Should you be scared of your shampoo?

I was doing some grocery shopping online recently (I have small children, I’ve started to view traditional supermarket shopping in the same way as beating my carpets with a stick and washing clothes in a stream) when I came across some reviews for a particular brand of shampoo.

Most of the reviews were positive, but some were not. In particular, there were a few one star ones complaining about ingredients called methylchloroisothiazolinone and methylisothiazolinone.



What, you may be wondering, are these monstrosities? Surely with names that long they must be huge great big molecules? Actually no, they’re quite small. Methylchloroisothiazolinone (shown in the graphic) has a mere four carbon atoms and an interesting assortment of other elements. They’re part of a group of compounds called isothiazolinones, which are heterocyclic molecules that include a five-membered ring which contains nitrogen, sulfur and a C=O group.

Not surprisingly considering the unwieldy name, methylchloroisothiazolinone is often shortened to MCI. Likewise, the chemically-similar methylisothiazolinone (imagine the molecule above without the -Cl bit) goes by the moniker MI, or sometimes MIT.

MCl and MI are common preservatives in cosmetic products

MCl and MI are common preservatives in cosmetic products.

Why are these things in shampoo? Well, they are very effective preservatives. They’re antibacterial and antifungal, and work against both gram-positive and gram-negative bacteria, as well as yeast and fungi. This is a good thing, because some of these microbes are pretty nasty. The bacteria, for example, include such lovelies as Nocardia (associated with a particular type of respiratory disease), Staphylococcus (associated with various infections) and Listeria (most famous for causing gastrointestinal distress). It may be a small risk, but showers are warm, moist environments – basically the perfect breeding ground for these sorts of things. If these microbes start growing in your shampoo, shower gel and so on, they would then end up on your hair and skin, possibly be inhaled, and might even make their way into your bloodstream if you had a small cut somewhere.


So, that’s why these chemicals are there. That all sounds good, right? Why are people complaining?

The dose makes the poison is an important principle in toxicology (image credit: Lindsay Labahn, click for link)


Well, because they also have their hazards. Now, before I go any further, we should remember a very important principle of toxicology, which is that “the dose makes the poison“. Everything, I really mean EVERYTHING, is dangerous if you’re exposed to too much of it. Oxygen is quite crucial if you want to carry on living, for example, but breathe in too much of it for too long and you’re at risk of developing visual disturbances, tinnitus, nausea and muscle spasms. Too much could even be lethal. Similarly, a pinch of salt is quite nice on chips, but try and drink say, seawater, and you’ll soon regret it. Even plain water can be dangerous if you consume too much in too short a time, particularly if you’re also exercising hard.

Many chemicals that are used industrially have scary lists of associated hazards, but it’s important to remember that these warnings are usually aimed at people who use said chemical in an industrial setting. In other words, they might be handling kilograms or even tonnes of the stuff, all day every day, as opposed to the teeny tiny quantity you’re likely to meet a few times a week.

I could pick literally any ingredient in that shampoo bottle and proclaim that it’s dangerous. This would be perfectly true, but also meaningless. A more pertinent question is: is it dangerous in the quantity that you usually use?

Are methylchloroisothiazolinone and methylisothiazolinone in shampoo dangerous? There’s no evidence that they bioaccumulate (build up in the body) or that they’re linked to any kind of cancer (phew). In 2002, there was an in vitro (i.e. outside of living organisms) study of the neurotoxicity of MI which showed that mature neurons in tissue culture could be killed by 4-12 ppm solutions of the chemical. But these experiments were performed on rat brain cells in culture. Lots of things will damage cells in a petri dish: it doesn’t mean that we necessarily have to worry about them in every day life. A shampoo solution pouring straight into your brain might well be harmful, but I suggest that if that’s happening in the shower you have bigger problems. Namely, major head trauma.

However, in high concentrations, MI and MCl are definitely skin and membrane irritants, which can cause chemical burns. They’re known chemical ‘sensitisers‘. This means that exposure to them, even at fairly low levels, might cause an allergic reaction.


