In reality, SPF 30 should be more than adequate to protect you from getting a sunburn. A sunscreen rated SPF 30 should block up to 97 percent of the sun’s harmful UVB rays when applied properly, and reapplied about every 2 hours during your time in the sun. What does properly mean when applying sunscreen? For your face and ears, it means an amount about the size of a nickel. For your neck, also about a nickel. For your body, an amount that would fill a shot glass, one ounce, or two tablespoons should be sufficient. Sunscreen sprays should be applied closely to the skin and evenly over the exposed areas.
The problem is that most people do not apply enough sunscreen and they end up getting burned, leading them to believe sunscreen does not work very well. In fact, most people apply only about one quarter of the necessary amount of sunscreen, and generally do not reapply while they are outside. So a sunscreen with an SPF of 30 ends up acting like an SPF of 8. They also commonly miss areas inadvertently. When you apply an insufficient amount of sunscreen, you do not achieve the protection factor the sunscreen is rated to provide. You may be able to get away with this for quick trips outside, but for longer exposures, this will result in a sunburn.
I generally recommend sunscreens with an SPF of 50 as a starting point so that even if you don’t apply your sunscreen heavy enough, you will still have more protection that if you had started with a lower SPF sunscreen. Most importantly, any sunscreen you apply on a daily basis is better than not applying any at all!