What affects air quality indoors after home improvement?
"Air quality" is an ambiguous phrase.

This baby monitor that you bought, on what basis does it determine the air quality in the room? Carbon monoxide, carbon dioxide, radon, particulates, pollen, VOCs, smoke?

There are literally thousands of things that can affect air quality. I don't think I would get too worried about a baby monitor, that doubles as an air quality sensor, giving you an overall poor reading without knowing why it is showing a poor reading.

Open the room (and your house) once a week and get fresh air into the room. If you are really concerned get a better piece of equipment to monitor your air quality that shows specifics about the content of the air. That would give you a better idea how to remedy poor air quality.

Good luck!