Air Handling Unit's MICROBE-filter's mesh size is 2 microns. Y?

Question

The Air handling units employed in the Pharmaceutical industries, usually have filters that prevent Bacteria or microbes to enter, as the production requires sterile air.

Usually the filters used have a size of 2 microns only.
* Why not lesser than that say 1 micron, to enhance better filtration?

Please, serious answers only.
Your help great/small will be appreciated :)

any Mech engg's or Microbiologists out there?

Answer 1

filtering too small organisms will clog the filter and the 2 micron filtration when stabilized acts as a filter to the smaller ones too.