I recently retrofit a new HVAC system and as a result have also gone on a bit of a deep dive on indoor air quality. The big thing I learned is that most homes -- even old "leaky" builds -- have rather poor ventilation, meaning all sorts of compounds build up in the air... CO2, formaldehyde, VOCs, PFAS, etc. Air purifiers work for particulate, but don't really make a dent for chemicals. IIRC, You would need like a 50 gallon drum of activated carbon to do anything on the chemical front.<p>A quick solution is to crack the windows, or if you live in a non-temperate climate, install an energy recovery ventilating system.