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