Depends on your climate/weather, local noise/crime, and construction of your house. Opening a window can be an option. So can an open bedroom door. The best o

Depends on your climate/weather, local noise/crime, and construction of your house. Opening a window can be an option. So can an open bedroom door. The best option to me is to just leave your air handler fan set to "on" rather than "auto". Of course you need forced air for this. My house is relatively newer and moderately sized. It still seems to keep the whole house CO2 levels like 50-100ppm over outdoor levels. The bonus here if you're looking for "air quality" and nor just CO2 is that your air handler filter should catch many particulates (although most aren't capable of high merv ratings, it still seems to help).