The sun protection factor (SPF) on all sunscreen labels is a ratio based on the amount of UVB (not UVA) radiation required to turn sunscreen- or sunblock-treated skin red compared to non-treated skin.
For instance, people who sunburn in five minutes and who want to stay in the sun for 150 minutes might use an SPF 30.
The formula would be: 30 (the SPF number) times five (minutes to burn) = 150 minutes in the sun.
Protection offered by sunscreens may be classified as follows:
Minimal: SPF 2 to 11
Moderate: SPF 12 through 29.
High: 30+.
Although some sunscreens claim SPFs higher than 30, the added protection at such higher levels is insignificant.