Lisa Drage, M.D., a dermatologist at Mayo Clinic, Rochester, Minn., answers some common questions about the sun's effects on your skin and what you can do to protect yourself.

What causes sunburn?

Sunburn is the result of overexposure to the sun's ultraviolet (UV) radiation. Repeated exposure to UV radiation both tans and damages your skin.

The UV content of sunlight varies. It's greater at higher elevations because it's unfiltered by clouds or haze. But reflected ultraviolet light from snow, sand, water and other highly reflective surfaces can burn as severely as direct sunlight. You can also get a sunburn on a cloudy day.

Two types of ultraviolet radiation — ultraviolet A (UVA) and ultraviolet B (UVB) — have an effect on your skin. UVA penetrates into the deeper layers of your skin and can impair your skin's immune system, which may contribute to cancer. UVA usually causes the leathery, sagging, brown-spotted skin of those who spend a lot of time in the sun. The longer waves of UVA can also penetrate window glass, including car windows. Tanning beds are a source of high doses of UVA.

UVB causes sunburn and plays a significant role in superficial skin cancers called basal cell carcinomas and squamous cell carcinomas. The relationship between sunlight and melanoma — the most serious type of skin cancer — is less clear-cut. Studies suggest that those exposed to occasional periods of intense sunlight, such as indoor workers who spend vacations lying on a beach, are at greater risk of melanoma than are farmers, roofers and others who regularly spend long hours in the sun.

How important is sunscreen in protecting you from UVA and UVB radiation?

Protection from the sun is very important in the prevention of skin cancer. To be most effective, protective steps should begin in early childhood. But all people, regardless of age, should practice sun protection. It's the sensible thing to do. Regular, proper use of broad-spectrum sunscreens — those that offer protection from both UVA and UVB radiation — is key in preventing sunburn, sun damage and skin cancer.

How do you choose a sunscreen?

Shopping for skin products that offer protection from the sun can be confusing. And labels that feature superlatives — such as total sunblock, waterproof, all-day protection and deep-tanning — provide little solid information for those seeking the best protection for their skin type.

Sun protection factor (SPF) ratings refer only to UVB protection. It's important to check sunscreen labels to ensure that the product is a broad-spectrum sunscreen. Products containing zinc oxide or titanium dioxide are among those that provide protection against both UVA and UVB.

SPF ratings are based on how much longer someone may be protected from sunburn than he or she is if no sunscreen were applied. For instance, if you normally burn in 20 minutes, a product with SPF 15 will allow you to stay out in the sun 15 times longer. You'll be able to stay out in the sun without burning for five hours, assuming you applied the sunscreen properly. The SPF number doesn't refer to a sunscreen's strength. For example, an SPF 30 is no stronger than an SPF 8 — it doesn't filter out more harmful rays than an SPF 8 does — but it does protect you longer.

What is proper application?

Most people use sunscreens too sparingly. Labels of most sunscreens call for liberal and frequent applications. A liberal application is 1 ounce — the amount in a shot glass — to cover all exposed parts of the body. Timing is important, too. To have the best effect, sunscreens need to be applied 30 minutes before you get to the pool or beach, not after you arrive.

Because of sweating, swimming and toweling off, sunscreen should be reapplied throughout the day. Even water-resistant sunscreens need to be reapplied every 90 minutes. Always use a sunscreen with an SPF of at least 15.

Tanning salons often advertise tanning beds as a good way to get a base tan before lengthy exposures to direct sunlight. Is this a safe way to prepare for a trip to Hawaii, for example?

There's no such thing as a safe tan. Avoid tanning beds — they aren't safe, and they may cause skin cancer. They're also associated with other nasty skin problems, such as infections and warts.

We know that UVB and sunburns are associated with increased skin cancer and melanoma risk. But that doesn't let the UVA radiation in tanning beds off the hook. UVA probably has an effect on your skin-based immune system and its ability to prevent melanoma. UVA radiation isn't safe. The tanning salon operators might say that UVA is good and UVB is bad, but research doesn't support that. UVA may actually increase the risk of skin cancer or melanoma.

What about so-called bronzers that make the skin look tanned?

I think that's an excellent way to get the tan look without paying the price. They're safe. The one caveat is that many don't have sunscreen in them, and the tan you get with them won't protect you from the sun. You still need to wear sunscreen. But overall, they're a much healthier way to achieve the look without the risk. With experimentation, you can find a level of color that looks best on you.

Should you avoid the sun altogether?

We don't want to scare people, but at the same time, behavior does have an effect on health — whether it's cigarette smoking, eating habits or sunbathing. You don't need to stay in a cave. Get out, be active and have a healthy life. However, be smart about your sun exposure — wear a broad-brimmed hat and light-colored clothing that covers your exposed skin, and use a broad-spectrum sunscreen. Limit outdoor sports and other activities for the early morning or late afternoon whenever possible.

Applying sunscreen to exposed areas is a good year-round habit in most localities. Make it a part of your daily routine, just like brushing your teeth.

Do certain medications affect your risk of sunburn?

Many drugs increase your sensitivity to sunlight and your risk of getting a sunburn. Some common ones include thiazides and some other diuretics, tetracycline and sulfa antibiotics, and nonsteroidal anti-inflammatory drugs, such as ibuprofen, in dosages used to treat arthritis. Ask your doctor about any medications you might be taking.

If you do develop a sunburn, take aspirin or ibuprofen (Advil, Motrin, others) for pain, apply cold compresses and avoid further exposure until the burn goes away. A sunburn spray may help relieve pain. A severe sunburn may require — and will likely benefit from — medical attention.

How can you tell if something is skin cancer?

Be on the lookout for new or nonhealing growths on your skin, which may indicate skin cancer.