SPF30 Fun Facts
SPF or Sun Protection Factor is only a measure for UVB (how well it protects you from sunburn… not a rating for how well it protects you from skin cancer (UVA).
SPF15 protects against 93% of the suns harmful rays. SPF30 protects against 97% of the suns harmful UVB rays
Regardless of what SPF number you are applying they all recommend reapplication after 2 hours.