Most of us have been moisturising our faces for a few decades, but that doesn’t mean we actually know what’s happening when we slap a product across our face. If we did, there wouldn’t be so much confusion around one particular category of moisture-boosting skincare: face oils. Face oils have been around but exploded in popularity recently as the clean beauty movement swept through our collective consciousness. One question that always comes up is whether or not face oils actually moisturise. And if they do, are they just for dry skin.

The short answer is yes, oils are part of the moisturising process. But the long answer is a little more complicated. First, one must understand the difference between an occlusive and a humectant, the two main types of skin moisturisers. An occlusive is a heavy cream or oil that seals moisture just like plastic wrap but for your skin. Because what good is moisture if it evaporates right off your face? A humectant is an ingredient that draws in water from the environment or the lotion itself, like glycerin or hyaluronic acid.

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“In order to be a really effective moisturiser, you need to have both the occlusive to lock in the moisture and the humectant to draw in that water. That’s what really makes a proper moisturiser,” says says Dr. Melissa Kanchanapoomi Levin, dermatologist and founder of Entière Dermatology in NYC. So while oils can condition the skin, they only retain water content not add it, which means they are moisturising but not hydrating.

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So occlusives and humectants are a dynamic duo, and one without the other isn’t going to deliver the kind of glowy results you want. Which is why most moisturiser formulas are made with a combination of both. “A cream has a lot more oil content, and a lotion has less oil content and more water,” notes Dr. Mona Gohara, an associate clinical professor of dermatology at Yale School of Medicine. But there are many people who like to replace their serum and moisturiser with just one bottle of face oil.

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“I do find that a lot of people who only use oils are actually really dehydrated or their skin barrier is compromised underneath all that application of oils,” adds Levin. “I’m not saying oils are bad, but it can’t be the only component when you’re talking about moisturising or restoring the skin barrier.” So in short, it’s also a myth that oils are only for dry skin, since alone they can actually contribute to dehydration. We all all need to find that perfect balance between occlusives and humectants. If you like to pat an oil on your skin, do so after a lightweight lotion. If you apply a humectant-heavy serum or lotion on top of an oil or oil-reach cream, all you’ll accomplish is wasting product.

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If you’re on the hunt for an oil to add into your routine, Levin recommends squalene, marula, or jojoba oil, since they can help with redness and won’t clog pores, given the molecule size. These natural oils will not only condition skin, but they’re also packed with essential vitamins. Gohara mentions mineral oil-based products like Vaseline are also a great choice for most skin types. Oils to avoid in skincare are argan and coconut oil, since the molecule size is larger and therefore may contribute to acne (but feel free to keep slathering them on your hair).

This story first appeared on Harper’s BAZAAR US.