How to Understand the Use of Sunscreen


No matter what the color of the sky is, whether bright blue or steely gray, the sun’s rays are raining down billions of photon particles per second on us. And, in addition to the warmth and light we crave, those kaleidoscopic rays contain ultraviolet (UV) radiation, the source of sunburn, premature skin aging and skin cancer, which more of us get than all other cancers combined.

Use of Sunscreen

What SPF Ratings Mean

SPF is a number that indicates how well a sunscreen shields unprotected skin from damage caused by a particular type of UV radiation: sunburn-causing, skin-cancer-promoting UVB rays. The scale isn’t simple and intuitive, though:

  • SPF 15 blocks 93% of UVB rays
  • SPF 30 blocks 97% of UVB rays
  • SPF 50 blocks 98% of UVB rays
  • SPF 100 blocks 99% of UVB rays

Keys to understanding SPF ratings. Keep in mind the following guidelines as you think about what SPF rating you want:

  • SPF 15 is the minimum rating dermatologists recommend for any sun exposure.
  • SPF ratings above 50 aren’t meaningful. Nothing blocks 100% of UVB rays, and a sunscreen that touts an SPF 100 rating, which sounds impressive, will actually only block 1% more UVB rays than an SPF 50 sunscreen.
  • SPF ratings don’t tell you how long you can wait before reapplying. Though protection time is a factor in SPF testing, the Federal Food and Drug Administration (FDA), which regulates sunscreens, recommends that you reapply any sunscreen every two hours, regardless of its SPF rating.

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