Surprised not to see more mention of MVHR in the comments. This approach constantly brings in fresh air while retaining heat pretty effectively and also filteri

Surprised not to see more mention of MVHR in the comments. This approach constantly brings in fresh air while retaining heat pretty effectively and also filtering incoming air for particles etc. I'm just in the process of retrofitting this to my old home (which definitely has elements of 'sick house' syndrome at the moment). It's not that cheap, and can be tricky to retrofit - usually alongside improving airtightness and insulation - but is one way to dramatically improve air quality in a building, especially over colder months. https://en.wikipedia.org/wiki/Heat_recovery_ventilation https://en.wikipedia.org/wiki/Heat_recovery_ventilation