As water continues to be lost from the skin, the skin deteriorates further with a loss of corneocyte cohesion causing increased cell detachment and scaling or flakiness. To the touch, the skin may feel rough with reduced elasticity. The latter is due to the fact that water helps maintain skin pliability and elasticity. Think of a dry kitchen sponge that is hard to the touch, with little elasticity. Once you wet that sponge it becomes extremely pliable enabling you to twist it into almost any shape you like.

What is Dry Skin?
When we look at a section of the epidermis and measure the water content through the different layers, we find that deeper layers (such as the stratum basale) have a higher concentration of water than the outermost layers. By the time we reach the outer stratum corneum layer, there is only about 10 percent of water remaining. When the water level drops below 10 percent, we end up with a dry skin condition. What causes the loss of water from our tissues? Intrinsic factors such as physiological disorders, illness, stress, etc. as well as extrinsic or environmental culprits are generally to blame. The latter category includes exposure to hot water, soap, chemicals (i.e. acetone), air conditioning, heating, frequent bathing, airplane travel, even friction from clothing, all of which can contribute to an increase in transepidermal water loss (TEWL) by compromising the skin's natural lipid barrier allowing excessive water loss. When the skin loses too much water, the outer layers may stiffen and develop fine cracks or fissures that become irritated or itchy; as you might expect this is exacerbated in areas of the body such as the arms, legs, and torso where there are fewer sebaceous glands present. Compounding the situation is the tendency for skin to dry more as we age; this is most likely due to a decrease in sebaceous gland activity and a slower recovery rate for lipids that comprise the barrier layer of the epidermis. Likewise, seasonal factors also exacerbate the situation; this is especially evident in fall and winter when humidity levels drop. Not only is the humidity level lower in winter, but exposure to indoor heat can aggravate the situation.
When the water content of the stratum corneum falls below 10 percent, there is a tendency for abnormal desquamation of corneocytes to occur.¹ Studies have shown that desmosomes, the snap like structures that hold the cells to one another, remain intact in the deeper epidermal layers but lose their connection in the outer, drier layers. This is attributed to impaired water levels that are required for proper enzyme activity and normal desquamation. Loss of water causes abnormal desquamation leading to cell clumping (i.e. flakes)of keratinocytes, which causes the skin to appear rough and dry. Patients with dry skin also displayed an abnormal complement of lipids making up the epidermal barrier lipid layer; there was a decreased level of ceramides, which is also seen in atopic dermatitis, a disorder characterized by dry skin.²
Ceramides comprise 40 to 50 percent of the stratum corneum lipids. Perhaps the most notable is Ceramide 1, which contains linoleic acid (a fatty acid); it is suggested that Ceramide 1 allows for the organized stacking of the lipid bilayers found in the stratum corneum. The function of this bilayer is to prevent water from evaporating out of the deeper tissues; it also helps control what molecules will penetrate the skin.³ One could speculate that a deficiency in ceramides would upset the organization of lipids controlling water loss in the tissues.

The Lipid Bilayer Functions as a Water Barrier
There is no question that the lipid bilayer in the stratum corneum is in part responsible for maintaining a barrier for water. The lipids that make up this lipid bilayer include about 40 to 50 percent ceramides, 25 percent fatty acids, and 20 percent cholesterol. Any alteration in the complement of lipids can disrupt the barrier function. Scientists now believe that no one lipid is more important than the others; ceramides, cholesterol, and fatty acids all play an equally important role in maintaining the barrier properties. Studies have demonstated that when the lipid bilayer is removed with acetone, it is only restored when a mixture of all three lipids are applied to the skin. Adding only one or two of the lipids is not adequate.⁴ Interesting, studies have shown applying lipid precursors to the skin increases ceramide levels indicating the precursors are incorporated into the skin's biosynthetic pathways.⁵ This finding could lead to the development of novel new moisturizers that enhance the ceramide concentration and optimize barrier function, assuming cholesterol and fatty acids are plentiful in the skin.
While lipids derived from sebaceous glands may form a film over the skin surface that can help prevent loss of water, low sebum lipid levels are not believed to be responsible for dry skin.⁶ Actually, the relationship of sebum lipids to dry skin is poorly understood.

