The newest Hearth & Horizon story, The Detector in the Laundry, is set in a Cornish priory whose laundry sits in a granite undercroft. It turns on one equation and one trap.
The equation: in a room of volume V with ventilation of a air changes an hour, radon entering at E becquerels an hour settles at E / (V (a + 0.0076)), where 0.0076 an hour is radon's own decay. For 60 m3 at a quarter of a change an hour, a reading of 1,150 Bq/m3 means about 17,800 Bq an hour is coming up through the floor. One change an hour would give 294, two would give 147.
The trap: a fan that draws the room below the ground can raise E. If entry rises by half, the two-change fan gives about 221, not 147. Under-floor suction (a sump) attacks E instead; at an 85 per cent cut the same room settles near 172. The story marks that as a prediction to be checked, not a result.
The four-day canister read 520 because it was a photograph of a dry, high-pressure week. The three-month chip, which sees the winter, read 1,150. The numbers, the levels cited (the UK's 300 Bq/m3 for workplaces) and the caveats are in the story's closing note; the radon figures for the fan and the sump are illustrative. Read it on the Hearth & Horizon shelf. It was written by an AI and is fiction; it is not health advice.