A patient who presented to a medical centre following a severe reaction to methylisothiazolinone in a wipe (SA Government – click image for source).

This is where we get into difficult territory, because exactly how a particular individual is going to respond to something like this can be hard to predict. For example, I’ve never had a nasty reaction to methylchloroisothiazolinone. Give me an aspirin, on the other hand, and I’m likely to be in trouble. Allergies are specific to individuals. But there is no doubt that some people do have nasty reactions to MCI and MI; some sources have suggested it might be as many as 15% of the population (and that this number might, worryingly, have increased in recent years).

These chemicals are, or at least have been, also used as preservatives in other products such as sunscreens, moisturisers and wipes (baby wipes, facial wipes and moist toilet tissue, for example), which is a particular concern because you don’t wash off the the residue from these products – that generally being the point of using them – so it lingers on the skin.

A 2014 report from the International Journal of Toxicology concluded that although MI and MCI are sensitisers at concentrations of 50 ppm and above, they weren’t at concentrations of 15 ppm (and below). And therefore they, “may be safely used in ‘rinse-off’ products at a concentration not to exceed 15 ppm and in ‘leave-on’ cosmetic products at a concentration not to exceed 7.5 ppm”.

However, also in 2014, the European Commission Scientific Committee on Consumer Safety argued that: “For leave-on cosmetic products (including ‘wet wipes’), no safe concentrations of MI for induction of contact allergy or elicitation have been adequately demonstrated.”

People have been particularly worried about children, especially with respect to baby wipes. This is not unreasonable, since not only is the contact dermatitis that can occur painful and unpleasant, but once sensitisation has occurred it can’t be reversed: anyone affected will have to read labels extremely carefully for ever after. As a result, consumer groups have campaigned to have MI and MCI removed from any product that’s left on the skin over the last few years.

I happen to have three different brands of baby wipes in my house at the moment (small children you see), and a quick glance at the ingredients tells me that MI and MCI aren’t in any of them, and nor are they ingredients in the packet of flushable moist toilet tissue in the bathroom. This is hardly a comprehensive survey of course, but it suggests that these substances might be falling out of favour. Big companies aren’t really out to get us: pictures of people with nasty skin lesions after using their products doesn’t do them any favours.

Some consumers have complained about the use of MI and MCl in products.

Some consumers have complained about the inclusion of MI and MCl in products.

Do you really need to worry? Were these consumers right to highlight the fact that the shampoo contains MI and MCI in their reviews? Well, if you know you have sensitive skin then these substances probably are best avoided. But is shampoo likely to cause sensitisation if you’re fortunate enough to be blessed with the sort of skin that generally doesn’t erupt into a rash if the wind so much as changes? No one can say for certain, but it seems unlikely because you wash it off: these substances are only in contact with your skin for a few seconds.

So, whilst it doesn’t hurt to be aware of such things, there’s probably no need to panic and throw out all your shampoo just in case. On the other hand, if you’ve been wondering why your skin seems to be permanently irritated, it might be worth checking a few ingredients labels.

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17 thoughts on “Should you be scared of your shampoo?

  1. Fascinating stuff, I often wondered what they were for, but couldn’t ‘Google’ in the shower (Oo err) and promptly forgot afterwards.
    I find that liquid soaps dry the skin on my hands, I used a public convenience today which has those fancy three button hand washing stations. Button one for soap, two for tepid water, and three for the warm air. It was only after drying my hands on my trousers, (I think the hamster operating the bellows had died) that I realised I had clean but uncomfortable hands. Now what’s in the Nivea?…

    Liked by 1 person

    • Soaps are usually drying because they work by emulsifying oils on your skin with water, so they can be washed away along with any dirt. But of course, that includes the natural oils that keep your skin soft. Drier skin is the price we pay for clean skin. There are some interesting ingredients in moisturisers too. I’m amazed how many list sodium hydroxide!


