At ordinary temperatures radon is a colourless gas. When cooled below the freezing point, radon exhibits a brilliant phosphorescence which becomes yellow as the temperature is lowered and orange-red at the temperature of liquid air. The main hazard is from inhalation of the element and its decay products which are collected on dust in the air. Recently, radon buildup in homes from the surrounding soil and rocks has become a safety issue and some areas around the world test homes for radon gas. It is the heaviest known gas.
Radon is present in some spring waters.
Isolation Here is a brief summary of the isolation of radon. Radon is present to a very small trace extent in the atmosphere and in principle could be obtained as a byproduct from the liquefaction and separation of air. This method gives 0.64 cm 3 of radon gas per gram of radium per month.