When you rub sunscreen on, you assume you’re guarding against sunburn, skin cancer, and wrinkles. But research indicates that consumers might be getting less than half the sunburn protection shown on labels because they don’t apply enough. Moreover, our tests of 19 sunscreens found that some provide minimal protection against ultraviolet A (UVA) radiation, which can cause skin cancer and wrinkles. UVA is not considered in the sun-protection factor (SPF), which refers to protection against UVB radiation, a different wavelength that can also cause skin cancer, plus sunburn.

We tested the SPF of all the sunscreens 15 minutes after application. Products claiming water resistance were scored again after immersion in water (scores for the rest are without immersion). We also tested all products for UVA protection, without immersion. And we took the eight products that claim to have zinc oxide or titanium dioxide, which absorb UVA, and had an outside lab test them for microscopic particles of those compounds. Whether such nanoparticles pose health risks is a subject of debate.

What we found. All products came within 10 percent of meeting their claimed SPF. In theory, an SPF 30 sunscreen lets you stay in the sun, unburned by UVB, up to 30 times longer than without it. But people typically apply about 25 to 75 percent less sunscreen than the amount used in most sunscreen tests, including ours. And in our UVA tests, several products scored fair or poor, even if they met their SPF claim. As a result, we’ve weighted UVA scores most heavily in the Ratings.

Of the eight products tested for nanoparticles, all contained them, but our tests found no correlation between their presence and sun protection.

What to do. Choose a sunscreen with maximum protection against both UVA and UVB rays--preferably one labeled very water resistant or waterproof, with an SPF of at least 30. Two fine choices: Neutrogena Ultra Sheer Dry-Touch SPF 45 and No-Ad Sunblock Lotion Maximum SPF 45, a CR Best Buy.

Be sure to use enough. To get the labeled protection, an adult in a swimsuit needs to use 2 to 3 tablespoons of sunscreen. Reapply it every two hours and after swimming or substantial sweating. If you’re out all day, find some shade or add protective clothing, including a hat.

More advice. For more information on sunscreen and other ways to protect yourself from UV radiation, check out our seven summer safety tips. You'll find advice on clothing, symptoms to watch out for and more.