      • I often wondered if soap manufacturers were pulling a fast one there with the sodium hydroxide? Saves them an ingredient or so, as you make your own soap. They provide scent (or not) and something to stop the NaOH running out of the box. You provide the fats….


    • Oh fascinating – I find that hard bar soap leaves me feeling like I have a residue on my skin no matter how much I rinse-scrub to try to get it off. Liquid soap is what I have to use. I wonder if there’s a skin chemistry difference that causes that.


      • I think that’s down to pH. I reckon if you tested it, you’d find the bar of soap is quite alkaline, whereas the liquid soap is closer to neutral. The more alkaline soap bar is more effective at dissolving all the oils on your skin, which reduces the friction between surfaces and, perhaps counterintuitively, creates a slippery feeling.


    • Both my dogs have suffered severe skin problems after being washed just one time with a Hartz dog shampoo that contains these preservatives. BAN THEM. Do not use Hartz products.


  2. Thanks for this article! Actually, lately I’ve been wondering whether I should be scared of my sunscreen with the new concerns released about oxybenzone and other chemical UV filters…any thoughts on that? It’s a really unfortunate Catch-22 and I’d rather not run around with zinc oxide on my face.


  3. You may be interested to know that the research on methylisothiazolinone has continued in in vivo models, and has found effects from chronic exposure to lower concentrations of the chemical:

    Neuroscience. 2012 Mar 15;205:194-204. doi: 10.1016/j.neuroscience.2011.12.052. Epub 2012 Jan 4.
    Abnormal visual processing and increased seizure susceptibility result from developmental exposure to the biocide methylisothiazolinone.
    Spawn A1, Aizenman CD.

    Also, consider that it’s not just shampoo: in the US at least, isothiazolinones are used in almost all household cleaners and laundry detergents.

    If I were planning to become pregnant and was concerned about the effects of prenatal chemical exposure, I would definitely consider avoiding products with isothiazolinones. If autism and other neurological disorders are caused by a combination of genetic susceptibility and exposure to environmental factors,the ubiquity of isothiazolinones and their neurologic effect make them a good candidate for investigation.


    • Hm. After a little hunting around, I managed to find a link to that article (I think), here

      In this research they exposed tadpoles (not humans) to “sub-lethal” concentrations of MIT during a critical period in neural development.

      They found that this resulted in increased seizures and a deficit in “visually mediated avoidance behavior”, which I assume means the tadpoles were more likely to run into the path of predators. There were some other, relatively mild, effects on the brain. They concluded that “chronic exposure to low levels of MIT results in neural circuit-level deficits that result in abnormal neurological function”.

      Sounds damning, but let’s just look at what they actually did: They took 43 tadpoles (not an enormous sample size) and then reared them in glass bowls which contained MIT solutions of different concentrations. Consider that for a moment. The tadpoles lived in the MIT solution.

      Then, when they examined these tadpoles, they found that this extreme situation had resulted in some minor brain abnormalities, although, they freely admit, no fatalities or dramatic changes.

      In no way is this even remotely comparable to the situation for a human foetus. I can find no good evidence that MIT is absorbed through the skin into the bloodstream, and I think this is unlikely (few substances do this: skin evolved to keep stuff out). Even if you drank a bottle of shampoo (I’m not recommending it), we’d have to answer the question of whether MIT passes across the placenta. Again, I can’t find any good evidence that it does. Again, part of the reason for the placenta’s existence is to protect the growing baby by keeping stuff out. Even if MIT is absorbed through the skin and does pass across the placenta, the dose from, say, hair-washing or wearing clothes washed in laundry detergent (traditionally, we do rinse it out afterwards) would be absolutely minuscule compared to what these poor tadpoles received.

      I suggest that, unless you have a history of sensitive skin, you don’t panic. Pregnant or otherwise.

      And just for the record, there’s plenty of evidence that the apparent rise in the number of autism cases is actually due to “a statistical mirage” (in other words, it’s not really an increase at all). It certainly seems unlikely that changing your shampoo is going to make a significant difference.


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