Someone asked me this question, and I researched the following answer.
Ultraviolet lamps are comprised of two electrode filaments (the spring looking wire at each end), a glass and quartz combination vacuum tube, several inert gases and approximately 3 mg of mercury (Hg).
These specialty lamps are designed to provide light energy in the germicidal spectrum of 254 nm (nanometer = one billionth of a meter). As they operate, they slowly loose their germicidal effectiveness, about 15% per year.
UV lamps should be exchanged after a maximum two year installation period for several reasons, including:
The disinfection effectiveness will be significantly compromised and the system will not be able to generate UV-C light as specified. Note that even though a visible blue glow can seen though the system viewport, the glow is not UV light (which is invisible) but actually a by-product of the operation.