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SPF, or Sun Protection Factor, refers to the time it takes your skin to get a burn from the sun when wearing sunscreen, compared to the time it would take you to burn when not wearing sunscreen. For example, if it takes you 450 minutes to burn while you are wearing sunscreen and 10 minutes to burn when you are not wearing sunscreen, the SPF = 450 / 10, or more simply an SPF of 45!
The SPF number measures UVB radiation from the sun. The sun, however, has both UVB and UVA rays, and both can damage the skin. So selecting a sunscreen based only SPF is not good enough. Make sure your sunscreen label says, “broad spectrum” and protects against both UVA and UVB rays.
Also, no matter which sunscreen you select, reapplication is key. Nothing is sweat proof and water resistance only lasts 40 or 80 minutes. If you’re at the beach, pool, on the golf course, or spending a significant amount of time outdoors, remember to take your sunscreen with you and re-apply every 1-2 hours.