Treating Dry Skin
Ideally to treat the symptoms of dry skin, one needs to increase the level of hydration of the stratum corneum. This is achieved with humectants that increase water content by altering the water flux from the dermis to the epidermis or bringing water in from the environment, emollients that smooth rough skin and treat itching/stinging/burning, and occlusives that prevent further TEWL. A good quality moisturizer should contain ingredients from each of these categories.
Humectants are molecules that act as water magnets helping to hydrate the layers of the skin. The list of humectants used in skin care products is quite extensive; some of the more popular humectants include glycerin, PCA, honey, amino acids, lactic acid, propylene glycol, and panthenol. Without a doubt, the most effective humectant is hyaluronic acid, also known as sodium hyaluronate or hyaluronan. A unique glycosaminoglycan (GAG), hyaluronic acid is found naturally in the cellular matrix of the dermis. Its efficacy is attributed to the fact that hyaluronic acid can hold a thousand time its weight in water, which helps keep the skin tissues well hydrated. Basically, the water-logged GAGs surround the dermal proteins (elastin and collagen fibers), helping to maintain their pliability and elasticity. The water content of the dermis is directly correlated to the amount of hyaluronic acid in the tissue.
Unfortunately, as we age the amount of hyaluronic acid in our dermis declines. The most significant decline occurs around our fifth decade which also coincides with a deterioration of the mechanical properties (i.e. elasticity) of our skin. If we want to increase the moisture content of the skin, then we must increase the concentration of hyaluronic acid, as well as the length of time that it is present. This means that we must slow the activity of hyaluronidase, an enzyme responsible for degradation of hyaluronic acid. For this reason it is important to include anti-enzyme agents like grapeseed extract that inhibit hyaluronidase activity.
Humectants are combined with emollients that help smooth and soften skin, alleviating dryness and scaliness. These are generally fatty molecules that help lubricate the skin, making it feel soft and pliable. Occlusives may also have a lipid like texture but not always. They form a barrier on the skin's surface helping to reduce evaporation of water from the skin. Molecules may be both emollients and occlusives; mineral oil, lanolin, and petrolatum are all examples of occlusive emollient ingredients that are often found in less expensive, mass market products. The more elegant (and expensive) formulas utilize silicones, shea butter, sweet almond oil, oil of evening primrose, avocado oil, olive oil, argan oil, and seaweed or algal extracts.
While all three of these ingredient types (emollients, occlusives, and humectants) are important components of a moisturizing formula, they are not the only critical components. To optimize the effectiveness, the moisturizer should also help restore the barrier layer of the skin. This is the lipid barrier layer in the stratum corneum that helps reduce TEWL from the skin and helps keep environmental chemicals from penetrating. Exposure to external agents that compromise this barrier layer allow chemicals from the environment (or even cosmetics) to penetrate and ultimately irritate our skin. It is a generally accepted fact that the sensitized skin condition may indeed result from a compromised barrier layer. When the barrier layer is compromised or removed from our skin, we lose moisture from the deeper layers leading to a dehydrated skin condition. This is why we usually see sensitized skin accompanied by dehydration. The biggest culprits that compromise our barrier lipid layer include soap and water, over-exfoliation, and exposure to harsh chemicals which can include skin care products. Products with a high pH, like soap, will literally strip the barrier lipids from our skin; that taut, well-cleansed feeling after using soap and water or even a cleanser that is too harsh for the skin is a perfect example of skin that has been stripped of its barrier lipid layer. Ingredients that help restore the barrier layer of our skin include silicones, ceramides, avocado oil, olive oil, sweet almond oil, oil of evening primrose, borage seed oil, essential fatty acids, such as linoleic acid, tocopherol (vitamin E), and other lipid substrates. 
Fortunately, we now have an arsenal of ingredients that can function as humectants, emollients, occlusives, and reinforcers for the barrier lipid layer. Obviously, the ratio of these functional ingredients will vary depending on the skin condition that the product is designed to treat. The professional skin therapist should conduct a proper skin analysis to correctly prescribe the correct moisturizer for the client. Regardless of what skin condition you are treating, make sure you spray a water- or botanical-extract based hydrating toner on the skin after cleansing followed by your moisturizer. Never use an alcohol-based toner as this will only dehydrate the skin more. Remember, to maximize the benefit of the moisturizer, you should follow the three minute rule: apply moisturizer within three minutes after cleansing face. Apply moisturizer to damp skin, preferably over a hydrating toner, to keep the skin smooth and flexible.
Clients need to understand that while we do have the means to treat dry skin today it is also important that we avoid exposing our skin to factors that may create or aggravate this skin condition. Evaluate the array of products that your client is using on their skin. Are they using products that are too harsh, high pH, or stripping the barrier layer from their skin? Are they cherry picking products from different lines? Are they using too many exfoliating products, too frequently? Are they using hot water instead of warm water? Do they protect their skin when exposed to cold, dry weather? Are they using a hydrating mask at least twice a week? The best treatment protocol will ultimately come as a result of a joint effort between you and your client.

