What causes skin moisture loss?

Stretch out an arm right now, preferably yours. Situate said arm so you can examine the skin on your knuckles. Do your knuckles look more cracked and parched than a desert tortoise's? They might. That's because your skin has dried out.

Why does your skin dry out? It helps to start by reviewing your skin's neat system for trapping moisture. The first stop is the epidermis, the outermost layer of skin, which covers you from your scalp to your toenails. Think of it as the body's outerwear. Although less than a millimeter thick, the epidermis has layers, just like a ski jacket [source: Whitton]. The stratum corneum, its top layer, is made of dead skin cells. These cells grab water using their filling -- natural moisturizing factors (NMFs), amino acids and other molecules that absorb water from the air and lock it inside the cell. In dry climates, our cells make more NMFs to stay moisturized [source: Rawlings].

A layer of fat sits below the stratum corneum. Ceramides, cholesterol and fatty acids are some of the fats you'll find here, arranged into layers of jelly and liquid [source: Bonté]. When water tries to evaporate from lower layers of the skin, it gets trapped in spaces between the fat molecules and can't rise any higher. The layers also give water too far to travel to reach the surface of the skin [source: Rawlings].

Next, we're on to the stratum granulosum. Its top half is almost watertight, the cells being sewn together by a type of protein thread [source: Bonté, Becker]. The bottom is waterlogged. Channels let water from deeper layers into this layer, but not out.

Last but not least is the stratum spinosum at the bottom. Cells in this layer sport hyaluronic acid, a sugar that binds to water, on their surfaces.

If our skin is built to hang on to moisture, then why does it dry out? Our genes, our age, evaporation and osmotic pressure all make a difference. And what a difference they make. Dry skin grays with dead cells and forms rashes and, just in case we didn't notice, itches and stings.

Our dry skin has a good reason to complain. It's troubled below. Housekeeping enzymes in our skin need water to work. When our skin dries, these enzymes falter, and the skin has trouble keeping water in and infections out. And for people with skin conditions, drying and cracking can open the already weak barrier, admitting skin bacteria like Staphylococcus species, for a gnarly infection [source: Aftergut].

Read on to learn more about some biological causes of dry skin.

Biological Factors in Dry Skin

As we age, our skin gets worse at retaining moisture. After our teenage years, we make less of the molecules that trap water in our skin. Almost every layer of the epidermis gets more barren: We make less natural moisturizing factors, less ceramides and less hyaluronic acid. By age 50 to 70, spots of dry skin show up on even the healthiest of hands, says Dr. Kent Aftergut, a clinical assistant professor of dermatology at the University of Texas, Southwestern at Dallas.

We can't stop our genes from making less moisture-trapping equipment in our skin as we age. But we can stop disabling the equipment we have. For instance, instead of lathering soaps, which yank fats out of our skin, we can switch to mild soaps that don't lather. We also can use creams that replace natural moisturizing factors that our genes have slacked off in making. Urea is one of those factors. But be careful: Some creams promise to replace waning moisturizing factors that the skin can't actually absorb [source: Bonté].

Age isn't the only genetic source of dry skin. Some people have chronically dry skin, starting when they are young. They may have a type of eczema, which stems from a genetic mutation related to one of the elements that holds water in the skin.

Just as for the elderly, part of the solution for eczema can be to replace the missing element in the skin. Scan the labels of moisturizing creams prescribed to people with eczema, and you'll often see ceramides and other fats that people with eczema don't naturally make [source: Aftergut]. The solution doesn't usually stop there, however. Because other bodily systems, such as the immune system, can contribute to the skin condition, patients may need other medications that deal with the broader issue.

Environmental Factors in Dry Skin

The climate, the air and the chemicals that surround us also exert pressure on our skin.

Dry air is the worst offender. It simply sucks water from skin, despite the skin's best attempts to keep it. It's a sad cycle: When dry air takes moisture from our skin, its structure changes. Those changes make our skin less able to hold onto moisture [sources: Bonté, Rawlings].

For instance, at low humidity, enzymes that cut connections among cells on the skin's surface, allowing such mild skin-shedding that we can't see it, start chopping deeper in our skin, and our skin flakes [source: Bonté]. Then, water escapes from our skin more easily because the flaking shortens the distance to escape. When the humidity drops below 10 percent, the enzymes that make some of our natural moisturizing factors don't work as well [source: National Weather Service, Bonté]. The stratum corneum dries.

Let's say we're in a city where it's dry and cold. Cold makes us switch on the heater, and the heater dehydrates our skin by dehydrating the air.

If the UV index is high, your skin is under further attack. Like dry air, ultraviolet rays from the sun shut off an enzyme that stocks the stratum corneum with natural moisturizing factors. You don't need a high dose of rays to see the effect. If your skin is reddened by the rays, you've experienced enough [source: Bonté].

You can fight back with common sense. The air can't burglarize skin that it doesn't touch, so when you're outside, shield yourself with a jacket, hat and gloves. If it's too warm for layers, moisturize exposed skin. Inside, if you use a heater, return moisture to the air with a humidifier or a pan of water in the room. Or warm yourself with a blanket. And in the sun (or snow), wear a UV-blocking sunscreen.

We're not done yet: Water can also be a problem. This life-giving liquid doesn't actually dehydrate you; the substances in it do, such as soap. To the teachers, nurses, doctors, dentists and other frequent hand-washers of the world, dermatology professor Aftergut says, "Don't wash your hands excessively. Obviously, if you've shaken hands with someone with the flu, wash your hands. When you start washing your hands 30 to 50 times a day, that's not good. When you do wash, use a gentle, nondetergent-based soap, and moisturize frequently."

In addition to hand-washing, swimming can be a problem. Swimming pools, at least hygienic ones, dry skin thanks to the chlorine. Fortunately, a shower after the pool to rinse off the chlorine usually takes care of it.

Now grab some moisturizer and keep reading for more links to skin stuff you might like.

Lots More Information

Sources

Aftergut, Kent. Personal interview. 12/7/2009.
Becker, Wayne M. et al. "The World of the Cell." San Francisco: Benjamin Cummings. 2003.
Bonté, Frédéric and Slyvie Verdier-Sévrain. "Skin Hydration: A Review on its Molecular Mechanisms." Journal of Cosmetic Dermatology. Vol. 6. No. 2. 2007.
National Weather Service. "Las Vegas, Nevada Average Relative Humidity." 2009. (12/10/2009) http://www.wrh.noaa.gov/vef/climate/page32.php
Rawlings, A.V. and C.R. Harding. "Moisturization and Skin Barrier Function." Dermatological Therapy. Vol. 17. Suppl. 1. 2004.
Whitton, Judi T. and J.D. Everall. "The Thickness of the Epidermis." British Journal of Dermatology. Vol. 89. No. 5. 1973.