[vhfpmr (OP)]

I didn't even notice at the time, but I recently picked up a mentholated version of the shampoo I normally buy.  At first I thought I was imagining things when my face and scalp felt freezing cold in the shower, but then I noticed the vapour catching in my windpipe too. Having decided I didn't like it, I made a mental note not to pick up any more after that bottle ran out.

Anyway, by the time I'd used half the bottle I developed a bleeding sore spot in my right ear, which I just put down to a zit or something. It wasn't until I developed a matching sore in my left ear that I put 2 & 2 together. The rest of the bottle's gone down the drain now, but am I just jumping to post hoc conclusions? From the look of this website, perhaps not:

https://www.poison.org/articles/what-happens-with-swallowing-or-inhaling-too-much-menthol--174

"Menthol can cause eye and skin irritation. When used on the skin, menthol is typically diluted into a "carrier oil", lotion, or other vehicle. If a high-percentage menthol product is applied to the skin, irritation and even chemical burns have been reported. There are a few reports of people being very sensitive to menthol and having severe skin reactions to even small amounts."

[Bored chemist]

m I just jumping to post hoc conclusions?

Probably.

[vhfpmr (OP)]

Well the sores disappeared quick enough as soon as I stopped using the stuff, so I think that was probably the cause, especially as I have a history when it comes to chemicals and skin reactions.

[set fair]

There are 8 menthol stereoisomers. Given the pharmacuetical companies hostility to ecigarettes I woud not be surprised to learn that they have found the most dangerous and only manufacture that one now.

[Bored chemist]

There are 8 menthol stereoisomers. Given the pharmacuetical companies hostility to ecigarettes I woud not be surprised to learn that they have found the most dangerous and only manufacture that one now.

Nobody in their right mind would "manufacture" menthol commercially.
They extract it from mint plants.

It's at least as likely the the OP's problem was caused by some other ingredient in the shampoo.
It's true that menthol (at high concentrations) can irritate the skin.
But it's not as good at irritating the skin as some detergents are.

[set fair]

Check your facts old son.

[Bored chemist]

Check your facts old son.

Why bother? They are correct.
Unless, of course you have something other than conspiracy claptrap.

[set fair]

Synthetic menthol production is thriving.

[Bored chemist]

Synthetic menthol

What do you know!
It turns out I was right for the wrong reason.
https://en.wikipedia.org/wiki/Menthol#Production

"Menthol is manufactured as a single enantiomer (94% e.e.) "
They do make it, but they are careful to make the right isomer.

So, my underlying point was valid.
This is drivel.

Given the pharmacuetical companies hostility to ecigarettes I woud not be surprised to learn that they have found the most dangerous and only manufacture that one now.

[vhfpmr (OP)]

It's at least as likely the the OP's problem was caused by some other ingredient in the shampoo.

Doesn't sound very likely to me. Apart from the menthol, all the other ingredients are in the non-mentholated version that I've been using for years without any problem.

It's Wash & Go, the 'Classic' version is ok, and the 'Sport' version that causes the sores. Here are the ingredients:

Classic version:
Aqua, Sodium Laureth Sulfate, Sodium Lauryl Sulfate, Glycol Distearate, Cetyl Alcohol, Cocamide MEA, Dimethicone, Guar Hydroxypropyltrimonium Chloride, Parfum, Trimethylolpropane Tricaprylate/Tricaprate, Hydrogenated Polydecene, Sodium Citrate, Citric Acid, Disodium EDTA, Sodium Chloride, Methylchloroisothiazolinone, Methylisothiazolinone, Sodium Benzoate, Benzyl Salicylate, Linalool, Hexyl Cinnamal, Citronellol, Limonene

Sport version:
Aqua, Sodium Laureth Sulfate, Sodium Lauryl Sulfate, Glycol Distearate, Cetyl Alcohol, Cocamide MEA, Dimethicone, Guar Hydroxypropyltrimonium Chloride, Menthol, Parfum, Sodium Citrate, Citric Acid, Disodium EDTA, Sodium Chloride, Methylchloroisothiazolinone, Methylisothiazolinone, Sodium Benzoate, Benzyl Salicylate, Linalool, Hexyl Cinnamal, Citronellol, Limonene

(Note that there are two ingredients in the trouble-free Classic that are not in the mentholated version.)

[Bored chemist]

One remaining issue "Parfum" can mean almost anything.

[vhfpmr (OP)]

So after I switched back to the original shampoo that had been trouble-free for years, the sores went away.....but then came back again a few weeks later. I've now switched to a completely different brand, and again the sores have gone away.
For the time being?