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.