The term “Moisturizer” is a generic term used to describe a multitude of skin care product formulations that vary in their ability to hydrate the skin, improve skin health, and improve the aesthetic appearance of the skin. Moisturizers sold in the marketplace today can be classified into four main types based on their ingredients:
- Occlusive based – These oily substances block water evaporation by providing a film on the skin’s surface and thereby trapping water in the upper most layer of skin, the stratum corneum. The most common occlusive ingredients include petrolatum, mineral oil, vegetable oil, and cetyl alcohol. Although very effective in sealing the skin’s surface, these ingredients are often sticky and greasy, and therefore, less cosmetically appealing. Oil-free occlusives, like silicone and its derivatives: dimethicone, cyclomethicone, and amodimethicone, are also commonly found in these moisturizers. The increase in skin hydration from the use of these occlusive agents hydrates dry and damaged skin, and results in increased skin smoothness and an improvement in barrier repair.
- Humectant based – Moisturizers containing these ingredients work by attracting water from below the epidermis and from the atmosphere, and drawing it into the stratum corneum. Common humectants include glycerin, urea, pyrrolidone carboxylic acid (PCA), lactic acid, propylene glycol, sorbitol and vitamins. Most often intended for daily use on normal skin, these moisturizers are usually water based and very aesthetically pleasing.
- Emollients – Moisturizers that contain emollients are designed to make the skin feel soft and appear smooth. These products often provide fragrance rather than increase hydration. Emollient based moisturizers are suitable for daily use on normal skin. Emollients are often lipids and oils that soften the skin and impart a smooth and silky feel. Although emollients are less effective at sealing the skin’s surface from water loss than occlusives, they do have some occlusive ability, and therefore, can improve the appearance of dry flaky skin. Common emollients found in moisturizers include lanolin, cetearyl alcohol and sunflower seed oil.
- Therapeutic Moisturizers – These formulations are designed to treat dry, damaged, and diseased skin conditions. They contain occlusives for water barrier effects, humectants to draw water into the stratum corneum, and emollients to soften the skin. These moisturizers often contain compounds found in the Natural Moisturizing Factor (NMF) like urea, lactic acid, and/or pyrrolidone carboxylic acid (PCA). In addition, they often contain lipids that mimic those found in the stratum corneum, such as ceramides.
Emollient based and Humectant based moisturizers may temporarily improve the appearance of dry skin, but they do very little to repair the skin’s barrier function. Occlusive based and Therapeutic moisturizers are more effective in decreasing water loss through the skin by improving the skin’s ability to hydrate.
Adequate hydration to the skin is essential for skin to maintain its flexibility and plasticity. As discussed above, cells constantly migrate from the bottom layer of the epidermis to the most superficial layer, and finally shed. The enzymes responsible for this shedding (desquamation of corneocytes) are dependent on adequate hydration. When this process is disrupted the skin enters a “dry skin cycle” giving it a rough scaly appearance. Imbalance of the water barrier is characteristic of: conditions like eczema, damage due to ultraviolent radiation, and the aging process.
The skin is the largest organ of the body and has several functions: Receptors in the skin provide sensory information like touch, vibration, and pain. The skin helps maintain a constant body temperature, and is a physical barrier to the elements of the environment, providing protection from bacteria, ultraviolet light, and chemicals. But it is the skin’s ability to create a barrier to water loss that is its most crucial function.
The skin is comprised of the following layers:
- Epidermis – outermost layer which consists of the following:
- Stratum corneum: The most mature non-living skin cells (corneocytes) are comprised primarily of a structural protein called keratin. These cells contain a collection of water-soluble compounds call Natural Moisturizing Factor (NMF.) Cells in this layer are surrounded by a fatty lipid layer making the skin virtually water repellent. These cells continually shed and are replaced by cells in the layers below.
- Stratum granulosum: Comprised of less metabolically active cells than the ones below them, these cells produce the lipids that are released into the stratum corneum.
- Stratum spinosum: Multiple layers of living polygonal skin cells (keratinocytes) produce keratin and ultimately mature to form the stratum corneum.
- Stratum basale (Basal layer): The deepest layer of the epidermis comprised of a single row of cells that divide to form new keratinocytes to replace the ones that are continuously shedding from the skin’s surface.
- Dermis – comprised primarily of collagen to give strength and flexibility. Also contains receptors for pain and touch.
- Fat layer (Sub cutis) – helps preserve heat and acts as a ‘shock absorber’ for protection from injury.
In simplest terms, a moisturizer is a mixture of chemical ingredients that is designed to hydrate the outer layers of the skin, improve the appearance of the skin, and/or improve the health of the skin. An increase in the water content of skin results in skin that feels softer and more pliable. It is this increased hydration that reduces the appearance of very fine lines!
The outermost layer of skin, the stratum corneum, is responsible for maintaining the appropriate water content of the skin. This paper-thin superficial layer of skin, comprised of approximately 20 cell layers, is an effective water barrier due to three primary characteristics:
- Individual skin cells in the stratum corneum (corneocytes) are surrounded by waterproofing lipids (produced in the stratum granulosum layer) that prevent the evaporation of water from the skin. The most common lipids found in the stratum corneum are ceramides, free fatty acids, and cholesterol.
- Collections of water-soluble compounds within corneocytes, collectively called the Natural Moisturizing Factor (NMF), absorb water from the environment and from the lower layers of skin to maintain hydration. NMF is comprised of amino acids such as pyrrolidone carboxylic acid (PCA), urocanic acid, lactic acid and urea.
- Protein bridges (called desmosomes) hold the corneocytes together, making it difficult for water to evaporate out of the skin.
When these mechanisms do not function properly and adequate water content is not maintained ‘dry skin’ ensues. Individual cells cannot shed appropriately and dry flaky skin results.