How Do Moisturizers Work? Humectants, Emollients and Occlusives Explained

Understanding these differences is important not only when choosing a skincare product, but also when developing one.

What does a moisturizer actually do?

The outermost layer of the skin, the stratum corneum, needs a certain amount of water to remain flexible and function properly.

Our skin is constantly losing water to the surrounding environment. If water loss becomes too high, the skin can become dry, rough and tight.

A moisturizer is designed to help counteract this process. It can do so by attracting water, helping the skin retain water, or improving the condition of the skin surface.

This is why moisturizers are usually discussed in three main categories:

• Humectants
• Emollients
• Occlusives

Although they work differently, they are often used together in the same cosmetic formulation. (Cosmetic Formulation: Principles and Practice, 2019, Heather A. E. Benson, Michael S. Roberts, Vânia Rodrigues Leite-Silva and Kenneth A. Walters, eds.)

Humectants: ingredients that attract water

Humectants are probably the easiest group to understand.

They attract and bind water, helping to increase hydration in the outer layers of the skin. They can draw water from the formulation itself and, depending on the circumstances, from deeper layers of the skin.

One of the best-known examples is glycerin, also called glycerol.

Glycerin has been used in skincare for a very long time and remains one of the most common moisturizing ingredients. Its ability to bind water makes it particularly useful in creams and lotions designed for dry skin. (Cosmetic Formulation: Principles and Practice, 2019, Heather A. E. Benson, Michael S. Roberts, Vânia Rodrigues Leite-Silva and Kenneth A. Walters, eds.)

Other humectants include ingredients such as urea and components related to the skin's natural moisturizing factor (NMF).

However, there is an important detail here: simply adding a humectant does not automatically guarantee the same result in every product. The concentration, the rest of the formulation and the conditions in which the product is used can all influence its performance.

Emollients: making dry skin feel smoother

Emollients work somewhat differently.

Rather than primarily attracting water, they help improve the feel, softness and smoothness of the skin. They can also interact with the surface and lipid structures of the stratum corneum.

This is why a good moisturizer often produces an immediate sensory change. After application, the skin may feel softer, smoother and less rough even before longer-term changes in hydration have occurred.

From a product-development perspective, emollients are also important because they have a major influence on the sensory character of a formulation.

Two moisturizers could contain similar moisturizing ingredients but feel completely different on the skin depending on the oils, emollients and other components used.

That difference is not accidental. Sensory performance is part of formulation design.

Occlusives: helping to keep water in

Occlusive ingredients take another approach.

Instead of attracting water, they help reduce water loss from the surface of the skin by forming a film over it.

Petrolatum is one of the best-known examples. Oils and other lipid-based materials can also provide varying degrees of occlusive effect.

This can be particularly useful when the skin is very dry because reducing water loss gives the outer layers of the skin more opportunity to retain moisture.

Research on moisturizers has shown that applying occlusive substances can reduce transepidermal water loss (TEWL) and increase hydration of the stratum corneum. (Cosmetic Formulation: Principles and Practice, 2019, Heather A. E. Benson, Michael S. Roberts, Vânia Rodrigues Leite-Silva and Kenneth A. Walters, eds.)

Why are the three approaches often combined?

This is where a moisturizer becomes more interesting from a formulation perspective.

A well-designed product does not necessarily have to choose between a humectant, an emollient and an occlusive. They can complement one another.

For example, a formulation might contain:

Glycerin → helps attract and retain water
An emollient → improves softness and skin feel
An occlusive component → helps reduce water loss

Together, these ingredients can address different aspects of moisturization.

The Dermatologic, Cosmeceutic, and Cosmetic Development book describes humectants, occlusives and emollients as the traditional foundation of cosmetic treatments for dry skin. It also discusses newer approaches involving ingredients such as ceramides, other lipid-forming materials and niacinamide. (Dermatologic, Cosmeceutic, and Cosmetic Development: Therapeutic and Novel Approaches, 2008, Kenneth A. Walters and Michael S. Roberts, eds.)