15, 25, 30, 50... Wherever you shop sunscreen, there will always be a number following 'SPF' on each label. SPF or 'sun protection factor’ tells us how much longer we can stay out in the sun without burning, compared to our natural protection. Dermatologists recommend using a sunscreen with an SPF of at least 30, which blocks 97% of the sun's UVB rays. Higher-number SPFs block slightly more of the sun's UVB rays, but no sunscreen can block 100% of the sun's UVB rays (that’s why it’s important to use an antioxidant serum underneath your SPF, which can scavenge UV rays that sneak through your sunscreen). It is also important to remember that high-number SPFs last the same amount of time as low-number SPFs. A high-number SPF does not allow you to spend additional time outdoors without reapplication. As many individuals only apply about 50% of the amount of sunscreen needed to achieve the amount of SPF on the label, application of high-SPF sunscreens help to compensate for this under-application. Sunscreens should be reapplied approximately every two hours when outdoors, even on cloudy days, and after swimming or sweating, according to the directions on the bottle.