Dry skin, or xerosis, is one of the most common skin care concerns and one of the most misunderstood. Before you can substantiate a hydration claim, it helps to understand what actually causes dry skin and what your formulation needs to do about it.
What Causes Dry Skin?
Dry skin can result from a wide range of factors, many of which have nothing to do with the product on the shelf:
- Low ambient humidity (below 40% relative humidity strips moisture from the skin, while levels above 40% allow the skin to absorb it from the air)
- Frequent or long showers and baths, which exhaust the skin’s natural moisturizing factors (NMFs) such as PCA, amino acids, and urea
- Harsh detergents, which denature surface proteins and strip NMFs
- Underlying skin conditions, including atopic dermatitis, eczema, and Sjögren’s syndrome
- Systemic diseases such as diabetes, hypothyroidism, and lupus
- Certain medications, which can affect dryness directly (by reducing sebum gland function) or indirectly (as with steroids)
When Dry Skin Becomes a Bigger Problem
Mild, transient xerosis typically shows up as visible flaking, tightness, roughness, fine lines, and a dull appearance. Left unaddressed, it can progress to moderate or severe xerosis, with cracking and fissuring (especially at the fingertips and heels), chronic inflammation, and skin thickening from persistent itching. In more severe cases, secondary infection can occur.
This progression is exactly why effective, well-substantiated moisturizing products matter and why understanding the formulation science behind them is the first step toward building a defensible claims strategy.
Humectants vs. Occlusives: The Two Core Strategies
Most moisturizing formulations rely on one or both of the following approaches:
Humectants
Humectants hydrate by drawing water into the stratum corneum, increasing its capacity to hold moisture. A small amount of water in the stratum corneum is also necessary for the enzymes responsible for exfoliation to function properly. Common examples include glycerol and hyaluronates.
Occlusives
Occlusives work differently. Rather than adding water, they reduce the rate of transepidermal water loss, allowing the skin to hydrate itself. Common examples include petrolatum, waxes, and oils.
Many effective formulations combine both strategies, along with emollients, which influence how moisturizing a product feels even when they aren’t directly responsible for clinical hydration.
Why This Matters Before You Test
The mechanism your formulation relies on, humectancy, occlusion, or both, directly affects which test methods will generate meaningful, substantiated results. Understanding the “why” behind dry skin and moisturization sets the foundation for choosing the right testing approach.