Several cosmetic ingredients help prevent moisture loss, but petrolatum is the single most effective one, reducing water loss through the skin by 98%. Other oil-based ingredients manage only 20% to 30%. That said, the best approach to keeping skin hydrated involves three different types of ingredients working together: humectants that pull water in, emollients that smooth the skin’s surface, and occlusives that seal everything in place.
Understanding how each category works helps you pick products that actually deliver on their hydration claims, rather than just feeling nice on the skin for a few minutes.
How Your Skin Loses Moisture
Your skin’s outermost layer, the stratum corneum, is only about 15 micrometers thick. Despite being incredibly thin, it’s your primary defense against water escaping from your body to the surrounding air. This layer is built like a brick wall: flat, tightly packed skin cells sit embedded in a matrix of natural fats. Water has to wind its way around each cell to escape, which slows the process considerably.
When this barrier is damaged, whether from harsh cleansers, dry air, aging, or skin conditions like eczema, water escapes faster. This is called transepidermal water loss, or TEWL. Cosmetic ingredients that prevent moisture loss work by either reinforcing this barrier, pulling more water into it, or physically blocking evaporation at the surface.
Humectants: Ingredients That Attract Water
Humectants are water magnets. They draw moisture from the air and from deeper layers of your skin up into the outermost layer, keeping it plump and hydrated. The two most common humectants in skincare are glycerin and hyaluronic acid, and they work quite differently despite having the same goal.
Glycerin is a small molecule that can actually penetrate through the skin barrier and hydrate down to the dermis, the deeper living layer of skin. It pulls water both from the surrounding air and from within the skin itself. Hyaluronic acid, by contrast, is a much larger molecule. High molecular weight hyaluronic acid (1,000 to 1,400 kDa) cannot pass through the stratum corneum at all. It sits on the surface, creating a hydrating cushion that holds water against the skin. Low molecular weight versions (20 to 300 kDa) can penetrate the stratum corneum, offering deeper hydration similar to glycerin.
Urea is another humectant worth knowing about. It occurs naturally in skin at low concentrations and plays a surprisingly active role in barrier function. At 20% concentration, topical urea reduced transepidermal water loss by 31% in a study of 21 healthy volunteers. At 10%, it triggered beneficial changes in gene expression related to skin barrier repair but didn’t significantly reduce water loss on its own. Lower concentrations (2% to 4%) still boost the skin’s natural urea levels several-fold and contribute to overall hydration.
One important caveat: humectants don’t prevent moisture loss by themselves. In very dry environments, they can actually pull water out of the deeper layers of your skin and let it evaporate. They work best when paired with an occlusive layer on top to trap the moisture they’ve attracted.
Occlusives: Ingredients That Seal Water In
Occlusives form a physical barrier on the skin’s surface that blocks water from evaporating. They don’t add hydration. They simply prevent what’s already there from leaving. This makes them the most directly relevant category for preventing moisture loss.
Petrolatum (the main ingredient in petroleum jelly) is the gold standard. Its 98% reduction in transepidermal water loss is unmatched by any other single ingredient. It works by creating a dense, semi-permeable film over the skin that mimics and reinforces the natural lipid barrier.
Dimethicone, a type of silicone, offers a lighter alternative. It sits on top of the skin and forms a temporary protective barrier that reduces water loss and provides some protection from environmental irritants. Unlike heavier oils and butters, dimethicone is less likely to clog pores, making it a practical option for people with oily or acne-prone skin who still need moisture protection.
Emollients: Filling Gaps in the Barrier
Emollients work differently from both humectants and occlusives. They fill in the tiny gaps between skin cells in the outermost layer, smoothing rough or flaky skin and helping restore the structural integrity of the barrier. When those gaps are filled, water has a harder time escaping.
Plant oils are common emollients. Jojoba oil is particularly effective because it’s rich in wax esters, compounds that closely resemble the natural fats in human skin. Research has confirmed its value for barrier repair, especially in conditions where the skin barrier is compromised, such as eczema and dermatitis. Squalane, derived from squalene (a compound found naturally in skin and in plant oils like argan), is another lightweight emollient that reinforces the lipid barrier without feeling heavy.
Ceramides and the Skin’s Natural Fat Ratio
Your skin barrier relies on three key fats: ceramides, cholesterol, and free fatty acids. These aren’t just moisturizing ingredients added to products for marketing purposes. They’re the actual structural components of the lipid matrix that holds your skin barrier together.
Research has shown that applying all three in an equal ratio supports normal barrier recovery. But a specific formulation performs even better: a 3:1:1:1 ratio with cholesterol as the dominant lipid significantly accelerated barrier repair in both young and aged skin. In chronologically aged human skin, this cholesterol-dominant mixture sped up barrier recovery at the six-hour mark compared to other combinations. Interestingly, a mixture where fatty acids dominated actually delayed barrier recovery in aged skin, suggesting that the ratio matters as much as the ingredients themselves.
If you’re over 40 or dealing with chronically dry skin, look for moisturizers that list ceramides, cholesterol, and fatty acids together. The combination outperforms any of these ingredients used alone.
How to Layer Ingredients for Best Results
The most effective moisture-retention strategy uses all three ingredient categories in sequence. Start with a humectant (glycerin, hyaluronic acid, or a serum containing them) to pull water into the skin. Follow with an emollient to smooth the surface and fill gaps. Finish with an occlusive to lock everything in.
Timing matters too. Applying moisturizer to damp skin, within a few minutes of washing your face or showering, gives humectants more water to work with. When the skin is already hydrated, the moisturizer seals that water in rather than sitting on top of dry skin. More occlusive products in particular may actually seal moisture out if applied to completely dry skin.
Many well-formulated moisturizers already combine ingredients from all three categories in a single product. A cream containing glycerin, ceramides, and dimethicone, for example, covers all three functions. You don’t necessarily need multiple products unless your skin is very dry or your barrier is significantly compromised, in which case layering a hyaluronic acid serum under a ceramide-rich cream topped with a thin layer of petroleum jelly can make a noticeable difference within days.