What is SPF?
You may have noticed that sunscreens always have an SPF rating. Sun Protection Factor (SPF) measures protection of skin from UVA and UVB rays offered by sunscreen products. The higher the SPF, the more protection it gives.
SPF is first calculated by measuring the amount of time required to produce the first signs of sunburn on unprotected skin, versus the time it takes to produce the first sign of sunburn on protected skin.
If your unprotected skin burns in 10 minutes, then an SPF 15 sunscreen will give you 150 minutes of protection. This is based on the sunscreen being applied properly and not rubbed off during the tine of sun exposure.
SPF 15 x 10 minutes = 150 minutes
This is an example only and the time it takes your skin to sunburn will vary from person to person.
SPF is only one element of sun protection and should not be considered alone but with water resistance, application amount and any other activities which may run the sunscreen away.