Look again, Pete. It is Kr-85m that has a 4.48 hour half life. Kr-85 ha a half life of 10.73 years so that which is released from nuclear power plants, nuclear weapons, etc. remains in the atmosphere for some time. It is a relatively innocuous radionuclide, but exeeding the DOT 2 nCi/gm criterion is quite possible. Charlie Willis caw@nrc.gov