Radon is a naturally occurring radioactive gas that comes from the radioactive decay of radium and uranium. It is most commonly found in igneous rocks and soil (every square mile of soil to a depth of six inches contains 1.0 g of radium) and is sometimes found in spring water. At 25C, radon is colorless. Below the freezing point, radon produces a brilliant phosphorescence that turns yellow as temperature drops and orange-red at the temperature of liquid air. It is a member of the Noble gas family and is the heaviest known gas. The gas and its decay elements can be hazardous when inhaled and testing for radon gas in homes is becoming more widespread.

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