Let’s cut to the chase. The infectious range for anthrax has been estimated at 6,000 to 8,000 spores/cu m of air. This means the hvac system — when properly maintained, given better filtration, increasing the number of air changes, and getting accessories like in-duct UV lighting — can be quite effective at reducing the total number of spores.
The bad news, of course, involves all those hvac systems that are not properly maintained; whose coils are dirty enough to breed and feed an army of anthrax; and whose mechanical rooms are too easily accessible.