Ultraviolet light producing bulbs loose their ability to produce ultraviolet frequencies capable of killing bacteria progressively from the first moment they are plugged in and running.  They loose approximately 1/2 their output over the period of 1 year. As almost all units are engineered on this basis, the effectiveness of the system dramatically reduces after the 1 year period.

There is also a protective coating put on the inside of the bulb that protects its and allows the output current from the ballast to remain constant. After approximately 1 year, the coating ‘burns’ off causing the ballast or power supply to drive harder and try to compensate for the lower output of the lamp.  This stresses the ballast unit and therefore can cause damage to it, effectively burning it out.