Out of the three classes of moisturizing ingredients (humectants, emollients, and occlusives), humectants are probably my favorite. Well, tied for first. But they’re incredibly cool.
First, a brief refresher: there are three types of ingredients that are considered “moisturizing”:
- Humectants: hold onto water and increase hydration within the epidermis
- Emollients: soften and smooth the surface of the skin
- Occlusives: lock in moisture to reduce transepidermal water loss (TEWL, aka water evaporating out through the skin)
For this post, let’s dive deep into humectants in all their moist glory.
How Do Humectants Work?
Humectants are pretty simple: they’re what are called hygroscopic, meaning they hold onto water. They’re usually compared to sponges, but that’s not quite right. A humectant molecule doesn’t absorb water into itself; rather, water molecules stick to the outside of a humectant. You can think of a humectant molecule like a sphere of velcro, where water molecules are coated on the other side of velcro.
That velcro analogy helps me bust a common myth about humectants. Some people caution against using humectants in dry climates, because it will draw moisture out from your skin and let it evaporate. That would only work if humectants were like magnets, physically drawing water toward itself. But that’s not the case here: humectants just catch passing water molecules and hold onto them. In a dry climate, you’re losing that water anyway through transepidermal water loss. Humectants just help keep that water from evaporating.
Why Are Humectants Important?
The reason humectants are important is obvious: they keep the skin hydrated. Part of that is simple; hydrated skin looks plumper, fuller, and more youthful. But there’s actually an incredibly cool biological reason that your skin needs humectants to stay healthy.
The outermost layer of your skin, the stratum corneum, is also called your moisture barrier. It’s what keeps your skin resilient and waterproof, and it helps keep the outsides out and the insides in. Technically, it’s “dead” – all the skin cells have lost their nuclei and are filled with keratin. Even so, it’s what’s called “biochemically active”, which means there are tons of important things happening on the molecular level.
Many of those important biochemical processes have to do with regulating desquamation, which is the official term for dead skin cells falling off. Your epidermis is organized in layers that a skin cell gradually rises through and naturally falls off over the course of about a month. It’s crucial to your skin health that those dead cells fall off easily, otherwise they can stick to each other and cause scaling, flaking, and clogged pores.
That skin desquamation is maintained by what are called “hydrolytic enzymatic processes”. That means enzymes in your moisture barrier help maintain normal desquamation, and those enzymes rely on water to function. If the water content falls too low, those enzymes can’t function normally, which means your skin cells start stacking up. That’s a big reason why dry skin is characterized by ashiness, flaking, and scaling.
By using humectants and keeping your stratum corneum at optimal skin hydration levels, you can not only help shed dead skin cells, but you can also improve your skin’s moisture barrier function. That helps reduce your skin’s sensitivity and can also help your skin stay clear.
Which Humectants Should I Use?
Not all humectants are created equal, but a lot of them are really great. As a rule, humectants tend to play well with others, so I recommend looking for formulas with multiple humectants to make sure all your bases are covered.
Glycerin is a member of an elite group of humectants known as “natural moisturizing factors” (NMF). These humectants are produced naturally by your skin to hold onto moisture within the epidermis.
At low concentrations, glycerin works as a traditional humectant, holding onto water within the upper layers of the skin. It’s cheap, effective, hypoallergenic, and doesn’t clog pores, so it’s used widely in all types of skincare. At higher concentrations (>~25%), glycerin also can enhance skin barrier function, reduce irritation, and speed up wound healing.
Another natural moisturizing factor, urea is an incredible humectant that doesn’t get nearly enough love. It’s not only great at increasing skin hydration, but it can also strengthen the skin barrier and help desquamation of dead skin cells. It even has some antimicrobial qualities.
At low levels (anywhere from 1-25%) it’s highly effective as a hydrating and softening ingredient. Above 25%, it’s often used as a keratolytic, meaning it breaks down the outer layers of the skin. This is particularly useful for conditions like psoriasis or calluses, which cause excess layers of skin to build up.
Yet another lovely NMF, sodium PCA is the sodium salt of pyrrolidone carboxylic acid, an amino acid. PCA actually makes up about 12% of the skin’s NMF, so it’s a great humectant to supplement with. It’s another non-irritating and bioavailable humectant for managing your epidermal moisture levels.
While hyaluronic acid isn’t a NMF, it is a naturally-occurring compound found throughout your body, particularly in your joints as a lubricant. The go-to fun fact about hyaluronic acid is that it can hold up to 1,000x its weight in water, which is absolutely true. It’s a fantastic ingredient for holding onto a whole lot of water with just a little bit of ingredient. As a molecule, hyaluronic acid is too large to pass through the moisture barrier, but it’s still a great option for topical hydration.
Panthenol is also known as pro-vitamin B5, which means your body converts it to vitamin B5 (pantothenic acid) once you apply it to your skin. In addition to its humectant properties, panthenol has been shown to increase moisture content in the skin by strengthening the skin’s moisture barrier and decreasing transepidermal water loss (TEWL).
Among its many great qualities, aloe vera is indeed a humectant, making it a great choice for hydration. In studies, aloe vera has demonstrated significant anti-inflammatory effects as well as wound-healing abilities, both of which explain aloe’s popularity as an after-sun treatment. It has also been studied to facilitate keratinocyte (skin cell) differentiation. Both functions enhance skin barrier health, making aloe a perfect addition to any skincare routine, especially those with irritated or sensitive skin.
Propylene glycol is an ingredient of many talents. It’s a great humectant, so it’s a nice ingredient for an extra moisture boost. It’s a solvent, so it helps ingredients stay dissolved in a formula. And it’s a gentle penetration enhancer, meaning it helps active ingredients reach deeper layers of the skin, making them more effective.