And the first thing you should know is that the below are categories of ingredients: each specifying a different set of qualities, board-certified dermatologist Keira Barr, M.D. tells me (who also medically reviewed this article). This is why you don’t see a serum called a “Humectant Serum”; rather, it’ll be identified as the specific ingredient itself, like “Hyaluronic Acid Serum.” The other thing to note is these products are usually formulated with some combination of the three—unless you’re putting a raw ingredient on your skin. If the humidity is high enough, they can even pull water out of the air, but in arid environments, they might actually be pulling in water from the deeper layers of the dermis into the stratum corneum. This can end up being drying because once water is on the top layer of skin, it can more easily evaporate. For this reason, humectants are usually formulated with an occlusive1 (more on that below). If a product is not—take, for example, a straight hyaluronic acid serum—it likely shouldn’t be used alone and instead should be layered under a cream or oil to help seal in the water.
