By- Dr. Amy MoviusIf youâ€™ve gone sunscreen shopping this summer (and I hope you have!) the options available are mind-boggling. A few years ago, it was rare to find a sunscreen SPF (sun protection factor) rating of more than 45: now there are many products with ratings of 70, 80, 90 and even 100! But what, exactly, do these ratings mean?Letâ€™s start with what the Sun Protection Factor does. An SPF rating refers to the sunscreen productsâ€™ ability to block ultraviolet B, or UVB, rays. UVB rays cause sunburn and contribute to the risk of skin cancer. Now, for what SPF doesnâ€™t do. SPF sunscreens do not block UVA or ultraviolet A rays. About 95% of the UV rays we are exposed to are UVA. UVA rays are closely linked to deeper skin damage (wrinkle causing) and also contribute to the risk of skin cancer. SPF rating measures time. A SPF rating of 10 means you would be protected from sunburn causing UVB rays 10 times longer when using the product than without using the product. As shown below, UVB protection does not increase in direct proportion to the SPF number:SPF UVB rays blocked15 94%30 97%45 98%Accordingly, SPF 15 products are generally fine if used correctly, but few people apply sunscreens as heavily or often as they should. Some providers recommend SPF ratings of 30 or so for this reason alone. No product blocks 100% of UVB rays, so distinctions between SPF 45 to 100 are tiny. In fact, the FDA is working on a new labeling system to limit SPF product claims to 50.Finding a sunscreen that protects against UVA rays is just as important, but a lot harder to do. Many sunscreens that block UVA rays often do so by chemical filters (avobenzone or Mexoryl). However, the protection may be marginal as these chemicals break down quickly and lose effectiveness in the sun, unless stabilized. Few companies have been able to prove they can stabilize these chemicals. Barrier sunscreens containing zinc oxide or titanium are preferred by some dermatologists but many people dislike the thick, pasty and opaque products that are over-the-counter. There are some medical-grade sunscreens that contain these barriers in a micronized form that isnâ€™t so thick or visible.No sunscreen will be effective, however, if not used properly. A water-resistant formula is recommended and should be applied generously about a half hour before going outdoors. It should be reapplied at least every 2 hours, or after swimming, drying off, or sweating. For more information on sunscreens, including specific products, go to www.wedmd.com, High-SPF Sunscreens: Are they Better?