The Sun Protection Factor (SPF) is commonly thought of as how much longer skin covered with sunscreen takes to burn compared with unprotected skin. Let’s say you would typically start to burn after 10 minutes in the sun, then using a SPF 15 rated sunscreen, would imply that you can safely remain in the sun for 10 min x 15 = 150 minutes before burning. Or with an SPF 50 rated sunscreen, you can safely remain in the sun for 50 times as long as compared to no sunscreen. If you'd do the math, that would be 10 min x 50, which would result in 500 minutes. Just be aware that sunscreen wears off during surfing, sweating and due to other activities. So the 50x times only can be assumed if you re-apply your sunscreen regularly.
Another way of thinking about SPF, rather than as time extended in the sun, is in dose/exposure-related terms. If you spend a certain time in the sun, wearing sunscreen with a given SPF would reduce the UV dose to 1/SPF of that which you would experience by spending the same time in the sun but without sunscreen – e.g. applying an SPF30 sunscreen results in a UV exposure of one-thirtieth of that which you would have received had you not worn sunscreen. Of course, one caveat applies, this assumes perfect application, which is rarely achieved. Most people in real life apply less sunscreen than the amount required, and they typically apply it less uniformly leaving patches of skin without adequate protection. As a rule-of-thumb, the protection actually achieved is only about one-third to one-half of the labelled SPF!
Answered by Alex Kelleher, MD