In the Straight Dope Classic #1, posted in the 1/01/2010 newsletter, you made the statement that, "There is unquestionably some risk involved in the use of a radioactive substance, but it is small . . . . ). This isn’t exactly untrue, but where very small radiation doses are concerned it should be qualified.
When the EPA promulgated its Standards for Protection for Ionizing Radiation in the 1970s, they were based on the “prudent” assumption that cancer risk is proportional to dose between zero and high levels of exposure known to cause cancer, and that there was no “threshold” below which the risk is zero. The modifier “prudent” was an admission that the assumption could possibly be wrong, but that it was best to use a linear nonthreshold model, “just in case”.
Since then (and beginning even before the 1970s) some scientists proposed that certain anomalous results of radiation response testing may result because low levels of radiation are actually beneficial (hormetic), and this is currently being vigorously debated among scientists (google “radiation hormesis”). A large group of scientists believe that low doses of radiation exposure by stimulating the human immune system and preventing it from going into a dormant state.