You would never think so looking at it, but the skin is an organ just like the heart or the liver. In fact, it happens to be the largest organ in the body.

The skin, as the border between the self and the outside world, is also the organ that defines us. It protects our internal organs from environmental threats. And for many people, the skin’s appearance, the presence or lack of wrinkles, for instance, is what defines aging, more so than the condition of their vital internal organs. Our skin is very important to us, but skin health is not something to which we give much (or any) thought.

The Skinny on Skin

Number of days it takes for the skin to renew itself: 28

Thickness of human skin in inches: 0.06 to 0.16

Average weight of adult male skin in pounds: 7

Surface area of adult male skin in square feet: 22

Millions of skin cells in average adult: 300

Number of hairs on a square half inch of skin: 10

Number of sweat glands in a square half inch of skin: 100

Number of feet of blood vessels in a square half inch of skin: 3.2

Number of days it takes for the skin to replace itself: 52 to 75

As that part of our body that is the most exposed to the environment, the skin’s greatest threat is the sun’s rays, and overexposure to these rays appears to be the most important factor in the development of skin cancer, which is on the rise worldwide. One out of every five Americans will develop skin cancer, and nearly 10,000 will die from it each year. Skin cancer was long considered a problem only for people over 50, but in the last couple of decades, the rate of skin cancer for people in their forties and younger has multiplied explosively. You need to think about your skin before it’s too late.

Sunny Myths

Sunscreen protects you against all ultraviolet rays from the sun.

FALSE. It protects you from the sun’s UVB rays, but no sunscreen product screens out all UVA rays, which are the cause of melanomas. So just because you put on sunscreen, you are not 100 percent protected.

You don’t need to reapply sunscreen when you come out of the water if you use waterproof sunscreen lotions.

FALSE. There’s no such a thing as a waterproof sunscreen. If you go into the water, you have to reapply these creams immediately because they get washed off.

Suntans are a sign of healthy skin.

FALSE. A tan is a sign of skin damage. The tanning occurs when the skin produces additional pigment (coloring) to protect itself against burns from the sun’s ultraviolet rays.

The sun produces two types of ultraviolet radiation. The ultraviolet A (UVA) rays, which penetrate deeper into the skin, are more responsible for melanoma, the most deadly type of skin cancer. The ultraviolet B (UVB) rays are responsible for sunburns and cause nonmelanomas, the squamous and basal cell skin cancers. Though melanomas account for only 4 percent of skin cancers, they are responsible for nearly 75 percent of all skin cancer deaths. Melanomas usually begin as flat, quarter-inch-sized, mottled, light brown to black blemishes with irregular borders. These blemishes can then turn red, blue, or white; crust on the surface; or bleed. Most frequently seen on the upper back, torso, lower legs, head, and neck, they can occur anywhere on the body.

At Greater Risk

Anyone can get skin cancer, but you are at increased risk if you

--have a family history of skin cancer

--have many moles or large moles

--have naturally blond or red hair

--have blue or green eyes

--have had five or more sunburns

--have been overexposed to the sun early in life

--are Caucasian with fair skin

While skin cancer is the most common type of cancer, and kills more young women than any other type of cancer, it is also the most preventable.

The best defense against skin cancer is sun protection. Because the sun’s ultraviolet rays can damage the cellular structure of the skin, the use of sun protection is important beginning at a young age and continuing throughout life. Whenever you are out in the sun, it’s a good idea to use a sunscreen with a sun protection factor (SPF) of 15 or higher. Apply it 15 to 30 minutes before going outdoors, and reapply it every two hours. If you can’t avoid being out and about in the hours of peak sunlight—ten a.m. until four p.m.—seek the shade whenever possible and wear a hat, visor, sunglasses, and other protective clothing during prolonged periods of sun exposure.

Since skin cancer has a 95 percent cure rate when detected early, by your late thirties you should have a skin physical every two years. This is particularly important if you are at greater risk for skin cancer. At a skin physical, a dermatologist will go over your entire body looking for atypical birthmarks, blemishes, and moles. This exam could save your life.

Skin cancers aside, chronic sun exposure also creates skin stains and premature wrinkling. Smoking, too, is bad for the skin. On the other hand, the aging of the skin can be slowed by a good diet. Certain foods, like salmon and the omega oils found in salmon, have been found to be extremely beneficial for skin health. On the other hand, while skin creams may reduce dryness, or improve excessive oiliness, they will not alter the overall health of the skin. Creams merely affect the superficial layer of the skin. Good skin health comes from within.

Click here to check out Dr. Manny's book The Check List (Harper Collins, 2007), from which this article was excerpted.

Dr. Manny Alvarez is the managing editor of health news at FOXNews.com, and is a regular medical contributor on the FOX News Channel. He is chairman of the Department of Obstetrics and Gynecology and Reproductive Science at Hackensack University Medical Center in New Jersey. Additionally, Alvarez is Adjunct Professor of Obstetrics and Gynecology at New York University School of Medicine in New York City.

Dr. Manny Alvarez serves as Fox News Channel's senior managing health editor. He also serves as chairman of the department of obstetrics/gynecology and reproductive science at Hackensack University Medical Center in New Jersey. Click here for more information on Dr. Manny's work with Hackensack University Medical Center. Visit AskDrManny.com for more.