite bad skin irritants. This is a real tragedy, because in every other respect they really are the ideal preservative system. It is a shame because this irritancy really isn’t a problem so long as you keep the concentration low enough. Unfortunately as you’ll see from my post about patch testing of cosmetic preservatives it is only too possible for a preservative to get an undeserved reputation. And this is particularly true of this pair.
The trade name is much easier to pronounce. It is Kathon CG. Kathon CG has been widely criticised for its sensitisation. I think it is impossible to dismiss the fact that it is a sensitiser, but it is easy to exaggerate just how bad it is. In particular if you look at the extensive literature it is pretty rare to find it studied at its typical use level in actual products.
Kathon CG is regularly slated on blogs extolling the benefits of natural products. It is so easy to find data about its skin sensitising properties it would be rather surprising if it didn’t. Needless to say this information is usually presented as if this is a major scandal. I don’t know what can be done about this. The simple statement ‘Kathon CG is a potent skin sensitiser’ is true. The more accurate ‘Kathon CG is a potent skin sensitiser at levels well above its normal use levels, but despite that it is still a good choice because it is so effective you can use it at really low levels so you get hardly any skin reactions in normal use’ doesn’t really trip off the tongue nearly as well.
The reality is that you are very unlikely indeed to have any problems with a product that contains this preservative. It is used in some very big selling products indeed and if it was really giving rise to a lot of reactions it would be changed. But once something gets a bad name it is a devil of a job to shift it. Recently it has become possible to get the methylisothiazolinone on its own. This is the least sensitising of the two and so this might be a chance to re-evaluate it. But the trouble is it is still going to look bad when tested at a high level in a patch test. And I don’t think many consumers pay that much attention to the details. But maybe I am too pessimistic and this will give us a new option. I hope so.
In the meantime, I think that Kathon CG and its derivatives are very close to being the ideal preservative. Whenever I need to preserve something in the lab – which is quite often, bugs are everywhere – it is my first choice. It hasn’t let me down yet.
This recent review by the EU’s scientific committee makes it clear just how much data, and just how positive that data is, for this combination of materials. The only issue of concern was sensitisation, which was considered as not an issue in rinse off products. It did not draw any conclusion about its suitability in leave on products.
A review paper from 2000 that finds reactions to this material to be comparable to other preservatives.
Israeli review of skin reactions which again showed relatively low levels of reactions.
Patch tests on a large sample of nearly 2,000 eczema patients only elicited 1.6% patch test reactions to Kathon CG. This was using a level much higher than would be used in cosmetics.