Everyone knows it's important to get regular exercise. A game of tennis, a bike ride, or a brisk walk all go a long way towards keeping healthy.  But these activities can result in the ravages of sun exposure — premature skin aging and increased risk of skin cancer .

Triathletes are a good example. Competing in three endurance sports in a row — swimming, bicycling, and running — they face long hours of training.

Scott B. Phillips, MD, in the Department of Dermatology at St. Elizabeth's Hospital in Chicago, IL, studied the skin injuries faced by endurance athletes. Of 100 triathletes surveyed, about half responded, all but one describing skin problems.

"Sun overexposure and sunburn are a major concern for endurance athletes," says Dr. Phillips.

"During races, triathletes can run into problems even when they use sunscreen, because it tends to wash off during swimming." Then, they may go on racing unprotected when back on land. Another problem in races for endurance athletes is that, for the sake of speed, they wear less clothing than they do in workouts, exposing more skin. "But overall, the greatest amount of exposure occurs during training, since athletes may work out several hours daily," says Dr. Phillips.

Fortunately, experienced competitors learn a host of methods to help avoid these threats. Dr. Phillips provides some key strategies for anyone who exercises in the sun:

Train early and/or late in the day, even if it means breaking workouts into two sessions. From 10:00 a.m. to 4:00 p.m., try to stay out of the sun. Do the bulk of your exercise before and after work, and sometimes during lunch hour, says Dr. Phillips. "But if you're out for hours on weekends, perhaps on a five- or six-hour bike ride, always use sun protection."

• Although you can't choose the time of day when races are run, encourage race promoters to take sun exposure into consideration in scheduling.

• During training, cover as much skin as possible, wearing sweatpants or at least long shorts, and a long-sleeved shirt or sweatshirt. Wear socks to soak up sweat and absorb impact as well as block the sun; wear them as high up on your leg as possible. And shield your eyes with UV-blocking sunglasses.

• While wide-brimmed hats are too unwieldy for endurance sports, wear a baseball-type cap whenever you can. It protects the forehead and front of the face. And always wear a helmet when biking, for both sun protection and crash protection.

• Use an SPF 15 or higher sunscreen on all exposed skin, up to 6:30 p.m. or later on a summer's day, even when it is cloudy. (The sun's harmful ultraviolet rays go through clouds.) "I recommend alcohol-based sunscreens for the face, because they're more resistant to sweating," says Dr. Phillips. "I also like to apply stick sunscreens around the eyes, since they are almost impervious to sweating. And I'm a big fan of lip balm."

• Even if sunscreen is labeled "water-resistant"or "waterproof," some washes off during swimming. Replenish it when you come out of the water, and every two hours after. In races, triathletes should keep sunscreen handy in the transition areas. Or, if you're bicycling next, keep sunscreen taped to the bike and reapply it to the arms, face, and neck while peddling.

"Taking these few moments makes a big difference in sun protection, and little difference in your competitiveness," says Dr. Phillips.