CO2, carbon dioxide, is a gas we exhale. It's used as a marker for indoor environmental quality because if our emissions of that are hanging out in the home, then any of our other odors, viruses, skin particles (which make up a lot of household dust), etc are also filling up the home. On an airplane, for example, which 'they' say is one of the most ventilated indoor places you can find, you can easily prove when they turn the outdoor air off while you're sitting on the ground, because the CO2 spikes upward of 2000 or even 3500 in our experience (and therefore, any of the COVID laden particles are also obviously building up). CO2 is not everything, in fact you might have a low CO2 level only because there aren't many people in the home, or the home is big enough to dilute it- you might have other markers showing the home suffers from poor ventilation, like formaldehyde, humidity, particles, TVOC, radon, and so on. But it's something that can be easily measured, so let's measure it and try to optimize for 1000 ppm as the max in homes, along with all the other factors. The two devices I use in this video, and everyday in our own home, are the Aranet4 (https://aranet.com/products/aranet4/) and the Airthings Wave Plus (https://www.trutechtools.com/AirThings-Wave-Plus-Indoor-Air-Quality-Radon). The 5 Factors of Ventilation are: https://www.youtube.com/watch?v=FOJ8fsAVUaI Learn WAY more about how to strategize the 5 Factors of Ventilation with our Ventilation Course at: https://buildingperformanceworkshop.com/ventilation See the Broan Overture system for controlling all of this: https://www.youtube.com/watch?v=slNPHuIXwnw Consult with Corbett on tuning any home, anywhere: https://buildingperformanceworkshop.com/video-consulting Watch the first-ever TV series about the Science of Homes: https://homediagnosis.tv/season-2-episodes