Q: I know that it’s a good idea to wear sunscreen if it’s a sunny day or I’m on the beach. But what if the sun is not out? Should I still use sunscreen?

A: If your aim is to prevent skin aging, yes.

The sun, not time, is our skin’s greatest adversary. While time causes chronological aging, the sun causes “photo-aging.”

The sun’s rays lead to more than 80 percent of the changes that result in wrinkles, age spots, dilated blood vessels, spider veins, red bumps, growths and raised dark spots.

And it doesn’t take much sun to do it. A 1997 article in the New England Journal of Medicine reported that only minutes of exposure here and there accumulate over the years and lead to premature skin aging. This is far less than the exposure required to produce a visible sunburn or even a tan.

HEALTH     MORE STORIES Diseased organ recipient: ‘You saved my life’   Jerry Trueba was tearful when he thanked the parents of Alex Koehne for saving his life twice: first by giving him their late son’s kidney, and second by alerting him when they learned it was cancerous.   The most toxic places in your home Sniff this! The shelf lives of 5 household items ‘Too fat’ model reveals fashion’s dark side You’re not crazy, but you might need therapy

In addition, even when the sun doesn’t appear to be shining brightly, its rays are there, working their damage.

You don’t need to be a physicist to understand how rays cause aging. We’re chiefly concerned with two types of rays.

The first type, Ultraviolet B rays (UVB), causes superficial and immediate skin damage by irritating the melanocytes in the bottom layer of the outer skin. Depending on the degree of irritation, we either get a tan or a burn.

A sunburn not only creates a cooked-lobster look but damages the skin’s immune system and increases the likelihood of potentially fatal malignant melanoma, which now occurs in 1 out of 90 Americans.

The second type, Ultraviolet A rays (UVA), penetrates more deeply into the skin. These rays are a product not just of direct sun but of all natural light, capable of reaching the skin through glass, clouds and smog.

HEALTH LIBRARY Addictions Aging AIDS Allergies & Asthma Alternative Medicine Alzheimer's Disease Arthritis Cancer Heart Health Children's Health Cloning Cold & Flu Cosmetic Surgery Diabetes Diet & Fitness Digestive Diseases General Health Genetics Health Care Infectious Diseases Men's Health Mental Health Pets Sexual Health Stem Cell Research Women's Health

In other words, there is no such thing as a sunless day.

(By the way, those damaging UVA rays are used in tanning salons, so don’t think that’s a no-risk way to get a tan.)

Of course, not everyone photo-ages at the same rate. This is determined by your skin type. You fall into one of six types, as defined by the FDA and the American Academy of Dermatology:

Type 1: You burn easily, never tan, and have red hair and freckles.

Type 2: You burn easily but can get a minimal tan. You have fair skin and hair, and blue eyes.

Type 3: You sometimes burn but can get a light tan. You are dark-haired and probably Caucasian or Asian.

Type 4: You burn minimally and tan to a moderate brown. You are probably Caucasian of Mediterranean descent, or of African, Asian, Hispanic, Indian or Middle Eastern ancestry.

Type 5: You rarely burn, and you tan well. You are probably of Middle Eastern, Asian, Hispanic, Indian or African descent.

Type 6: You never burn. You are probably of African descent, with deep skin pigmentation.

For those of us between Type 1 and Type 4, there is no such thing as safe, or non-aging, exposure to sunlight. Wear protective clothing, stay out of the sun from 10 a.m. to 3 p.m. and always use sunscreen.

Only on TODAY.MSNBc.com! Dr. Judith Reichman addresses your questions about women's health. LAST WEEK: Will removing my ovaries prevent cancer? GO TO ALL HEALTH TUESDAY COLUMNS

Not all sunscreens are created equal. You need to block both UVB and UVA rays. Look for the term “broad-spectrum” on the label; this means it blocks both UVB and UVA rays. Products containing zinc oxide, titanium dioxide or Parsol 1789 are broad spectrum.

The minimum sun-protection factor, or SPF, for daily wear, should be at least 15. Since sunscreen begins to work 30 minutes after it goes on, apply it well before venturing outdoors. Even waterproof sunscreen loses its effectiveness within 90 minutes if you sweat or swim, so reapply it often. Don’t be stingy — apply sunscreen to all exposed skin.

Dr. Reichman’s Bottom Line: If you want your skin to look good, cover it up. Do not walk out that door without protecting all exposed skin from the sun’s visible and invisible rays.

Dr. Judith Reichman, the “Today” show's medical contributor on women's health, has practiced obstetrics and gynecology for more than 20 years. You willl find many answers to your questions in her latest book, "Slow Your Clock Down: The Complete Guide to a Healthy, Younger You," published by William Morrow, a division of HarperCollins.

Print this Email this Blog this  IM this

MORE FROM HEALTH

Health Section Front

Diseased organ recipient: ‘You saved my life’   The most toxic places in your home Sniff this! The shelf lives of 5 household items ‘Too fat’ model reveals fashion’s dark side You’re not crazy, but you might need therapy 28 million women face unwanted pregnancy Group fears step taken toward ‘designer babies’ Overeater sheds 240 pounds — naturally Lose 10 pounds in 6 weeks! It’s easy Oh, baby: The year’s most popular names Health Section Front   Add Health headlines to your news reader: