From what I understand (and please correct me if I'm wrong), the SPF applies only to UVB rays -- these are the rays that cause burning. UVA rays are the ones that cause wrinkles, cancer, etc. and the SPF does not apply to these rays. I THINK that some of the sunscreens reported as ineffective don't have enough UVA protection in them. We go through life thinking we've been effectively protecting our skin 'cause we don't burn, blissfully unaware that the UVA rays are silently doing their below-the-surface damage . . .