What is a humectant? "Humectancy or hygroscopy is the tendency of a substance to attract water from the surroundings by absorption and adsorption at defined conditions (temperature, humidity)." (page 26, this review).
Is glycerin a humectant? Yes. "Pure glycerol absorbs its own weight in water over 3 days." (page 23, this review)
Does glycerin remove water from your skin in low humidity environments? Let's review what I've found so far...
From SW Gloves News: "Glycerin is also a humectant, which means it attracts moisture to your skin. However, if the air is less than 65% humidity, glycerin will instead draw moisture out of the lower layers of your skin. The moisture is held on the surface of your skin, effectively drying it from the inside out. This results in the upper layers of skin feeling soft and conditioned, while in reality the newly formed inner layers are drying out." (They are referencing this site Natural Health Information Centre, which adds, "It doesn't take a rocket scientist to work out that if you keep drying out the skin from the inside out, it will get progressively worse!") There is no citation from the Natural Health Information Centre about where they found this information.
From Dermatology Times, we get this quote,"While glycerin is a highly effective moisturizing ingredient in low-humidity climates (such as those found in Arizona, Nevada and New Mexico), it can leave skin with a sticky feel in humid environments (such as those found in Florida, Louisiana and Mississippi.) This is because glycerin can also attract water from the air." I'm inclined to go with the words of Zoe Diana Draelos, M.D., over SW Gloves News and Natural Health Information Centre, but let's not stop here!
From The Biology of Women, from a chapter entitled, Cosmetics: The Multibillion Dollar Put-On, we see this quote, "Advertisements imply that such agents have the ability to draw water out out of the air and bring it to the skin surface. It is very unlikely that humectants actually transfer water out of the atmosphere to the skin. They are, in fact, nondirectional. If humectants were used alone or in high enough proportions they would be just as likely to pull water out of the skin. The only way that a high concentration of glycerine (or glycerol, which is the same substance) can be useful for dry skin is under conditions of very high atmospheric humidity of 90% or more. Of course, if the humidity is very high, dry skin would not be a problem."
I had wanted to present these quotes without comment, but I don't think I can in this case as most of the preceding paragraph is simply inaccurate. I want to highlight the sentence "If humectants were used alone...they would be just as likely to pull water out of the skin" because this is the crux of the entire argument about glycerin drawing water out of your skin when it isn't humid. Remember that part - if humectants were used alone - and ignore most of the rest of the paragraph. I just think things need to be put into context, and to take that one phrase out of this wildly inaccurate paragraph would be cherry picking information to make my case, and I hate that!
"They [humectants] are able to attract water from the atmosphere (if the atmospheric humidity is greater than 80%) and from the underlying dermis. Although humectants may draw water from the environment to help hydrate the skin, in low humidity conditions, they may take water from the deeper epidermis and dermis, resulting in increased skin dryness. For this reason, they work better with occlusives." (Cosmetic Dermatology: Principles and Practice by Leslie Baumann, page 94.)
"Creating an occlusive barrier is more effective in combination with a humectant to draw water to the stratum corneum. Using a humectant alone will draw water to the outer surface of the skin, which will be lost to the environment with an impaired [skin] barrier. However, moisturizers without humectants may also make the skin more susceptible to irritation." (Lippincott's Primary Care Dermatology, page 31).
this study, "In every experiment, the temperature was held constant at 25˚C, and the starting relative humidity (RH) was set to 0% RH. The RH was programmed to step in 10% increments ending at 90% RH. The RH was incremented to the next level only when the mass change was less than 0.005%/min. NaPCA [sodium PCA] was a more effective humectant at high relative humidities (above 60% RH); while glycerin performed better at humidities below 40% RH. Washing the skin with sodium lauryl sulfate (SLS) reduced the skin’s ability to absorb water more so than washing with Tween 80, a milder surfactant. Vaseline petroleum jelly enhanced the water-retention properties of untreated skin." From page 39, "Glycerin outperforms NaPCA at humidities below 40%." Page 40, "Clearly, Table 3 shows that the skin samples treated with Vaseline experienced a lower rate of water loss." Page 40, "Of the moisturizers that were examined in this study, glycerin was one of the most efficient at absorbing water at low relative humidities." Page 41, "Because occlusion prevents water evaporation, occluded skin has a higher water content compared with unoccluded skin. Occlusion can be achieved mechanically or chemically, by applying a hydrophobic material or film."
"...glycerol does not act as humectant at such a low humidity..." (This review, page 27, near the bottom)
In this study, the abstract notes that "In vitro studies have shown glycerin to prevent crystallization of stratum corneum model lipid mixture at low room humidity."
What can we conclude from these studies and writings? Humectants draw water out of the atmosphere to skin, but when it is less humid, humectants may draw water from under the skin to outside the skin (remember the idea of osmosis!). Not just glycerin, but any humectant, including honeyquat, sodium lactate, sodium PCA, tamarind seed extract, and so on: Any humectant has this potential. If we include an occlusive ingredient with the humectant, this should stop the water from evaporating and will help moisturize our skin.
A couple of questions...
- Why is the idea of drawing water out of your skin only about glycerin and never about the other humectants?
- How much glycerin do I need to use for this effect to make my skin really dry?
- Why do the authors of scientific studies of glycerin - and other humectants - never mention the humidity in their part of the world? Wouldn't this have a huge impact on the results of the study?
- At what humidity level does it draw water from your skin? If it is a well known fact that glycerin draws water from your skin, wouldn't the humidity level at which this starts to happen also be a well known fact? There should be some kind of chart or information showing that at this humidity and below, glycerin is a dessicant and above this humidity and higher, glycerin is a humectant. I can't find anything like that!
There is evidence demonstrating that although glycerin might not be a good humectant at low humidity levels, it offers other benefits - for instance, helping to keep our stratum corneum lipids from crystallizing - more about this in a near future post - which makes it very valuable, indeed! And there's some evidence a'brewing that use of a humectant can increase mildness in our products. (I will be writing about this in the very near future!)
Would I say that glycerin draws water from your skin when the humidity is low based on what I've read? No. I think there is that potential from a chemistry perspective, but I think there's strong evidence that using an occlusive ingredient with glycerin - or any humectant - will prevent this from happening in real life. I think there's way too much evidence showing that glycerin performs amazingly well as a moisturizer that can offer a ton of benefits beyond drawing water to my skin to not use it. I do think that most of the information about glycerin drawing water from our skin comes from popular media or websites, and I don't think this claim is substantiated.
Would I use glycerin in my products at 20% humidity? At 10%? At 5%? Yes, and I have. I would not hesitate to suggest using glycerin in products for low humidity environments. I think there are other humectants that might perform better in those environments, but it sounds like none of the humectants we can access - see list above! - will work well in a really dry environment. You'll have to get your moisturization from somewhere else!
Join me tomorrow for more fun with humectants!