When school's out for the summer, kids love to play outside. This makes it even more important for infants and children to be protected from the sun. Over time, sun exposure can cause changes in skin cells that can lead to cancer. Sun exposure also speeds up the development of wrinkles and can cause noncancerous spots on the skin.

Your risk depends on both. The damaging rays of the sun, called ultraviolet (UV) rays, are most intense between about 10 a.m. and 4 p.m., but the intensity also increases as you get closer to the equator. On a given day, a person in Florida will receive more than twice as much sun exposure as a person in Maine. UV rays are also more intense at higher altitudes: The intensity increases about 5 percent for every 1,000 feet above sea level.

Some people can become sunburned with as little as 15 minutes of unprotected sun exposure. Fair-skinned people are especially at risk. If you can, avoid being in the sun in the middle of the day, when the sun's rays are most intense. You also can become sunburned while sitting in the shade. That's because the sun's rays reflect off concrete, sand, water and other surfaces. They can still reach you, even if you're not in direct sunlight.

The sun's ultraviolet (UV) rays reach the earth even on cloudy days, so use sunscreen and protective clothing no matter what. Reflections from snow, water and light-colored sand can intensify the rays, so be particularly careful at the beach and, in the winter, while you're out skiing or sledding. The intensity of the sun's rays at noon is measured by the UV index, a number published in many newspapers and weather Web sites. Numbers range from 2 (or less) to 11 (or more), and take into account cloudiness and altitude. The National Weather Service considers an index of less than 2 to be low, 3–5 to be moderate, 6–7 to be high, 8–10 to be very high, and 11 and over to be extreme.

Apply sunscreen 20 to 30 minutes before you go outside, and reapply it at least every two hours. If you are swimming or sweating, remember that "waterproof" sunscreen lasts about 80 minutes, and "water-resistant" sunscreen lasts about 40. Sunscreen is effective if used correctly; the problem is, most people don't use it correctly. People don't use enough of it, don't apply it frequently enough, or miss spots (particularly the back of the neck and the ears). Apply sunscreen to all uncovered skin except eyelids. To protect lips, use a lip balm or lipstick with a sun protection factor (SPF) of 15 or higher. Wear sunglasses to protect your eyes and eyelids. Do not use sunscreen on infants younger than 6 months. Instead, keep them out of the sun altogether or use a combination of shading, clothing and a hat to protect them. Also, check the expiration date of your sunscreen before you use it. If there's no expiration date, expect the sunscreen to last only about three years — less than that if it's exposed to extreme temperatures.

Almost all skin cancers occur on areas that have been exposed to the sun, most commonly the neck, face, ears, hands and forearms. The three most common types of skin cancer are basal cell cancer, squamous cell cancer and melanoma. More than one million cases of basal cell and squamous cell cancers are diagnosed each year; both of these cancers are highly curable. Melanoma will be diagnosed in about 62,000 people this year and will cause over 10,000 deaths. It is often curable if caught early, but can be fatal if it isn't.

The higher the number, the better the protection. An SPF 15 sunscreen lets about 7 percent of the sun's burning rays through to your skin; an SPF 30 sunscreen lets about 3 percent through. The number of the SPF is the number of minutes it will take, properly using the sunscreen, to absorb the amount of UV rays you would absorb in one minute without the sunscreen. If you normally burn after 20 minutes in the sun without protection, properly using an SPF 15 sunscreen would allow you to stay in the sun for 300 minutes (20 x 15), or five hours, before you started burning. If you used an SPF 30 sunscreen, you could theoretically stay in the sun for 600 minutes (20 x 30), or 10 hours. But it is likely that the sunscreen will not stay on the skin for that amount of time. So don't forget to reapply sunscreen at least every couple of hours, especially if you go for a swim or sweat heavily.

Invisible sunscreens block only ultraviolet light; zinc oxide and titanium dioxide creams block all types of sunlight. These creams are good for high-risk areas (your nose, tops of your ears, your shoulders and lips).

Tightly woven clothing (such as long pants and a long-sleeved shirt) and a wide-brimmed hat can protect you from the sun, and you should wear them whenever possible. Most T-shirts, however, have an SPF rating less than 15, so you should still use sunscreen and avoid the midday sun. A dry T-shirt also is more protective than a wet one. When choosing a hat, avoid straw and similar materials that can let sunlight through.