Actually anything over 50 spf is bad for you... 50 is the highest you should ever go otherwise it is toxic to your body in large doses. *is a certified tanning booth attendant* We had to listen to that shit for 6 hours...
You have to apply it every hour to 90 minutes. But also, getting a small tan will help boost the sunblock. the darker your skin the higher the boost.. think of it as a multiplier.