If you put a raisin in a glass of water and leave it for a few hours it will swell up. The concentrated contents of the raisin, full of sugar and other small water soluble molecules pulls water in across the skin of the raisin. This process is called osmosis and for some reason it seems to be one of the things that people remember very clearly from their science lessons. I think it is because it is easy to understand and can be illustrated with very nice examples and diagrams. You probably remember looking at diagrams showing the concentrations on either side of a semi-permeable membrane with dots indicating the concentration difference.
A typical diagram illustrating osmosis looks something like this.
I think they should make a point of teaching is that the skin is also a semi-permeable membrane. Just like the raisin it has the ability to pull moisture into itself. Think of the wrinkly fingers you get after a really long soak in the bath. It’s the same process as in the raisin. The skin also uses small water soluble molecules, the best known and most significant is glycerin.
Humectants and the NMF
But it isn’t alone. There is a whole set of molecules that the skin uses to maintain its moisture balance. These are sometimes referred to collectively as the skin’s natural moisturising factor, or NMF for short. This term comes and goes in fashionability. It isn’t really a particularly useful description for a scientist – it doesn’t really give you much of a clue as to what is going on. But it seems to appeal to marketers, who frankly just like the sound of it. After all, if you have dry skin wouldn’t you like to have a natural moisturising factor to add?
Scientists prefer to call anything which helps hold on to water as a humectant. This is an ugly term and rarely makes it out of the lab, but nearly every skin product has some kind of humectant in it. Glycerin is the most popular choice.
Another individual component of the skin’s natural moisturising factor that gets talked about a lot is hyaluronic acid. This is one of the body’s many building blocks and gets used for all sorts of things throughout the body. In the skin as far as I have been able to tell, it acts as a humectant and nothing else.
Hyaluronic acid comes with a high price tag and sounds very impressive. Whereas glycerin can be relatively easily extracted from just about any kind of oil, hyaluronic acid needs elaborate production techniques. Hyaluronic acid does work pretty well, but given that it is pricey there is a tendency to use it at a low level. When it comes to humectancy, concentration is very important. It simply doesn’t work very well at a low level. Glycerin on the other hand is a low cost material that you can afford to put in at a meaningful level. My feeling is that you can achieve as much as you are ever likely to just using glyceinl. I have yet to see a compelling demonstration that hyaluronic acid has any real benefits over glycerin for the end user.
How much is enough? I have had very good results indeed at 4% and that is the level I generally use. I don’t think using more does any good to the end user in most cases. However there are a small number of people out there who seem to really thrive on products with a high glycerin content. If you find that you have dry skin but most standard skin creams don’t work for you, then you might fall into this group. A good cream to try to find out is Neutrogena Norwegian fomula, which has an extremely high glycerol content and a cult following of people who have worked out that is works for them. You should be able to feel the difference in less than half an hour if it is going to work. My friend Pedro from Brazil recommends Yu Be Moisturising Skin Cream which has glycerin listed as the first item on the ingredient list. I haven’t tried it myself but it must have a pretty high glycerin content.
Humectants – the Benefits
But for most people the benefits of humectants are real, but pretty modest. I have found that when I have tried to evaluate humectants on their own I can hardly notice the effect at all. They can add a bit to a formulations moisturising power, but most of the work is done by other components.
There are a number of alternative humectants to glycerin. The most common is propylene glycol. This is a derivative of the petrochemical industry and is very cheap – though as you might expect it does vary with the price of oil. It is one of those chemicals that has somehow acquired a bad reputation without having done anything particularly wrong. It can be irritating at very high levels, but so can a lot of things – more on that later. Some websites and blogs have pointed out that it is used in antifreeze. Well, diamonds are used in mining. That doesn’t make a diamond ring any less attractive.
But although there is nothing particularly wrong with propylene glycol, there isn’t much to recommend it either. It doesn’t work as well as a humectant as glycerol. In a skin cream designed to moisturise it is basically a cheap substitute. In some other products it has a technical function that justifies its inclusion. It is for instance one option when you want to solubilise something. Personally I would avoid any moisturiser that contains propylene glycol, and I would be suspicious of its inclusion in most other types of product. But I am willing to concede that there might be cases where it is the best available option. Deodorant formulations for instance might well need to use it, and some other specialised preparations.
One reason some people give for avoiding propylene glycol is that it is a product of the petrochemical industry. This is a perfectly respectable choice to make. It is an industry that I would love to be able to live without myself. It is polluting and unsustainable. Propylene glycol production and use represents only a fragment of fragment of the overall issue, but we’ll have to make do without it when the oil runs out.
But what if there were a sustainable non-polluting source of propylene glycol? As it happens there is something quite close to it. There is a commercially available material called propandiol which is very similar indeed to propylene glycol. This is made from vegetable sources – corn oil at the moment but in principle any vegetable oil – so is carbon neutral and non-polluting. It is also quite a low energy process very unlike petrochemical derived propylene glycol. The drawback is that it is made by a technology that a lot of greenies don’t like – it is fermented by genetically modified e-coli bacteria. This makes it impossible to get it certified to any organic standard so a lot of the natural brands steer away from it.
(It is incidentally, technically perfectly feasible to make propylene glycol by biotechnology and propandiol by conventional cracking in a petrochemical plant. As far as I know, at time of writing nobody is making either of these, so if you see propylene glycol on an ingredient list you are looking at a petrochemical derived material and any propandiol is made from natural feedstocks using biotechnology and genetic engineering.)
It is a shame that so many in the Green movement are so opposed to genetic engineering. This is a good example of how it can help reduce the impact humans make on the planet.
As for safety, there really is no particular issue with any of the humectants. But bizarrely, glycerin is the only cosmetic raw material I personally have ever had a skin reaction to. I got a 40% solution of glycerol on my arm once. A few hours later I had a bright red patch that was very itchy. I imagine the osmotic pressure of the glycerin disrupted my skin barrier temporarily. The concentration is the key thing here. Going back to our raisin. If you add some glycerin to the water the raisin is in, you can alter the way it behaves. Put in just the right amount of glycerol and the raisin will not swell up because the osmotic pressure of the water balances the osmotic pressure in the raisin. Put still more glycerin in and the already shrunken raisin can be shrunk still more – and this is I think what happened to my arm. I have never had the slightest issue with glycerin at lower concentrations.
What this shows is that any humectant can also be an irritant if you use too much of it. I have seen websites and blogs that warn people off glycerin and propylene glycol because they have read material safety data sheets that describe them as primary irritants. I sigh, and think to myself if only things were that simple my job would be a lot easier.
Humectants – Notes for chemists
If you are a chemist who doesn’t work on cosmetics you may be confused by the nomenclature of the glycols used in cosmetics. It has certainly baffled me often enough. Ingredient lists on cosmetic products are required to use standard names called INCI names. I have chosen to refer to glycerin throughout this post because that is the INCI name for glycerol. I think everyone can follow that. The INCI name for 1,2 Propandiol is propylene glycol – the petrochemical derived one. 1,3 Propandiol has the INCI name propandiol. This is the bioengineered stuff I referred to and is made by DuPont under the trade name Zemea. The two chemicals are isomers. The isomers are known to at least some chemists, myself included, as alpha propylene glycol and beta propylene glycol. You may have noticed a product labelled ‘propylene glycol free’ but which has propandiol on the ingredient listing. While this is misleading, and deliberately misleading, it isn’t technically incorrect.