Humectants, Emollients & Occlusives: What's The Difference?

So let's dive into the specifics, shall we? 

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. 

An emollient is something that softens the skin and makes it feel more comfortable by filling in cracks. This is why emollients are often used for irritated, inflamed, or reactive skin2. Butters, certain oils, lipids, and fatty acids are all emollients. Emollients are also usually formulated with a humectant and occlusive, in order to perform its job better (as well as other ingredients entirely, like antioxidants; Barr notes that alone, they likely don't do enough for the skin). These products can range from light lotions, which contain more humectants, to thick creams, which likely contain more occlusives. 

The thickest end of the spectrum, these seal moisture into skin. In the natural world, these are your waxes like candelilla, carnauba, and palm kernel, beeswax, or lanolin ("traditional" examples include silicones and petroleum). They are also many types of oils, like jojoba or olive oils. Given they act as a water retention barrier, they also act as a barrier to outside forces: This is why they should be the last step of your routine. After you slick on an occlusive, anything layered on top isn't getting past it.