Using
UV Germicidal Irradiation is excellent for forced air systems to prevent build up of mold and mildew and bacteria on the surfaces of the HVAC system, especially in parts of the system that become damp, like condenser coils. The UVGI is constantly bombarding the coils keeping anything from growing on it.
However, UVGI effectiveness for the forced air passing by has little evidence of effectiveness because the air, with whatever germs are in it, pass by too quickly. Some testing in hospitals and other close quarters (like army barracks) have had mixed results -- and this is in small, contained spaces with the same people using the space. For a large space in which you have lots of new people coming in and out, you'd need *a lot* of UV units throughout almost the entirety of the ventilation system, which starts to become prohibitive in execution. One unit behind each intake vent ain't gonna cut it.
en.wikipedia.org
iaqscience.lbl.gov