More on filters:

Most HVAC filters are designed to keep the system clean enough to avoid problems – they aren’t for purifying air, they’re to keep gunk from building up on equipment and vents. Using a higher rated filter in a system that wasn’t designed for it will cause expensive problems down the road. The restricted airflow (which only gets worse as the filter gets clogged) is hell on the equipment: it’s less efficient and can’t exchange heat at the rate it was designed to. E.g.

• Blower motor.
• AC evaporator coil.
• Heating elements or burners.

Unless your HVAC system was purposefully designed to act as an air purified (and you’re following the manufacture’s directions) you’re better off using a “low efficiency” filter for it and purifying the air with purpose build devices … like a Corsi-Rosenthal box.