Answer to Question #2498 Submitted to "Ask the Experts"

Q

Thank you for giving nonradiation experts the opportunity to get accurate answers to questions about radiation and health. I have two questions that concern the health risks of certain exposures to radiation.

First, I have a small plastic disc in which is embedded 10 microcuries of cesium-137, which is used to test the operation of a Geiger counter. We have always been afraid of the cesium-137 disc and have stored it outdoors in a metal container. What are the health risks of this device if one were exposed to it at perhaps 12 inches for eight hours per day? What would be the risk of holding it in your hand for a period of hours or carrying it in your pocket for about the same period?

Second, a family member was recently given a thallium-201 cardiac stress test. I assume this test was given using thallium-201, but I am not sure of the isotope. When she arrived home, my Geiger counter indicated almost continuous counts at four meters away. Two centimeters from her body the counter registered 10 milliroentgens per hour. Having used the Geiger counter for the testing of radioactive rock samples, I had never before encountered a dose this high. What are the risks to those who might be near her for extended periods such as in bed for eight to ten hours? What is the half-life of this isotope, and what type of radiation does it emit? Is radiothallium basically a gamma emitter similar to cesium-137? How long will this material remain in the body, and what are the risks to the patient who was administered the radiothallium intravenously and will presumably be exposed to well over 10 milliroentgens per hour for several days or weeks? Clearly the risk is less than that of an untreated cardiac condition, but since we are dealing with radiation the risk would have to be at least somewhat higher than no exposure.

A

To answer your first question, radioactive check sources for verifying the day-to-day operation of survey meters are low-activity, low-hazard sources. You say that yours is a 10 microcurie cesium-137 source (of course we need to know 10 microcuries on what date? Cesium-137 has about a 30-year half-life, so half of the activity decays away every 30 years). A quick calculation of the exposure rate at 1 meter from a 10 microcurie source would be about 3 microroentgens per hour (by comparison background radiation is of the order of 10 microroentgens per hour).

In answer to your second question, thallium-201 is widely used for cardiac tests in nuclear medicine. Thallium-201 emits a number of x rays and gamma rays (the most important one around 70 keV) and a series of moderate to low-energy electrons (which will be absorbed in the body of a person who has received an injection of thallium-201). The effective whole-body dose equivalent to the subject receiving the test is around 1,200 mrem (12 mSv). By comparison, our annual effective whole-body dose equivalent from all forms of background radiation is around 300 mrem (3 mSv). The dose is much lower to other people around the subject. I don't think this has been well characterized for thallium-201 studies. The release of patients receiving "therapeutic" levels of radiopharmaceuticals is carefully controlled, as this can result in an appreciable radiation dose to family members and others. The dose from "diagnostic" tests such as this one are known to be fairly low and are not controlled. Even though you can detect some of the gamma rays coming from a person who has had a thallium-201 test, using a sensitive survey meter, this does not necessarily indicate that there is a significant hazard to worry about.

(I hope that the units used on these quantities are meaningful to you. If they are strange or confusing, see some of the material at the "Ask the Experts" Radiation Terms and Definitions page.)

Michael G. Stabin, PhD, CHP