Hi,
Hopefully you are fine,
any one can share me reference document (WHO/FDA/GAMP/cGMP/ISO14644) )regarding to define frequency of HVAC system (Air balancing of Clearoom) /HEPA filter DOP test
WHO, FDA guides, EU GMP guides are available for free on related agency web sites. Just google little bit!
ISO14644 is not for free as all the other ISO’s, you need to purchase it.