Measures radon to identify if your home should have a radon mitigation system.​
https://www.cdc.gov/radon/radon-numbers-ld.html

This is the current historical data for my (Tim's) home.  I installed a radon mitigation system 10/31/22 as I saw levels get as high as 18 pCi/L.

Airthings app provides clearly defined air quality metrics with color coding to indicate whether these numbers are good, ok, or bad 
https://www.airthings.com/contaminants

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Real time air quality at any moment.  We use this information to ensure our clients are safe and that the fan speed and operation are dialed for their circumstances. This typically means running the  furnace fan at all times to keep co2 levels from building up. The most critical areas are typically the bedrooms overnight.  Some homes only need the fan on and fan speed adjusted while others require fresh air to be brought into their home with the fan on. If fresh air ventilation is required, we can then bring in fresh air anytime co2 levels are beyond a safe threshold and only as needed.