When it was announced that 2004 olympic bronze- medalist marathoner Deena Kastor had been diagnosed with melanoma, the deadliest of skin cancers, athletes across the world shuddered. Though few people train as hard or long as an Olympian, there was a collective fear that countless hours on trails, chairlifts, and sunny decks put everyday enthusiasts at a similar risk.

Most adults — even athletes — do not take skin health as seriously as they should. Statistics from the American Cancer Society show that skin cancer is the most common of all cancers, accounting for half of all new cancer diagnoses each year. While the vast majority of skin cancers are not fatal, projections suggest that there will be more than 60,000 new cases of melanoma in the United States in 2008, and more than 8,000 deaths.

Winter sports enthusiasts often dismiss warnings for skin protection as relevant only to summer athletes. But skiers and snowboarders should pay attention: There is actually an increased risk of developing sunburn at higher altitudes, since less atmosphere is present to filter ultraviolet (UV) rays. For every 1,000 feet above sea level, there is a 4 percent greater penetration of UV rays. In other words, vacationing in Aspen exposes you to 32 percent more harmful rays than visiting the San Diego Zoo.

In addition, snow reflects up to 85 percent of UV rays, which can lead to sunburn on parts of the body that usually do not see much sunlight, such as under the chin. A study in Vail, Colorado, determined that the typical skier’s exposure to UV radiation could not only cause immediate skin damage but also increase later risk for skin cancer. Ski instructors especially were noted to have exposure levels 10 times greater than the recommended UV exposure limit.

According to Dr. Pai Lao, clinical assistant professor of dermatology at Weill Cornell Medical College, the best advice is to avoid sun exposure between 10 a.m. and 3 p.m., when UV rays are most intense. If this isn’t possible — and certainly it would put a massive dent in your daily vertical — it’s important to use something that protects your skin.

Most people choose a skin product based simply on the SPF (sun protection factor), which indicates how long a person can remain in the sun before being burned by UVB rays. However, outdoor enthusiasts must also beware of UVA rays, which unlike UVB are present from dawn to dusk — even on cloudy winter days — and penetrate deeper into the skin. UVA rays have been shown to cause DNA damage (a prelude to skin cancer), skin aging, fine lines, and wrinkles.

Then there’s the choice between sunscreen and sunblock. Sunscreen absorbs UV rays and converts them to harmless heat; it’s best to use a water-resistant sunscreen applied liberally 15 to 20 minutes before participating in outdoor activities, then reapplied at least every 90 to 120 minutes or after excessive sweating (which can be difficult to gauge on the slopes due to quick evaporation of perspiration). In contrast, sunblock uses physical blocking agents such as zinc oxide or titanium dioxide to scatter and reflect UV radiation away from the skin, and can be applied immediately before outdoor activities without needing extra time for absorption. Sunblock is less likely to cause skin irritation and is excellent for children or those with sensitive skin, but it may feel slightly heavy or appear opaque, a look that hasn’t been in fashion since the mid-1980s.

So whether hitting the hill or training for a marathon, select a product that offers excellent protection from both UVA and UVB rays, and apply it well and often. And maybe find a shady spot on the deck occasionally.