Moisturizers are a complex mixture of chemical agents specially designed to make softer and more pliable the external layers of the skin (epidermis), by increasing its hydration (water content). Naturally occurring skin lipids and sterols as well as artificial or natural oils, humectants, emollients. lubricants, etc. may be part of the composition of commercial skin moisturizers. They usually are available as commercial products for cosmetic and therapeutic uses, but can also be prepared at home using common pharmacy ingredients.

Besides imparting or restoring normal levels of hydration to the skin, moisturizers can have several additional intended and unintended effects on its users, including building a barrier against the loss of water through the epidermis, repairing scaly, damaged or dry skin resulting from external environmental aggressions or internal changes (such as in acne or naturally dry skin), repairing or postponing age effects on the skin, etc.

Mechanism of Action

Moisturizers act on the most external of the skin layers, the so-called corneal stratum (stratum corneum, the anatomical term in Latin), which is largely formed by squamous cells or keratinocytes (an epithelial layer which is quite dynamic, i.e., it is being constantly shed and replaced by the growth of new cells coming from the deeper layers of the skin, therefore its name). Most, if not all, agents present in moisturizers are unable to penetrate these deeper layers such as dermis and hypodermis.

The stratum corneum has approximately 30% water, of which a third is tightly bound to hygroscopic molecules and lipids in the skin. This fraction of water content is proportional to external relative humidity, and the thickness and flexibility of the stratum corneum increase with added water content. Evaporative loss of water of the skin increase in certain circumstances, especially if relative air humidity is decreased (in the dry season, in air-conditioned spaces, etc.). The remaining two thirds of water content are part of the biological tissue, such as keratin, and usually do not change in non-pathological conditions. Impairment of the bound fraction may occur in response to endogenous or exogenous conditions such as hormonal level, toxins, disease (such as hyperkeratosis), etc.

Composition

Although simple and effective moisturizers can be prepared from two or three simple chemicals, such as stearate, olive oil, water and glycerin, commercial preparations are astoundingly complex and varied in composition and may include:

• Humectants, such as glycerin, urea, lactic acid and sorbitol;
• Natural moisturizing factors (NMF) include low molecular weight substances such as ammonia, aminoacids, glucosamine, creatinine, citrate and ionic solutions such as sodium, potassium, chloride, phosphate, calcium and magnesium.
• Emollients, such as lanolin (the earliest complex organic substances used in facial and body moisturizers, which is extracted from wool). Lanolin act as a barrier (occlusion effect) against loss of water and also as a softener of stratum corneum, by means of lubrication and smoothing. Other emollients are oil-water emulsions of varying composition and may include several esters and oils such as octyl dodecanol, hexyl decanol, oleyl alcohol,decyl oleate, isopropyl stearate, isopropyl palmitate, isopropyl myristate, hexyl laureate, and dioctyl cyclohexane.
• Emulsifier, preserving and fragrance agents are also part of commercial preparations.

Since moisturizers are among the most used and prescribed products for the skin, unfortunately the cosmetics industry many times advertise loudly for scientifically unsubstantiated effects. Physicians, cosmeticians and consummers alike should be aware of the real science behind skin moisturization, and know what is possible to achieve and what is not. For example, the addition of vitamins (A, B, C, D and E), nutritive agents, proteins and phytotherapeutic agents has been common in the industry, supposedly in order to add to the moisturizer the capability to treat several skin conditions such as cellulitis, age and photo damage, edema, loss of collagen, wrinkles, etc., with little or no scientific evidence for such. Of course, this has the effect of increasing the tag price of moisturizer creams to incredible market values, although the basic and really effective components are very inexpensive.

 

Adverse Effects

Despite claims to the contrary by the cosmetics industry, complex moisturizers may cause a number of adverse effects, including allergic reactions to some of its components, skin irritation, contact dermatitis, characterized by redness, itching, burning and stinging sensations, or even may cause a contrary effect to the desired, i.e. they may actually increase dehydration. When used near sensitive spots, such as the eyes, lips and genitals, these effects may be enhanced in some persons. Use of plant extracts, some alcohols and proteins may increase the danger of adverse effects.

Cosmetic and therapeutic moisturizers should be accompanied by the printed formula in order to inform consumers adequately, as well as physicians, in order to easily and quickly identify the offending component.