Geiger counters can not detect radon gas because:
a) Radon is radioactive because of radium that emits Alpha particles, which unlike Beta or Gamma, are "too large" and can be stopped by a sheet of paper or a layer of skin, hence they won't pass through the plastic case of the unit or the Geiger-Muller tube (the sensor)
b) Activity within radon gas is very slow, with particle release every 2-3 hours, meaning there is very little data for the geiger counter to make its calculations.
There are special Radon Gas detectors used for these purposes and can be found on Amazon for about $150.
They are left in the basement overnight and readings are checked in the morning.
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By Quarta-Rad, Inc. SELLER on July 18, 2015