Radon monitor. Equipment for monitoring the radioactive gas radon in the home. The detector is left for extended periods of time in enclosed rooms, such as cellars. Radon gas is produced by a series of radioactive decays initiated by the decay of uranium, an element which occurs in many rocks. Radon levels are low in the open air, but in poorly-ventilated buildings the gas can accumulate in concentrations hundreds of times higher than out of doors. The radioactive decay products of radon can induce lung cancer. The threat from radon can be countered by improving a building's ventilation.