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www.verywellhealth.com/what-is-spf-1068889

SPF stands for sun protection factor. Simply put, an SPF rating tells you how long you can stay in the sun without getting burned while wearing that sunscreen, compared with how long you can stay in the sun before you burn without wearing that sunscreen . For example, if it typically takes you 15 minutes to burn without sunscreen and you apply an SPF 10, it will take 10 times longer (2.5 hours ...

www.webmd.com/beauty/features/high-spf-sunscreens-are-they-better

"SPF is not a consumer-friendly number," says Florida dermatologist James M. Spencer, MD. "It is logical for someone to think that an SPF of 30 is twice as good as an SPF of 15 and so on. But that ...

www.healthline.com/health/what-spf-should-i-use

SPF is short for sun protection factor. In sunscreen, SPF helps to block your skin from the sun’s radiation. ... so sunscreens that use them as a base have a good natural SPF foundation.

www.medicalnewstoday.com/articles/306838

Sun protection factor (SPF) is a number, for example, SPF15. It indicates how much protection a product offers against UVB light. A product with a higher SPF number will offer greater protection.

health.howstuffworks.com/skin-care/beauty/sun-care/spf.htm

To determine a sunscreen's SPF, testers round up 20 sun-sensitive people and measure the amount of UV rays it takes them to burn without sunscreen. Then they redo the test with sunscreen. The "with sunscreen" number is divided by the "without sunscreen" number, and the result is rounded down to the nearest five. This is the SPF.

www.skincancer.org/blog/ask-the-expert-does-a-high-spf-protect-my-skin-better

The SPF number tells you how long the sun’s UV radiation would take to redden your skin when using the product exactly as directed versus the amount of time without any sunscreen. So ideally, with SPF 30 it would take you 30 times longer to burn than if you weren’t wearing sunscreen.

www.mayoclinic.org/healthy-lifestyle/adult-health/in-depth/best-sunscreen/art...

Experts recommend using a sunscreen with an SPF of at least 30. Sunscreens with SPFs greater than 50 provide only a small increase in UV protection. High-number SPFs last the same amount of time as low-number SPFs.. Sunscreen is often not applied thoroughly or thickly enough, and it can be washed off during swimming or sweating.

www.webmd.com/beauty/features/whats-best-sunscreen

While SPF 15 filters out 93% of UVB, SPF 30 filters out 97%, only a slight improvement. UVA protection. There is no rating to tell you how good a sunscreen is at blocking UVA rays, says Leffell.

www.skincancer.org/skin-cancer-prevention/sun-protection/sunscreen

The number tells you how long the sun’s UVB rays would take to redden your skin if you apply the sunscreen exactly as directed compared with the amount of time without sunscreen. So, if you use an SPF 30 product properly, it would take you 30 times longer to burn than if you used no sunscreen.

www.oprahmag.com/beauty/skin-makeup/g26719162/best-sunscreen-for-dark-skin-tones

While you don't necessarily have to reach for that bottle of SPF 100 (SPF 30 is sufficient), yes, you should wear broad-spectrum sunscreen every day, even if you have a darker skin tone. The trouble is, so many options on the market leave a white cast that can be unflattering and chalky.