Moved into new (to us, built in 1980) house 2 years ago. Installed an Airthings radon meter a year ago. Levels get up to 400 Bq/m3 when all the windows are closed. Finally had a radon mitigation system installed. Will hopefully get to much safer level below 100 Bq/m3.
It gets cold where I live, but it’s never been quite that cold! Roughly -15 to -30C most years.