List of Emollients, Occlusives, and Humectants Used in Skin Care Products:

Moisturizers are indeed some of the oldest forms of skin care products. While they may seem like a rather old fashion means of treating the skin, today we can formulate them as cosmeceuticals that can alter the structure and function of the skin while hydrating the tissues. We can use them as a vehicle to deliver actives such as sunscreens that ultimately protect the skin from photoaging, brighteners that treat hyperpigmentation, and peptides that address the signs of aging. So while their original intent is to help us end the drought, they in fact are some of the most important weapons in our skin care arsenal.

Definitions:
Emollients – substances that soften and soothe the skin. They are used to correct dryness and scaling of the skin. They are a key component in the manufacture of lipstick, lotions, and other cosmetic products
Occlusives – that tends to occlude; to obstruct, cover, or otherwise block an opening
Humectants – a hygroscopic substance. It is often a molecule with several hydrophilic groups, most often hydroxyl groups, but amines and carboxyl groups, sometimes esterified, can be encountered as well; the affinity to form hydrogen bonds with molecules of water is crucial here
Glycosaminoglycan – (GAG) or mucopolysaccharides are long unbranched polysaccharides consisting of a repeating disaccharide unit
transepidermal water loss – (TEWL or TWL) defined as the measurement of the quantity of water that passes from inside a body (animal or plant) through the epidermal layer (skin) to the surrounding atmosphere via diffusion and evaporation processes

References:
1 (Wildnauer RH et al. Stratum corneum biomechanical properties: I. Influence of relative humidity on normal and extracted human stratum corneum. J. Invest. Dermatol 56:72, 1971)
2 ( Rawlings, A. et al; Skin Dryness-what is it? J. Invest. Dermatol. 100:510, 1993)
3 (Downing, DT et al; Skin Lipids: an update. J. Investig. Dermatology 88 (Supple):2s, 1987.)
4 (Man, M.Q. et al. Exogenous lipids influence permeability barrier recovery in acetone treated skin. Arch Dermatol 129:728. 1993.)
5 (Davies,A. et al. Increased stratum corneum ceramide levels and improved barrier function following topical treatment with TAPs. J. Invest Dermatol. 106:918,1996.)
6 (Downing, DT et al; Skin Lipids: an update. J. Investig. Dermatology 88 (Supple):2s, 1987.)

As Vice President of Technical Development for The International Dermal Institute and Dermalogica, Dr. Diana Howard travels the world as a researcher and speaker, sharing her knowledge of ingredient technology, with a special emphasis on how ingredients impact the structure and function of skin. Dr. Howard earned her doctorate degree from The University of California Los Angeles where she specialized in plant biochemistry. For more than 29 years, Dr. Howard's interest in research on skin and the development of novel ingredient complexes has made her an invaluable asset to many of the major cosmetic companies. Today, she oversees all aspects of technical services for The International Dermal Institute which researches and develops the Dermalogica skin care line.