[mangeletti]

I don't think the goal is to provide a bio-weapon defense system (I think it's more for bad smog), but this truthfully is how you would defend against a bio-weapon in a vehicle; use the only safe air source (the HEPA filter) to pump air in at a rate that maintains positive pressure in the cabin, preventing air from leaking into the vehicle from unsafe sources (e.g., a small crack in a seal somewhere).