On an episode of the sitcom "Seinfeld," Jerry and Newman discover Kramer asleep on the roof, shirtless, on a very sunny day.

"Oh man," Kramer says when they wake him up. "I think I cooked myself."

"Kramer! Look at your skin!" Jerry exclaims.

"Oh, stick a fork in me, Jerry. I'm done," he replies.

Bad sunburns may fuel funny TV scenes, but you know they're no fun to experience — and they can do long-term damage.

But even if you're not getting a blazing burn, too much exposure to the sun's rays may be putting you at risk for skin cancer.

That's why a new study out of St. John's Institute of Dermatology in London offers such important info: The thin sheen of sunblock most of you put on provides only about 40 percent of the cream's labeled protection, so harmful rays can still damage your skin's DNA.

The rating you see on the bottle is the protection you get when you apply 2 milligrams of sunscreen on every square centimeter of skin. That turns out to be a pretty thick layer, and most people don't use that amount.

So from now on, choose a sunscreen with micronized zinc and/or titanium oxide, and an SPF of 30 to 50. And lay it on thick. That gives you the best protection against UVA rays, the kind that penetrate deeper into the skin.

Don't get products with any other chemical sunscreens. Make sure to lather up toddlers. And infants shouldn't get sunscreen or direct sun.