Kansas City, Mo. ? Who knew that sun protection could be so mystifying?

First we’re told that we should slather on sunscreen every time we go out. Then, we were told too much of it could deprive us of vitamin D – increasing our risk of certain kinds of cancer.

And now, a new curveball: Sunscreens, if used but not applied often enough, can actually cause more damage than if not used at all.

According to a new yearlong study, some of sunscreen’s UV filtering ingredients can be more dangerous once they are applied and absorbed deeper into the skin – and another application of sunscreen is not reapplied later.

Scientists found that if three widely used FDA-approved UV filters are left to penetrate the skin’s outer layer, they can generate free radicals that have been shown to cause damage to the cell walls and DNA. Such damage potentially increases the risk of skin cancer. Reapplying sunscreen helps protect inner layers of skin.

“If you went into your bathroom medicine cabinet, most likely the sunscreen you are using would contain at least one of these,” said chemist Kerry Hanson of the University of California, Riverside, a co-author of the study with Chris Bardeen. The findings are in an upcoming issue of Free Radical Biology & Medicine.

The study took a mixture of those three UV filters – octylmethoxycinnamate, benzophenone-3 and octocrylene – mixed in a simple oil-and-water-based cream, rather than testing sunscreens on the market, she said.

“What you buy at the drugstore tends to be a more advanced cream,” she said. And more advanced formulations might have properties that don’t penetrate the skin as quickly.

“The ideal sunscreen acts as a latex paint on the outermost layer of your skin and doesn’t penetrate to the inner layer of skin cells,” she said.

The skin’s inner layers are more vulnerable because they contain nuclei and DNA that could be harmed by free radicals created by the UV filters.

But Hanson emphasizes that the study doesn’t mean that the rules of sun protection are shifting. Rather her findings simply reinforce advice from such experts as the Skin Cancer Foundation: Apply liberally and reapply often – every two hours or after swimming or exercising, she said.

“The standard recommendation is the way to go,” she said. “Most people apply sunscreen at 10 a.m. when they go to the lake and beach and they forget to reapply it.”

Sukumar Ethirajan, an oncologist at the Kansas City Cancer Center, said the study was evidence that physicians not only need to continue to emphasize the importance of using sunscreen, but also that “we need to be more conscious that the protection needs to be continuous and not just short-term.”

That’s an important message for anyone who spends much time outdoors.

Jan Marcason uses sunscreen when she visits the pool but might not apply any when she goes for a walk after work. She doubts that many people remember to keep reapplying.

“I can’t imagine that they do, especially with the busy lives we lead,” she said.

The study did not test all the UVB filters on market, nor did it test UVA filters, Parsol 1789 or recently FDA-approved Mexoryl, which block the deeper-penetrating UVA rays, Hanson said.

More research is needed to create advanced formulations of sunscreen that stay on the skin’s surface, she said.

Hanson also recommends using sunscreens that contain vitamins E or C to reduce free radicals in the skin as added protection.

She admits that all the research on skin protection can seem a little mind-boggling to the average consumer. But her study’s findings simply underscore the conventional wisdom, use sunscreen and reapply it regularly. And don’t let those reports about the deprivation of vitamin D scare you away from sunscreen, she said.

“Walking to your car or getting the mail every day could be enough” exposure for vitamin D, she said.