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    What do I need to know about sunscreens and sun blocks?

    Cosmetic Facial Surgeon, WebMD Medical Expert
    USC, School of Medicine

    Often I am posed with questions about the significance of the sun protection factor (SPF) number. There is an assumption that the higher the SPF number, the more protected you'll be from the sun. But you need not seek the highest SPF available, because beyond an SPF of 30, you will be getting little additional protection. The American Academy of Dermatology recommends using sunscreen that has an SPF of 30 or greater, that protects against UVA and UVB rays, and that's water resistant.

    It is important to know that UVB rays are those that cause bad sunburns and are associated with skin cancer. UVA rays tend to be the culprit for skin aging but can also influence the development of skin cancer, just not as ambitiously. Therefore, I recommend using a product that protects against both UVB and UVA rays.

    It's important to know what happens to your sunscreen when you get in the water. Water will dilute sunscreen, so you'll need to reapply it reasonably often. While sunscreens can be labeled as water resistant, they can not claim to be waterproof. The recommendation is that water resistant sunscreens be reapplied after 40 minutes spent in a swimming pool or the ocean. 

    Sun protection products used to be labeled as either "sunscreen" or "sunblock." However, without evidence to support a difference between the products, the FDA has ruled that companies can no longer call their products "sunblocks."

    Remember that the sun allows the body to perform a self-conversion for vitamin D, which is essential for the calcium metabolism that impacts the strength of your bones. Vitamin D is very important, and in fact may be critical in the prevention of certain cancers.