SPF 101

A sunscreen’s SPF or “sun protection factor,” refers to its ability to shield from the sun’s ultraviolet rays. It is a measure of the time it would take for a person to start getting red if they were not wearing sunscreen.
In general, it takes about 10 to 20 minutes without sunscreen for a person’s skin to start burning. An SPF 15 product would prevent skin from burning for 15 times longer – so about 150 to 300 minutes, or about 2.5 to five hours. However, that doesn’t mean you’re fully protected for that five hours. Dermatologists highly recommend reapplying sunscreen every two to four hours, as sunscreen can rub off or get washed off in the water.
Some brands are better than others in terms of the range of ultraviolet radiation (UVA, UVB and UVC rays) they protect against. More brands in recent years have broad-spectrum or multi-spectrum protection, meaning they shield against more than one type of ultraviolet radiation. All-natural sunscreen alternatives containing zinc oxide are also effective. Look for brands with those descriptions, as UVA rays are the number one cause of long-term skin damage, including wrinkles and cancer.
Source: IB Times