The microbial population of air is very variable.
Bacteria in soil can be generlly regarded as 10E8 to 10E10 per gram so
the amount of "dust" per cubic meter (most of which will be soil dust)
can give an idea of the likely burden. Added to that will be a
contribution of mainly fungal material(spores) from plant surfaces
together with any aerosols from such things as water surfaces (fountains
are a very powerful source of aerosol borne bacteria and fungi.
The main point is that the air burden will be very location/seasonal
Center for Articulated Contextualisation,
The University of Reading,