People in glass houses really do need to be careful-and not just about throwing stones. Those folks are also in danger of long-term skin damage from the sun.

Dermatologist Dilvid Sawcer, M.D., Ph.D., assistant professor of clinical medicine at the Keck School of Medicine of USC, says that people who spend time in the sun-even behind glass windows or windshields-should make sure they wear sunscreen to protect against the sun's damaging ultraviolet light.

Ultraviolet light comes in different flavors, with the shorter wavelength rays-UV-C and UV-B-being the most dangerous and UV-A being the closest to visible light," Sawcer says. While glass blocks transmission of UV-C and UV-B rays, the longer wavelength UV-A rays sail through unimpeded.

The good news is that glass protects against the most dangerous ultraviolet radiation and can prevent sunburns. The bad news is that sunburns are an important indicator of skin damage, and with that signal absent, people assume nothing is wrong.

"For example, I see a lot more precancers on the left side of people here in the U.S. because drivers sit on the left side, and that is where the greater sun exposure occurs. When I was living in England, people had more precancers on the 'right," Sawcer says. "That little bit of exposure adds up."