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

Rules implemented last year by the U.S. Food and Drug Administration say that only sunscreens that are broad-spectrum protecting against both UVA and UVB rays and that have an SPF of 15 or higher can claim to reduce the risk of early skin aging and skin cancer.

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The UVA component is important as window panes at home or office or car often block UVB but not UVA rays.

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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.

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