The SPF (Sun Protection Factor) 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. Even lower SPF numbers can be very effective at blocking the vast amount of UV rays. In fact, as long as your sunscreen has an SPF of 30 or higher, you will be protected from at least 97% of the sun’s rays! However, if you want the most protection, SPF 50 blocks about 98% of the sun’s rays while SPF 100 blocks about 99%. Unfortunately, no amount of SPF can protect you from 100% of the sun’s rays, so using umbrellas or sun hats can provide additional protection from sunburns and skin cancer.