Both UVA and UVB rays contribute to skin cancer, but the damage caused by UVA rays is harder to measure.

WSJ: European Sunscreen Roadblock on U.S. Beaches

New sunscreen labels in effect this year require a pass-fail test for UVA and UVB protection.

WSJ: European Sunscreen Roadblock on U.S. Beaches

On top of that, sunscreens can be labeled "broad spectrum" only if they protect equally against UVB (the main culprit of skin cancer) and UVA rays, which cause aging.

CNN: Everything you need to know about sunscreen this summer