1. SPF 30 × 15 minutes = 450 minutes
Until now, many people have misunderstood that UV(ultraviolet) index indicates the time lasting for protecting skin from the UV rays. 450 minutes is almost 8 hours. So it is easy to think that sunscreen of SPF 30 protects skin for almost 8 hours. If you go outside at 10a.m. it lasts until 6p.m. It is enough to apply it once a day. However, this is false. The UV index shows an intensity of blocking UV rays. Therefore, even though you use sunscreen of high index, if you don't re-apply it frequently, it is hard to be effective.