I got a good point made to me via e-mail responding to one of my posts about methylchloroisothiazolinone and methylisothiazolinone.   He or she desribes themselves by the pathos ridden name foreverbotheredbythisallergy and referring to some products that readers have identifed as free of those two chemicals –

Some the products above have the benzylisothiazoline which can give you the same reaction as the methyl and chloromethylisothiazoline. I have been using Tide Free for years and just now have developed the skin rash. As it turns out it is the benzylisothiazoline in the product. Be warned, the benzylisothiazoline allergy can occur after use over a period of time with a product that has this chemical in it. good luck.

And they are absolutely right.  The medical term for this is cross sensitivity.  If people with allergies didn’t already have enough trouble, it turns out that if you are sensitive to one chemical it might predispose you to get the same reaction to a related one.

In the case above the similarity is reasonably easy to spot from the way the names sound.  But it isn’t necessarily all that easy to work out.  A common pair of cross sensitisers for example is caffeine and nicotine.  Lots of chemists wouldn’t spot that one.

The reason for this is fairly easy to understand.  The reaction is caused by a particular bit of a molecule rather than the whole thing.  So when a molecule with the same shape turns up, it triggers the same reaction.   Two people who react to one chemical might well be reacting to different bits of it.  So if you have the same reaction as foreverbotheredbythisallergy to MI and/or MCI you won’t necessarily also react to benzylisothiazoline – though I for one would forgive you if you didn’t want to try it and found it.

Although cross sensitisation is easy to understand it is almost impossible to offer any way this knowledge can help.  It is probably a good idea to pick products with the shortest possible ingredient list, if you have a choice.  You rarely will though.  Avoiding natural products might be wise too since they tend to have more different ingredients than more conventional ones.  That is very much a theoretical suggestion.  My personal experience is that natural products seem to attract pretty much the same rate of reports of skin reactions as more mainstream ones – though that might simply reflect the particular set of products where I have seen the data.  I’d love to hear if anyone has any actual data on this  (I tried product x and I came out in a rash doesn’t count as data here.)

In the long run the best thing for everyone is to have as diverse a set of products formulated in as many different ways with as many different ingredients as possible.  This would give people who suffer from allergies more chance to find products that suit them.  In this respect, every single scare story is a step backwards.  Do your best not to spread them.   In the meantime, my sympathy is with people like foreverbotheredbythisallergy.  I know it isn’t easy and good luck with your search for products that your skin can cope with.

You might be interested in my post on products free of MethylChloroIsothiazolinone and MethylIsothiazolinone

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