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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. The recommendation is that a water resistant sunscreen be reapplied after 40 minutes spent in a swimming pool or the ocean. Sunscreens that claim to be waterproof tend to be a little heavier and more oily, and should keep you protected for up to 80 minutes.

The difference between sunscreen and sun block is that sunscreen protects your skin by absorbing and reflecting UV rays. Sun block reflects and scatters UVB rays and creates a barrier between your skin and the sun.  

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.