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.
This section gives some data for naturally occurring isotopes. Copyright 1997 Mark Winter Department of Chemistry at the University of Sheffield , Sheffield S3 7HF, England.