So why are patch tests a problem for preservatives?
The trouble is the way they are done. The preservatives get put straight onto the skin, or at least not diluted to anything like the extent they are in an actual product. Preservatives are intended to kill germs so they tend to be a bit on the aggressive side as chemicals go and a lot of them are quite irritating. So reactions to them in patch tests are common. Also there aren’t that many permitted preservatives, and not all the permitted ones are actually any use practically. So the same preservatives get used in huge numbers of products.
Dermatologists are scientists, and like all scientists they love data and often publish reviews of the results of their patch tests. This is a good thing, the more information the better. But it does mean that lots of data is available on irritant reactions and preservatives tend to get over represented in this data. They get patch tested again and again thanks to being so widely used. And they are usually being patch tested at concentrations way higher than the formulator put them into the formulation. And on top of that, the people being patch tested are ones with sensitive skin – by definition.
This isn’t to say that preservatives don’t cause skin reactions. They do. But the impression you would get from looking at the published literature on skin reactions is more than a little misleading. For a start, the more widely a preservative is used the more likely it is to end up in a patch test. The most widely used preservatives are the parabens. There is a good reason for this. Most formulators have worked out that they get the least number of complaints about skin reactions when they use parabens. But they feature quite strongly in the numbers of skin reactions reported in the scientific literature.
With the advent of the internet it is possible for anyone to do their own research and to reach quite wrong conclusions about just how bad some preservatives are.
The trouble is that all this makes the idea of launching a new preservative a deeply unappealing project for chemical manufacturers. If it is successful it is only a matter of time before it starts appearing in patch tests and starts to get a bad name. And it is getting to be more and more of a problem for formulators as we lose more and more options to a combination of enthusiastic regulators and bad press for the remaining permitted molecules.
But I think there is some good news on the horizon. If the upcoming regulations on cosmetic vigilance are implemented carefully, I am hoping that they will bring some perspective back. I have said that preservatives cause reactions. But what you wouldn’t realise from most of the coverage of them is that the numbers of reactions are really really low. Perhaps when there are publicly available statistics on overall rates it will become clear that they really aren’t that much of a problem.
Nobody is ever going to love them – they just aren’t that lovable – but I hope with more information people will come to realise that they really aren’t all that scary.
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