This is a device I use in my office: it draws outdoor air from outside the window into the room, passes it through a filter, and then vents it indoors. The excess air pressure pushes indoor air out through the gaps around the door. This way, indoor carbon dioxide, particulate matter, and VOCs can be reduced to very low levels. Even an air purifier with activated carbon cannot achieve this effect, because air purifiers cannot lower CO2 concentration.
This device is modified from an AirFanta 3Pro. I reversed the fan installation and connected two reducing adapters so that the final diameter matches that of this plastic expandable duct. Finally, I sealed the window with a sealing fabric, leaving only an opening the same size as the duct.
Its only drawback is the lack of heat exchange, so it requires some additional air conditioning load. It would be unsuitable in very cold or very hot places.