Water is essential to maintain skin plasticity and barrier integrity. Xerosis or dryness of the skin is the result of decreased water content in the stratum corneum which leads to abnormal desquamation of the corneocytes (top-most layer of the skin). For the skin to appear and feel normal, the water content of this layer should be more than 10%.

Hydration of the skin is achieved by three sources: diffusion from underlying dermis, from sweating and from surrounding atmosphere. The hydration is maintained by a barrier in the top most layer of the skin that guards against trans-epidermal water loss (TEWL) and this barrier is made up of fats released by the sebaceous glands (oil glands) and layers of the skin. Any factor that leads to either excessive loss of water through defective barrier or decreased hydration from the different sources can result in a dry skin (xerosis). Xerosis is seen clinically as flaky or ash-like skin. Dry skin is not only aesthetically unappealing but also itchy.

Defective barrier can be due to:

• Atopics (those with family history of asthma, allergic rhinitis or dermatitis)
• Skin diseases like psoriasis
• Use of harsh detergents
• Nutritional deficiency (essential fatty acids, vitamins)
• Drugs (lipid lowering agents)
• Aged skin

Decreased diffusion can occur in:

• Elderly
• Dehydration
• Chronic photo damage
• Decreased sweating (hypothyroidism, decreased exercise)
• Other factors: Low humidity in the atmosphere
  • Winter
  • Central heating
  • Air-conditioning etc.

Dry skin is the most important cause of eczema that starts as itching followed by rash. Itching in individuals with dry skin may be precipitated by the following:

• Woolen clothing
• Detergents
• Water (bathing for long hours)
• Environmental changes (winter, areas with low humidity)
• Friction (use of scrub, rough clothing)
• Stress
• Excessive use of talcum powders

Moisturization can be achieved by using the products containing the following:

1. Occlusives: Occlusives prevent trans-epidermal water loss. Many chemicals act as occlusives; the most occlusive is petrolatum, others are vegetable and animal fats, silicon oils, vegetable waxes and phospholipids.

2. Humectents: Humectents draw water from environment and deeper tissues to rehydrate the horny layer. They may also allow the skin to feel smooth by filling the holes. Glycerin, honey, urea, propylene glycol are examples of humectents

3. Emolients: Emolients function by filling the spaces between the desquamating skin scales and provide a smooth feel to the skin.

4. Hydrophilic matrices: Hydrophilic matrices provide blanket against water evaporation. Hyaluronic acid is an example. 

5. Sunscreens: Sun screens prevent cellular damage and prevent dehydration. Many of the repair and replenishing moisturizing formulations contain sunscreens.

Selection of moisturizers based on body sites and skin type:

Ideal moisturizer is a combination of an occlusive, a humectent and an emolient. Selection of moisturizer should be done on the basis of the site of application, type of the skin and acceptance.

Facial moisturizers should be chosen correctly depending on the skin type. With proper use of facial moisturizers, fine wrinkling due to skin dehydration and roughness due to skin scales can be improved.

• Oily skin moisturizers contain water and silicon derivatives such as cyclomethicone or dimethicone.

• 'Oil control' moisturizers contain oil absorbing substances such as talc, clay or starch and methacrylate.

• Normal or combination skin moisturizers contain predominantly water, mineral oil and propylene glycol with very small amounts of petrolatum or lanolin.

• Anti-wrinkle moisturizers contain sun-screen agents and alpha hydroxy acids in addition.

• Dry skin moisturizers contain water, mineral oil, propylene glycol and larger amounts of petrolatum and lanolin.

Body moisturizers should be non-greasy and easily spreadable. Lotions are preferable. Creams and ointments are preferred in extreme cases of dryness.

Hand moisturizers – simplest is petrolatum jelly. Those with silicon derivatives can render the hand-cream water resistant.

Proper moisturizer selection can aid in the treatment of disease, whereas poorly formulated, poorly selected products can initiate the disease process.