Ultraviolet radiation is divided into three categories: UVA, UVB and UVC, only one of which is blocked by conventional window glass. UVA has the longest wavelength, followed by UVB and UVC.
The longer the wavelength of light, the greater its penetrating power. UVC, with the shortest ultraviolet wavelength, is readily filtered out by Earth's ozone layer and generally doesn't reach the surface. UVB has a longer wavelength, and it does penetrate the atmosphere. UVB is not able to penetrate deep into human skin, but it does cause sunburn on the skin's outer layer and is a contributing factor to skin cancer. UVB is not able to pass through glass. UVA has the greatest penetrating power, and it causes damage to the skin's deeper layers. UVA passes unimpeded through most glass.