Q. There are lots of sunblocks out there with SPF numbers on them. What exactly do these numbers mean?

(Sun exposure is an extensive and important subject that is worth two columns. This is the first one.)Sunblocks or sunscreens work to prevent the damage of ultraviolet (UV) rays, an invisible component of sunlight. There are three types of UV rays: UVA, UVB and UVC.UVA is the most abundant of the three ultraviolet rays at the earth's surface. These rays penetrate through the outer skin layer. Many of the UVB rays are absorbed by the stratospheric ozone layer, so there aren't as many of these at the earth's surface as the UVA rays. UVB rays don't penetrate as far as UVA rays but are still harmful. UVC radiation is extremely hazardous to skin, but it is completely absorbed by the ozone layer.Sunburn and suntan are signs of skin damage. Suntans appear after the sun's rays have already killed some cells and damaged others. UV rays do more harm than damaging skin. They can also cause cataracts, wrinkles, age spots, and skin cancer.Sunscreens are given SPF (Sun Protection Factor) ratings that tell you how well they protect you from damaging rays from the sun. The SPF ratings can be as low as 2 and as high as 100+.Here's how the ratings work: If you apply a sunscreen rated at SPF 2, you will double the time it takes for your skin to burn. A sunscreen rated at SPF 15 will multiply the burning time by 15.Dermatologists strongly recommend using a broad-spectrum (UVA and UVB protection) sunscreen with an SPF of 15 or greater year-round for all skin types.The SPF number indicates the screening ability for UVB rays only. Research is being to done to establish a system to measure UVA protection.There is a point of diminishing returns with sunscreens. Here's how it goes:A sunscreen with an SPF of 2 screens 50 percent of UVB rays.A sunscreen with an SPF of 15 screens 93 percent of UVB rays.A sunscreen with an SPF of 30 screens 97 percent of UVB rays.A sunscreen with an SPF of 50 blocks 98 percent of UVB rays.A sunscreen with an SPF of 100 blocks 99 percent of UVB rays.Not applying enough sunscreen can seriously reduce your protection. You should use an ounce about a palmful on your body to gain the full protection indicated by the SPF on the product. Also, dermatologists advise reapplication every two hours or after swimming or sweating.It seems logical that, if you use half the required sunscreen, you will get only half the protection, but that doesn't seem to be true. A study in the British Journal of Dermatology found that you get the protection of only the square root of the SPF. So, in theory, if you use a half ounce of sunscreen rated at 64, you won't get the protection of an SPF 32, but only the protection of an SPF 8.In addition to applying a sunscreen, you should protect yourself by avoiding the sun between 10 a.m. and 4 p.m., wearing protective clothing and wraparound sunglasses, avoiding sunlamps and tanning beds, and checking your skin regularly for changes in the size, shape, color or feel of birthmarks, moles and spots.If you have a question, please write to

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