When you hear the word "Radon." I hope you think of us here a Radon Controls, Inc. We have made it our business to make homes and businesses safe for over 22 years. One interesting property of radon is that it glows when cold. Radon gas is clear and colorless. The liquid is transparent. The element starts to glow yellow, then orange, then red, as cooled into its opaque solid form at -71°C. The glow is 'Cerenkov radiation', which is light produced as charged subatomic particles interact with their immediate environment. Below is a photo of Cerenkov radiation.