I just wanted to post this out there incase anyone has any ideas how to do this.
Radon detection was brought up briefly in the DIY Kig v3 feedback thread. It seemed to be dismissed as lack of hardware. It appears though that may not be true as i came across this device in on other forums.
Looks to be made by Radon FTLAB though and they have their own Home sensors. Not sure if there is any viability there, but it is a interesting thought.
Interesting. Do you know how much it cost?
The link doesn’t work anymore here, but I’ve posted information about detectors in another thread:
Seconding this—right now I use an AirThings Wave, because outside of that, there’s precious else for under $300+ that can monitor Radon and easily integrate with any other systems.
I would love to add on a Radon monitor to my completely-local setup at some point, and AirGradient seems like a place that could make it happen 
Does anyone know what sensor AirThings is using to measure Radon? I haven’t see any under $100 that could even be considered to add to AirGradient, and the cost of the Wave that Jeff mentioned seems to indicate there must be something out there.
For what it is worth, looks like you could setup ESPHome to get data from your AirThings over bluetooth and report it to HomeAssistant or whatever else, giving you local processing.
AirThings BLE Sensors — ESPHome
I think AirThings designed their very own radon module.
Okay so how long before you design one too haha 
I was actually thinking about it but currently no idea how difficult it is.
Specs:
Cost:
https://www.globalsources.com/product/real-time-radon-sensor_1157701110f.htm
Not cheap
5310
That one also needs 12v which isn’t available on current versions of the AirGradient since they are 5v from USB
It seems a radon sensor can be DIYed.
Maybe a great project to try?