Aranet4 Home: Wireless Indoor Air Quality Monitor for Home, Office or School [CO2, Temperature, Humidity and More] Portable, Battery Powered, E-Ink Screen, App for Configuration & Data History

You don't need to guess. Get an indoor CO2 monitor like the Aranet4 and find out!

Nope, carbon dioxide is absolutely what I meant. It is a waste product from exhaling every time you breathe and is one of the biggest reasons why you should open a window every now and then. You can buy a monitor to track it as part of your air quality. Because it is denser tham air, it tends to linger.