Ich kopiere mal etwas aus der instyle hier rein:

How effective are sunscreens with SPF?

In general dermatologists agree that a broad-spectrum sunscreen with an SPF of 15 is adequate for most people. However, if you have fair skin or spend a lot of time outdoors in direct sunlight, a higher SPF can give you added protection—just not as much as you think. An SPF of 15 gives you 94 percent to 95 percent UVB coverage, while an SPF of 30 only bumps up your protection to 97 percent, according to David J. Leffell, M.D. a professor of dermatology at Yale School of Medicine. And the higher you go, the smaller the percentage increase. The extra protection also comes at a cost: When more sunscreen is added to the formula it becomes thicker and potentially more irritating. But the reason the FDA has discussed capping SPF levels at 30 is that superhigh ratings tend to breed a false sense of security. "Using an SPF 60 is not like putting on an SPF 30 twice," warns Leffell. The bottom line: No matter how high the SPF, you still need to reapply every two hours.

Also ein höherer Lichtschutzfaktor bietet keinen höheren Schutz (zumindest keinen wesentlichen) sondern nur einen längeren.