@J12t AIUI anything above MERV 13 is likely to harm your air circulation machinery. I don’t understand the details, but it’s something to do with the pressures required to push air through the filters; the pressure drop on the outgoing side isn’t enough to get good circulation, and the higher pressure against the fans will (possibly dramatically) shorten the life of the motors. I’d guess stand-alone filters don’t use MERV ratings because that’s designed for protecting the equipment