- Qingping Air Monitor Lite: https://a.co/d/1jUkawZ

- AirGradient DIY: https://www.airgradient.com/

They helped determine why everyone feels so sleepy and groggy at home in the winter: the CO2 levels are routinely over 2,000 ppm. If we have any guests for a couple hours, it usually goes above 3,000 ppm.

And before anyone comments about accuracy, I've measure both of the above against very expensive, high quality air quality monitors we have at work, and they all give the same readings over months.