[williamtrask]

Also high CO2 in working and sleeping areas. Basically any older building without HVAC and with upgraded (double pane) windows.

[surfmike]

Most home HVAC systems don’t bring in fresh air, right? Just recirculate air in the house.

[kubanczyk]

V in HVAC is for "ventilation" and although home AC unit might not provide it itself, I'd be surprised if there really wasn't any. Unlike AC, V is mandated by law in many jurisdictions.

[kqr]

The ones I know suck air out of the building (well, really, create a mild low pressure), and rely on fresh air getting in elsewhere (for example through dedicated intakes around windows in apartments.)

[gruez]

Newer homes are supposed to have heat recovery ventilators, which do bring in fresh air even if all windows are closed.