 The only types of charged particle radiation produced in the process of a criticality accident (fission reactions) are alpha particles, beta particles, positrons (which all come from the radioactive decay of unstable daughter products of the fission reaction) and energetic ions which are the daughter products themselves.Of these, only beta particles have sufficient penetrating power to travel more than a few centimeters in air.Since air is a very low density material, its index of refraction (around "n"=1.0002926) differs very little from that of a vacuum ("n"=1) and consequently the speed of light in air is only about 0.03% slower than its speed in a vacuum.