What you need to know to protect your skin this summer

By now we all know that excessive sun exposure causes most of the more than one million cases of skin cancer diagnosed each year in the United States. Most of us can go on to recite a litany of other sun-related skin woes—liver spots, wrinkles, sagging skin, and increasingly uneven pigmentation. We've heard endless admonitions to take precautions to limit our exposure to the sun's ultraviolet rays, starting with the regular use of sunscreen.

But judging from the number of sunburns MIT Medical dermatologist Linh Lu, M.D., Ph.D., treats each summer, many of us still don't follow that advice.

Perhaps this is a case of having too much information rather than not enough, Dr. Lu acknowledges. UVA? UVB? SPF? It can be hard to make sense of it all. And then there's the ever-expanding variety of sun-protection products, each new sunscreen boasting "improved" ingredients, ever-higher SPF values, and "better protection" against different parts of the ultraviolet spectrum. Where do we even begin? To help us make sense of this skin-protection puzzle, Dr. Lu agreed to answer some of our most perplexing questions:

In ads for sunscreen, I hear about both UVB and UVA rays. What's the difference? Do we need to be protected from both?

Dr. Lu: Ultraviolet (UV) rays are classified into different bands based on wavelength. UVA rays have a wavelength of 320-400 nanometers, and UVB rays have a shorter wavelength, 290-320 nanometers. UVB rays cause direct photochemical damage to DNA and are more likely to cause sunburn. But UVA could have indirect effects on DNA by creating reactive oxygen molecules. Some evidence suggests that UVB is involved in squamous cell carcinoma, one form of skin cancer, while UVA has a greater role in sun damage and aging. It is probable that both UVA and UVB play a role in the cancer pathway or process. Therefore, we do need "full-spectrum protection"—protection from both types of UV rays, in other words.

Do different types of sunscreen offer different types of protection? How do I read a sunscreen label, anyway?

Dr. Lu: Sunscreens are divided into two types: those that use inorganic/physical agents and those that use organic/chemical agents:

• Inorganic agents, such as titanium dioxide and zinc oxide, coat your skin with a film of inert metal particles that reflect and scatter ultraviolet and visible radiation. These agents provide full-spectrum protection.
• Organic agents, on the other hand, act by absorbing ultraviolet radiation. These chemical agents can be classified as either UVA absorbers, UVB absorbers, or broadband absorbers.

Because we need protection from both UVB and UVA rays, you want to look for a sunscreen containing either an inorganic agent or an organic broadband absorber. Read the label to make sure your sunscreen contains one of these four ingredients: titanium dioxide, zinc oxide, ecamsule (trade name: Mexoryl SX), or avobenzone (trade name: Parsol 1789).

What's with these astronomically high SPF numbers? Does anyone really need to use SPF 70, or is that just a marketing ploy? What number should we be looking for when we buy sunscreen?

Dr. Lu: SPF stands for "sun protection factor." It can be defined as the minimum amount of UVB sunlight needed to produce skin redness when using the product as compared with unprotected skin. An SPF-15 product can filter out about 93 percent of UVB rays. An SPF-30 product can filter out about 97 percent of UVB rays.

After SPF 30, the SPF-versus-UV-protection curve flattens out dramatically. For example, SPF 64 filters out 98.4 percent of UVB rays, but that's only 1.4 percent more than SPF 30.

I usually feel that SPF 30 is a good target number. Of course, if someone wants to spend the money to buy a sunscreen with a higher SPF, that's fine. They just need to understand they are gaining only about one to two percent more UVB protection for their money.

What's the correct way to use sunscreen?

Dr. Lu: Most people don't apply sunscreen soon enough before sun exposure. It takes time for sunscreen to be absorbed by your skin, so you need to apply it to all exposed skin about 20 to 30 minutes before you go outside. Once outside and exposed to the sun, re-apply sunscreen every two hours, especially after swimming, towel drying, or heavy sweating. There's really no such thing as a "waterproof" sunscreen or one offering "all-day protection."

People also tend to apply sunscreen unevenly and too thinly. SPF numbers are based on an application of 2 milligrams of sunscreen per square centimeter of skin, which is a fairly liberal coating. The amount of sunscreen it would take to fill a shot glass—about one ounce—is roughly the amount you should be using each time you apply sunscreen to all exposed skin.

And don't forget your neck, ears, and any exposed parts of your scalp! Be sure to use a sunscreen-containing lip balm as well.

What if I use other skin products along with sunscreen, like makeup, moisturizer, or topical medicated creams? In what order do I apply these products?

Dr. Lu: If you're using any kind of medicated cream, it needs to be applied to your skin first, before any other product. Moisturizer should be next. Follow moisturizer with sunscreen, and then apply makeup last.

It's important to note that many moisturizers and makeup products now contain sunscreen. If you like a certain brand of moisturizer or makeup, check to see if it comes in a formulation that includes sunscreen.

Does sunscreen block the absorption of vitamin D from sunlight? Are we avoiding skin cancer only to come down with rickets?

Dr. Lu: It is true that sunscreens can block vitamin-D-producing UV rays, but this is not a concern for most people. In practice, the skin is likely to synthesize some vitamin D even when it is protected by sunscreen. Furthermore, the best way to obtain vitamin D is through food, such as milk, cereal, or fish, or supplements, like multi-vitamins or even cod fish oil.

If I have naturally dark skin or tan easily, do I still need to use sunscreen?

Dr. Lu: Darker-skinned people should also use sunscreen, because they can also get skin cancer. Furthermore, sunscreen helps prevent "photoaging," skin damage that occurs in both lighter- and darker-skinned individuals as a result of sun exposure.

Can I forgo sunscreen in winter or on cloudy days?

Dr. Lu: UV radiation can penetrate clouds, haze, and smoke, so sunscreen should be used on cloudy days. It should also be used in the winter, even though the sun is not as strong.

What else do we need to know about protecting our skin from sun damage?

Dr. Lu: Sun avoidance is the best way to protect your skin from harmful UV rays. If at all possible, avoid being outdoors between 10 a.m. and 4 p.m. when the sun is at its most intense. If you do go outdoors during those hours, use sunscreen, and also wear clothing with a tight weave, sunglasses, and a hat with a four-inch rim all around.

It's also possible to buy clothing made of SPF-rated fabrics. Or you can wash your clothes with Rit SunGuard, which actually adds sun protection to your clothes.