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For the love of moisture, please stop saying that oils can hydrate skin and hair

There are a number of words commonly used in beauty-industry marketing that sound good but don't really mean anything, scientifically speaking: nourishing, clean, revitalizing, etc. But certain terms have definitive meanings in the context of chemistry and biology, and you'd think — considering skin-care and hair-care products are all about ingredients and how they impact said skin and hair — that brands would, oh, I dunno, use those words correctly? But for as long as I can remember (and especially since face oils have risen in popularity) beauty brands have been telling shoppers that their oils are hydrating.

When I see this — especially when "hydrating" is right in the name of a face or hair oil — it's a record-scratch moment for me. I might have failed honours chemistry in high school (got an A in summer school, though — just FYI), but even I'm pretty sure that oils don't hydrate. I definitely didn't fail English, so I also know that you're not supposed to start an essay with "The dictionary defines…," but we're technically in the second paragraph, so I'm going to throw caution to the wind and go for it: The dictionary (Oxford, in this case) defines hydration as "the process of causing something to absorb water."

Oil is devoid of water. In fact, it's what scientists call hydrophobic — literally afraid of water. So, it would seem, it's certainly not adding or attracting it to your skin or hair.

To get a better understanding of if oils can, in any way, hydrate — and what exactly they do do — I spoke to cosmetic chemists and dermatologists to clear up the confusion (and perhaps encourage brands to reconsider their wording).

Can oils hydrate?

No, oils technically cannot hydrate. To put it simply, "There are no oils that add water," says Ron Robinson, a cosmetic chemist.

So what on Earth are brands trying to say when they claim their oils are hydrating? "Oils are part of the moisturizing process," says cosmetic chemist Vince Spinatto. "So while oils can condition the skin and hair, they only retain water content — not add it — which means they are moisturizing but not hydrating."

Cosmetic chemist Krupa Koestline emphasizes that hydration and moisturization are two very different things, and brands that say their oils are hydrating are incorrectly using the two words interchangeably. "Hydration in skin care means applying water or water-binding ingredients to your skin," like hyaluronic acid or glycerin, she tells Allure. "Moisturization, on the other hand, means lubricating the skin with emollients, thereby reducing dryness and transepidermal water loss."

Why some brands insist on erroneously calling their oils hydrating instead of or in addition to moisturizing is unknown, but there's no pressure — other than me yelling about it here — on them to change anything. "Marketing terms can be defined by the brands pretty much however they want," Spinatto says.

OK, so… oils moisturize then?

Oils do, indeed, moisturize, and for the most part, they're all very similar in composition, according to cosmetic chemist Nick Dindio. "The general structure of oil is called a triglyceride, which is basically a glycerin molecule with three different fatty acids attached," Dindio says, adding that oils moisturize by supplementing and strengthening the skin barrier, which in turn will help trap moisture.

Despite their similar structure, however, not all oils moisturize the same way. It depends on their lipid content, Koestline says.

One of the ways oils moisturize is "by serving as an occlusive to help lock moisture in," according to New York City-based board-certified dermatologist Marisa Garshick, M.D. However, that's not the only way they work.

Oils can also work as emollients to help skin feel smoother and seal in hydration, Robinson says. "That is assuming that the skin is already hydrated before the oil is applied."

Dindio likes to use the brick-and-mortar analogy, wherein the bricks are the skin cells and the mortar is composed of lipids, to explain oil's role in moisturizing skin. "The lipids — mortar — can be stripped away under hash conditions which render the skin dry. Oils can help replenish this lipid layer that is stripped away to moisturize the skin," he explains. "A strong skin barrier will prevent water from escaping, therefore keeping the skin hydrated."

But it's also worth noting that, while they moisturize, they do so differently than cream and lotion moisturizers, which typically contain both oils and hydrating ingredients together in a single formula. "[Traditional] moisturizers penetrate through the outer layers of the skin and function to improve skin hydration by drawing in water and by creating a barrier that decreases water loss through the skin surface," says Boca Raton-based board-certified dermatologist Jeffrey Fromowitz, M.D. Although oils also act as a barrier, he says, they stay on the surface of the skin. "I don't believe that they should replace traditional moisturizer use."

So what kind of product should you use?

In order to get both hydration and moisturization, you can't use just an oil. "Consumers should look for products that do both — have humectants that can attract water and have emollients that help seal moisture in," Robinson says, adding that you can accomplish that in two ways: by using separate products for each action or products that do both in one formula.

The latter will never be just an oil alone, but other types of moisturizers can be — especially emulsions. "Emulsions are best of both worlds," says Koestline. "They are designed to give the skin hydration as well as moisturisation. So in order to maintain your skin and hair, make sure to use moisturisers and conditioners that contain both oil and water-based ingredients." She especially loves KP Away Lipid Repair Emollient, which features coconut oil and purified water, because its lightweight texture absorbs quickly and moisturizes for a long time.

We also love Eau Thermale Avène Cleanance Mattifying Emulsion, which provides moisture and hydration with a combination of sunflower oil, spring water, and glycerin while reducing unwanted shine.

But that doesn't mean you shouldn't use an oil.

Although Dr. Fromowitz doesn't believe that face oils should replace traditional moisturizers, they do have a rightful place in a skin-care routine. "Oils can be used a couple of days a week, typically in the evening after cleansing, to keep the surface of the skin hydrated and help keep other products where they belong," he recommends.

Koestline makes the case for using an oil not only because it has a long-lasting effect on skin compared to hydrating ingredients, but because "many oils specialize in a variety of benefits." She gives camellia, rosehip seed, grapeseed, and marula oils as examples of lightweight, nutrient-rich options.

You can find rosehip oil in one of Dr. Garshick's favorite face oils, Omorovicza Miracle Facial Oil, which also features seabuckthorn, sweet almond, and evening primrose oils. Although it may not "instantly hydrate," as the brand claims, Dr. Garshick says it reduces dryness and smoothes the appearance of fine lines without irritation. She's also a fan of SheaMoisture's 100% Pure Argan Oil for both hair and skin because "it helps to control frizz and provide shine while helping to moisturize skin and hair."

Something to keep in mind while you're shopping for your next face, body, or hair oil: Although it's annoying that many brands misuse "hydrating" to describe their oil products, that doesn't mean those products don't do the other things a quality oil is supposed to do. Go ahead and try a so-called "hydrating" oil — just be sure to actually hydrate your skin with humectant ingredients first so the oil can seal in that hydration.

This article was originally published on Allure.

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