A sunny day at the beach has plusses and minuses for your health.

A little bit of sun can help your body produce vitamin D, but the sun's ultraviolet radiation raises your risk for skin cancer. And, it turns out, UV radiation poses another threat — it physically weakens your skin.

As far as barriers go in the biological world, healthy skin is a pretty awesome one. And it's one you definitely want to keep intact, which can be a challenge.

Skin cells have to stick together to keep your insides in and bend with your body as you move. Skin has to be permeable enough for you to sweat but impenetrable to noxious chemicals, bacteria and viruses. The skin's top layer — the stratum corneum — is just 15 microns thick (about half that of wax paper) and is your first line of protection against the outside world.

"Every single day it's insulted by all sorts of abrasions and contact with surfaces," says Reinhold Dauskardt, a bioengineer at Stanford University. "It's actually a remarkably good barrier, which is why getting medications through the skin is so difficult because many molecules simply can't penetrate."

Dauskardt's lab studies the biomechanics of skin. How much does it stretch when you stress it? How stiff is it? That sort of thing.

In a study just published in the Proceedings of the National Academy of Sciences, Dauskardt and his colleagues exposed pieces of stratum corneum from a human cadaver to UVB radiation. That's the type of UV light that the top layers of your skin absorb. In the experiments, the researchers used UVB equivalent to 12, 24, or 60 consecutive days in Florida sunlight.

Then, they tested the treated skin. How easily did it crumble when pressed? How much force did it take to tear or crack it. How hard was it to separate the layers of its cells.

Bottom line: UV light reduces skin's strength and dries it out, putting a strain on the cells. Skin exposed to UV light breaks, cracks and chaps more easily.

"UV radiation really is a double whammy," says Dauskardt. "On one hand you're making skin weaker and easier to break, and on the other hand you're actually increasing the stresses in the skin so there's more stress available to cause it to break."

At the microscopic level, UV radiation changes the structures of integral proteins, namely keratin, and fat molecules called lipids that serve as the glue that holds skin cells together. These lipids become less cohesive. So, even in deeper skin layers, the cells just don't stick together as well.

The damage depends on the dose, and the researchers used particularly high doses. They estimate it takes at least 2 days of sun exposure for the skin's top layer to start weakening.

Can goops and lotions help? Maybe. Most moisturizing products would help prevent the skin from drying out, Dauskardt says, but "depending on what moisturizing product you use, it potentially could even be damaging because it could aid in the absorption of UV light rather than blocking it."

Your best bet is sunscreen. Dauskardt's group has found in preliminary tests that most sunscreens block the physical damage. "What we're trying to figure out now is whether some sunscreens and some molecules or nanoparticles that are typically used in sunscreens are more effective than others," he says.

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