A Sun Protection Factor (SPF) extends the time it takes for the skin to burn by a certain factor. For example, a person who gets sunburnt after 30 minutes of sun exposure without sunscreen would not get sunburnt for 300 minutes using a sunscreen in high enough amounts with a sun protection factor of 30. However, it is best to only stay in the sun for up to 60% of this time, and remember that skin damage can occur even before sunburn is visible.