They don't work in ducts. They work inside the cabinet.
The reason they don't work in ducts is because the amount of time UV
light takes to kill things in the air is longer than the amount of
time the air is rushing by.
As for working inside the cabinet, where the air is still, not only
would you need lights surrounding whatever surfaces you are aiming at,
but if you have things to kill on those surfaces, that means there is
something wrong with the system in the first place.